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UBC Theses and Dissertations

Coexistence in the Gerridae Jamieson, Glen Stewart 1973

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n m . C-l COEXISTENCE IN THE GEBRIDAE by GLEN STEWART' JAMESON B.Sc. (Agr.)# M c G i l l U n i v e r s i t y , 1967 M. S c , U n i v e r s i t y of B r i t i s h C o l u m b i a , 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT CF THE REQUIREMENTS FOR T BE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Zoology We a c c e p t t h i s t h e s i s a s con f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA June, 1973 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p urposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada Date i i ABSTRACT A comparative a n a l y s i s has been made of the b i o l o g i e s of f i v e s p e c i e s of the w a t e r s t r i d e r G e r r i s , G. i n c u r v a t u s , G. buenoi, G. n o t a b i 1 i s , G. i n c o g n i t u s and G. r e m i g i s , which occur t o g e t h e r i n southwestern B r i t i s h Columbia. The aim of t h i s study was to i n v e s t i g a t e how c o e x i s t e n c e i s achieved, and through the c o n s t r u c t i o n of a computer model, to simulate the i n t e r a c t i o n s a r i s i n g from t h i s c o e x i s t e n c e . The t h e s i s i s d i v i d e d i n t o s i x main s e c t i o n s : f i e l d b i o l o g y , temperature p r e f e r e n c e s on a g r a d i e n t , food consumption, r e a c t i v e d i s t a n c e and movement, g e r r i d s as predator and prey, and the model. F i e l d s t u d i e s i n 1971 and 1972 i n d i c a t e d that the f i v e s p e c i e s of G e r r i s s t u d i e d are not e c o l o g i c a l homologues. Eoth s p a t i a l and temporal d i f f e r e n c e s were d e t e c t e d , although c o n s i d e r a b l e o v e r l a p does occur. The s p a t i a l s e p a r a t i o n r e s u l t s from the f a c t t h a t G. r e m i g i s p r e f e r s a stream h a b i t a t , G. i s c o ^ n i t u s p r e f e r s a l i t t e r e d water s u r f a c e h a b i t a t , and although G. buenoi, G. n o t a b i l i s and G. i n c u r v a t u s a l l p r e f e r an open water h a b i t a t , the d i s t a n c e s from shore p r e f e r r e d by each s p e c i e s are r e s p e c t i v e l y g r e a t e r . Temporal s e p a r a t i o n of the s p e c i e s r e s u l t s from d i f f e r e n c e s between the s p e c i e s i n t h e i r response to temperature. The optimum temperature f o r growth i n G. r e m i a i s and G. buenoi i s 22°C. and i n G. i n c u r v a t u s , G. iU£2gnitus a n ^ -* n o t a b i l i s 2 6 ° C The t h r e s h o l d temperatures below which growth ceases are 9.3 0C* f o r G. i n c o g n i t u s , 10.3°C. f o r G. n o t a b i l i s , and 12.6°C. f o r G. buenoi, G. r e m i ^ i s and G. i n c u r v a t u s . i i i Temperatures vo l u n t a r i l y selected by each species on a water gradient d i f f e r e d . G. remigis selected a temperature of about 2 0 ° C , G. incognitus temperatures between 14-30°C, and G. incurvatus, G. buenoi and G. n o t a b i l i s temperatures from 25-30 °C. The sig n i f i c a n c e of these temperature preferences in the context of those temperatures encountered by each species i n the f i e l d s i t u a t i o n was discussed. , Food consumption in a quantitative manner was described for the larvae and adults of the f i v e species. The following parameters being determined: s a t i a t i o n time, the mean length of feeding and non-feeding periods i n the presence of excess food, the effect of food deprivation on amount consumed, maximum gut capacity, the e f f e c t of temperature on food consumption, and the effect of developmental state on food consumption. A s i g n i f i c a n t difference in digestive rate between l a r v a l and adult gerrids was found, with the time required by an adult gerrid to achieve maximum hunger being about twice that required by a larva. There were no s i g n i f i c a n t differences among the larvae or among the adults. Temperatures affected food consumption, and within the temperature range tested, 5-32°C., the amount consumed increased with increasing temperature for every species except G. remigis. In thi s l a t t e r species, food consumption peaked at 20°C. Within any one instar, food consumption was found to peak about H0% of the way through the stadium, following a rapid r i s e from the previous ecdysis. After peaking, feeding declined gradually u n t i l the occurrence of the next ecdysis. i v The two parameters which determine the rate at which prey are encountered by the instars of the f i v e species studied, namely the distance at which they respond to prey items and thei r rate of movement, were measured. These parameters allowed ca l c u l a t i o n of the swath the predator covered as i t moved across the water surface. Results suggest that -Gerris species prefer to attack l i v e prey in front of them, and tend to ignore prey i f the attack requires a turn of more than 100°. The state of hunger was found to a f f e c t the vi s u a l angle required to e l i c i t an attack by G« £§IA2i§» a n <* regardless of species, smaller gerrids reguired the prey to be closer before an attack was i n i t i a t e d . The rate of movement in Gerris was measured as a function of s t r i d e length and the number of strides made per unit time. Stride length varied according to the length of the mesothoracic leg, and the frequency of movement was observed to be species s p e c i f i c . G. remic|is f a stream species, moved 4 to 6 times as often as the four other species studied, a l l of which are c h a r a c t e r i s t i c a l l y found on non-moving water surfaces. The propensity to move in G. remiqis was s i g n i f i c a n t l y influenced by hunger, with a maximally starved gerrid moving only one sixth as much as a satiated gerrid. Within a species, gerrid size had no s i g n i f i c a n t e f f e c t on the frequency of movement, although there was a tendency for smaller gerrids to move less. The significance of the di f f e r e n t propensities to move i s discussed for the f i v e species studied. V Predation studies demonstrated that no s i g n i f i c a n t differences existed among the species in the e f f i c i e n c y with which they k i l l e d prey. However, they did show that differences existed among the species in t h e i r propensity to attack, and that these differences resulted in s i g n i f i c a n t l y d i f f e r e n t numbers of prey k i l l e d by the species. G. remigis i n p a r t i c u l a r , both as a larva and as an adult, k i l l e d more prey than any of the other four species. The a b i l i t y of each instar to prey upon gerrids i t s own size or smaller was also investigated to permit assessment of the extent of cannibalism. Preference studies where the larger gerrids were presented simultaneously with a l l the prey size classes they could capture e f f i c i e n t l y , demonstrated that the prey gerrids most preferred were those most e f f i c i e n t l y handled. The e f f e c t of d i f f e r e n t "habitats" on k i l l success was also investigated. These studies indicated that the ease with which certain gerrids are captured approaches that with which t e r r e s t r i a l insects trapped on the water surface are captured, suggesting that cannibalism may be an important mechanism in population regulation and perhaps i n the a b i l i t y of d i f f e r e n t species to coexist. The re s u l t s of t h i s study were incorporated into a mechanistic computer model based cn an experimental components analysis type of approach (Holling, 1966), which was expanded to include a number cf species and to simulate an entire season under f i e l d conditions. Predictions by the model, which was based on laboratory-derived data, were compared to f i e l d observations, and were found to agree favourably. Cannibalism v i was predicted to be an important phenomonen in a l i the situations studied, and mean hunger l e v e l of the gerrids affected only s l i g h t l y the predicted number of progeny produced at the end of the summer. The model when run with only single species present showed l that each species completed i t s l i f e cycle with the observed number of generations each year and with the temporal spacing of these generations much as observed i n the f i e l d . When a l l fiv e species were simulated to coexist in the same habitat, G. i££03£ii5S was found to v i r t u a l l y eliminate a l l of the ether species present. This prediction i s suggested to be the reason l i t t l e overlap in f i e l d d i s t r i b u t i o n occurs between th i s species and the other pond species. The open water pond species, G. incurvatus, G. buenoi and G. n o t a b i l i s , were a l l predicted capable of coexisting in the same habitat. The s p a t i a l separation among these species in the f i e l d may thus be concerned with long term coexistence rather than the short term problem of occupying the same habitat. v i i LIST OF CONTENTS ABSTRACT ...2 LIST OF TABLES , ...11 LIST OF FIGURES ...............11 ACKNOWLEDGED! EN TS ..19 GENERAL INTRODUCTION ...1 SECTION 1. FIELD BIOLOGY GROWTH AND FECUNDITY ...4 I n t r o d u c t i o n ..................4 M a t e r i a l s and methods 5 1. Species d i s t r i b u t i o n s .............................5 2. The study area .................................... 5 3. Environmental temperature measurements ............ 9 4. G r i d sampling ......9 5. L a b o r a t o r y s t u d i e s ..11 A. E f f e c t o f temperature on i n s t a r d u r a t i o n ........ 12 B. F e c u n d i t y 12 R e s u l t s • ... 14 1. F i e l d sampling .................................... 14 A. Environmental temperature 14 B. General b i o l o g y of the g e r r i d s 15 C. S p a t i a l d i s t r i b u t i o n and m i c r o h a b i t a t s ..........22 D. Temporal occurrence and v o l t i n i s m ............... 29 2. Laboratory s t u d i e s .............41 A. E f f e c t of temperature on i n s t a r d u r a t i o n 41 B. Fecundity ....................................... 47 D i s c u s s i o n .51 SECTION 2. MOVEMENT ON A TEMPERATURE GRADIENT 59 In t r o d u c t i o n ....................... 59 v i i i Materials and methods 60 Results .................... 63 Discussion 68 SECTION 3. FOOD CONSUMPTION 74 Introduction ....74 Material and methods 75 1. General methods 75 2. Satiation time ..80 A. Effe c t of gerrid size 80 B. Effect of starvation ............................ 80 3. Digestive pause .....82 4. Food deprivation 83 5. Maximum gut capacity 84 6. E f f e c t of temperature on feeding .................. 84 7. Variation of feeding during development ........... 85 Results .....86 1. Satiation time 86 2. Digestive pause ................................... 88 3. Food deprivation ..................................91 4. Maximum gut capacity 96 5. Effe c t of temperature on feeding .....99 6. Variation of feeding during development ...........101 Discussion .............................................. 103 SECTION 2. REACTIVE DISTANCE AND RATE OF MOVEMENT ........113 Introduction .....113 Materials and methods 114 1. Reactive f i e l d ................114 2. Movement 120 Results ..................................123 1. Reactive f i e l d .................123 2. Movement .....................131 Discussion ..............................................138 SECTION 5. GERSIDS AS PREDATORS AND PREY .....150 Introduction ..............150 Materials and methods 151 1. F i e l d hunger levels ...............................151 2. Gerrids as predator and prey ...152 A. The s i z e classes of gerrids 152 B. Ef f e c t of species and hunger l e v e l on predation .152 C. Relative predator-prey s i z e and k i l l success ....154 D. Environmental complexity and k i l l success .......154 E. Effect of predator choice of prey on predation ..156 3. Other insect predators of gerrids ................. 158 Results ........160 1. F i e l d food a v a i l a b i l i t y and gerrid hunger l e v e l s ..160 2. Gerrids as predator and prey 162 A. Effect of species and hunger on predation .......162 B. Relative predator-prey size and k i l l success ....172 C. Environmental complexity and k i l l success .......175 D. Ef f e c t of predator choice of prey on predation ..179 3. Other insect predators of gerrids .................184 Discussion .............................................. 186 SECTION 6. THE MODEL ..195 Introduction ....195 Materials and methods .........195 A. Growth and moulting .............................198 X B. Predator-prey u n i t s ............................. 198 C. Number of predators and prey ......199 D. Temporal p a t t e r n of hunting bouts ...............201 E. C a l c u l a t i o n of food i n the gut ..................201 F. Estimated h a n d l i n g time and area searched 202 6. C a l c u l a t i o n of p o t e n t i a l number of prey k i l l e d ..203 H. C a l c u l a t i o n of a c t u a l number of prey k i l l e d .....203 R e s u l t s ....... 204 D i s c u s s i o n ......................218 GENERAL DISCUSSION 222 REFEREHCES .........227 APPENDICES 232 L i s t of tables Table 1. The c h a c t e r i s t i c s of the areas sampled in 1972 9 Table 2. The proportions of the various adult morphs of the d i f f e r e n t species observed in the f i e l d s i t u a t i o n . .............................................. 22 Table 3. The numbers of gerrids marked and the subsequent recaptures i n the areas sampled on Marion L. in 1971. 29 Table 4. Occurrence of female gerrids with chorionated eggs. .......................................30 Table 5. The temperatures for each of the f i v e species below which growth ceases. ......................48 Table 6. The values of the unknowns in the equation used to describe fecundity in Fig. 19. .................50 Table 7. A summary of the biologies of each of the f i v e species of Gerris. .................................53 Table 8. F i e l d temperature measurements of gerrids on a sunny day with a microthermocouple inserted into the body. .67 Table 9.the temperature zone c h a r a c t e r i s t i c s of the f i v e species of Gerris. ,.72 Table 10. Theoretical dry weight c o e f f i c i e n t s of ££2§2£iii23 w e t weights following various time periods with gerrids. 79 Table 11. Mean lengths i n minutes of consecutive feeding and non-feeding periods of G. remjLgis i n the presence of excess food. 90 Table 12. Values of constants i n the equation used to describe the rates of digestion i n Gerris. ..............93 Table 13. The gerrid species-instars, the mean body s i z e , and the states of hunger used in determining the e f f e c t of body size on mean reactive distance. ..........1 Table 14. The species-instars used in determining the effe c t of siz e on freguency of movement, and th e i r mean number of s t r i d e s per unit time period. ............ 1 Table 15. The mean distance at which the d i f f e r e n t s i z e classes of gerrids w i l l attack each other at maximum starvation. 1 Table 16. Mean percent of the t o t a l time spent by Gerris species either moving, grooming or remaining stationary. 1 Table 17. The mean distance which each of the species' i n s t a r s would be l i k e l y to move in 1 hour. .....1 Table 18. The mean area of the swath each of the species' in s t a r s would cover per hour as i t moved across the water surface. 1 Table 19. The number of prey presented form each size c l a s s i n unequal prey number choice sit u a t i o n s . .........1 Table 20. Gerrid hunger l e v e l s i n the f i e l d s i t u a t i o n . 1 Table 21. F-values f o r the ef f e c t of hunger on arcsin t o t a l percent k i l l and arcsin percent k i l l per attack for adults of each of the f i v e species. ................. 1 Table 22. F-values between species pairs for arcsin t o t a l percent k i l l by adults at each of the three hunger l e v e l s tested. ..1 Table 23. F-values between species pairs for arcsin percent k i l l per attack by adults at each of the three hunger l e v e l s tested. ...................................1 Table 24. Mean values of t o t a l percent k i l l and x i i i percent k i l l per attack observed for f i f t h i nstar larvae ............................................169 Table 25. F-values between species pairs for both a r c s i n t o t a l percent k i l l and arcsin percent k i l l per attack by f i f t h instar larvae. .......................... 170 Table 26. The mean values of percent k i l l weighted for predator species aggressiveness for each predator s i z e class tested in each of four microhabitats. ....,..,176 Table 27. Hean values for instar and adult percent k i l l s i n each of the four microhabitats. 177 Table 28. The percent k i l l in the open water microhabitat i n both the dark and l i g h t . ................ 178 Table 29. The predicted number of adult female progeny produced at the end of the summer when the species are considered separately 209 Table 30. The predicted number of progeny produced from varying numbers of overwintered G. n o t a b i l i s . .,,..,211 Table 31. The predicted number of progeny produced when the f i v e species coexist together. .................213 Table 32. The predicted number of progeny produced when only the four pond species are considered together. ....214 Table 33. The predicted number of progeny produced when only the open water pond species are considered t ogether. ........216 Table 34. The predicted number of progeny produced when the three small species and the two large species are each considered separately. ................218 L i s t of Figures Figure 1. Marion Lake showing the pattern of vegetation. .............................................6 Figure 2. The mid and south sections of the D.B.C. Research Forest and the areas i n i t that were sampled in 1972. ... ....8 Figure 3. The mean daily temperatures of the water in Area 3 on Marion L. 15 cm below the surface of the water and the accumulative number of degree-days attained there derived from this data. . .............15 Figure 4. The continental and B r i t i s h Columbia d i s t r i b u t i o n s of the f i v e species of Gerris studied in the present study, and of two other species found i n B r i t i s h Columbia. 16 Figure 5. The r e l a t i o n of leg length to wet weight for a l l the in s t a r s of the f i v e species studied. ........19 Figure 6. The numbers of the i n s t a r s of each of the species summed which were c o l l e c t e d in each of the six areas sampled on Marion L. in 1971. ..................... 23 Figure 7. The numbers of the in s t a r s of each of the f i v e species summed which were collected in each of the s i x main sampling areas i n 1972. .................... 25 Figure 8. The temporal occurrence and numbers of f i r s t instar larvae of the four species collected on Marion L, i n 1971 .........32 Figure 9. The temporal occurrence and numbers of f i r s t instar larvae of the four species collected on Marion L. in 1972. 35 Figure 10. The temporal occurrence and numbers of f i r s t i n s t a r larvae of the four species collected on Gate Pd. in 1972. 37 Figure 11. The temporal occurrence and numbers of f i r s t i n s t a r larvae of the four pond species collected X V on Jacobs Ck.., Placid L. and 0 In l e t 37 Figure 12. The mean stadium length of each of the gerrid i n s t a r s of the f i v e gerrid species studied. .,..,.42 Figure 13. The mean duration in degree-days above 0 °C. of the f i r s t i n s t a r l a r v a l stadium at d i f f e r e n t constant temperatures. .................................. 44 Figure 14. The reciprocals of the mean duration i n days of the f i r s t i n s t a r l a r v a l stadium of each of the f i v e species at di f f e r e n t constant temperatures. .....,,.45 Figure 15. The accumulative mean number of young hatched from eggs l a i d by females of the f i v e species of Gerris. ...................47 Figure 16. The number of gerrids of each of the species i n each section on the. water gradients studied. ., 64 Figure 17. The a i r and water temperatures on each section on the hot and cold gradients studied. ....,....,66 Figure 18. The ef f e c t of feeding time on the amount eaten by a third i n s t a r G. remigis starved for 20 hr. ...82 Figure 19. The e f f e c t of starvation and gerrid size on the maximum duration of the feeding period. ..........87 Figure 20. f The pattern of feeding a c t i v i t y over 4.5 hr as observed i n ten ind i v i d u a l G. remigis following starvation f o r 48 hr. ,.7....7...7.............. 89 Figure 21. The e f f e c t of starvation on the amount eaten by l a r v a l G. remigis and adults of each of the f i v e species. 92 Figure 22. The r e l a t i o n between maximum stomach capacity and gerrid s i z e . ...............................97 Figure 23. The r e l a t i o n between maximum amount consumed at a single feeding to the average amount consumed per day for l a r v a l G. n o t a b i l i s and the males of a l l f i v e species. 98 Figure 24. The e f f e c t of temperature on the amount consumed by males of each of the f i v e species of G err i s . .......102 Figure 25. The e f f e c t of developmental state on food consumption. ............112 Figure 26. A flow diagram to i l l u s t a r t e how the parameters r e l a t i n g to food consumption may be used. ....112 Figure 27. The r e l a t i o n between species-instars and leg length and their grouping into the ten size classes used. ...........................................118 Figure 28. The eff e c t of prey location r e l a t i v e to the predator on the propensity of adult G. remigis to attack stationary and moving prey, and on the a b i l i t y of the prey to successfully avoid capture. 124 Figure 29. The ef f e c t of hours of food deprivation on the mean subtended angle required to e l i c i t an attack by adult G. remigis. ............126 Figure 30. The r e l a t i o n of the amount of food i n the gut to the mean visual angle required to e l i c i t an attck by adult G. remigis. ...127 Figure 31. The r e l a t i o n of gerrid size and the mean visual angle which a prey must subtend before an attack i s e l i c i t e d . .....................................128 Figure 32. The r e l a t i o n between str i d e length and the mean length of the mesothoracic leg for the ten siz e classes. 129 Figure 33. The r e l a t i o n between Gerris species and the mean number of s t r i d e s made per i n d i v i d u a l . .........132 Figure 34. The r e l a t i o n between gerrid size and the mean number of s t r i d e s made per i n d i v i d u a l . .............133 x v i i Figure 35. The eff e c t of hours of fcod deprivation on the frequency of movement by G. remigis. 135 Figure 36. The r e l a t i o n of the amount of food in the gut to the mean number of stri d e s made per i n d i v i d u a l . ..136 Figure 37. The t o t a l percent k i l l and percent k i l l per attack observed for each of the f i v e species at four hunger l e v e l s . 163 Figure 38. The t o t a l percent k i l l and percent k i l l per attack of each of the gerrid s i z e classes when attacking Drosophila. ...........172 Figure 39. The t o t a l percent k i l l and percent k i l l per attack of each gerrid s i z e class when attacking gerrids of i t s own size or smaller. ..................... 173 Figure 40. The numbers of each prey size class k i l l e d by adults of each of the f i v e species when one of each of the prey s i z e classes which that size c l a s s adult can k i l l e f f i c i e n t l y are presented 181 Figure 41. The numbers of each prey s i z e class k i l l e d by adult gerrids when the numbers of the prey size classes presented were ratioed as to t o t a l leg length present. ................................................182 Figure 42. The numbers of each prey size class k i l l e d by adult gerrids when the numbers of the prey s i z e classes present were ratioed as to the t o t a l biomass present. 184 Figure 43. The t o t a l percent k i l l and k i l l s per attack of gyrinids, fourth i n s t a r larvae notonectids and adult notonectids when attacking each of the siz e classes of gerrids. .....................................185 Figure 44. A simple flow diagram describing the model constructed. ............................................196 Figure 45. The predicted occurrence and numbers of f i r s t i n s t a r larvae of each of the fi v e species when each species was considered separately and the guts x v i i i were always 75% f u l l 205 Figure 46. The manner of presentation of predicted numbers of adult female progeny produced during the season by any one species. ..............................208 Figure 47. A summary of the c h a r a c t e r i s t i c s of the species important in t h e i r coexistence ....225 xix ACKNOWLEDGEMENTS The work des c r i b e d i n t h i s study was done with the support of a grant i n a i d of r e s e a r c h to Dr. G.G.E. Scudder. A p p r e c i a t i o n i s expressed to Ms. Barbara Hoye and Ms. Shiona Whyte f o r t h e i r a s s i s t a n c e i n the c o l l e c t i o n of the data, Ms. E l l a Korinck f o r the p r e p a r a t i o n of the D r o s o p h i l a media, and Drs. I . E. E f f o r d and J. Krebs f o r p r o v i d i n g equipment. Dr. B. White and Mr. M. L j u b i s i c {Faculty of Education, U n i v e r s i t y of B r i t i s h Columbia) k i n d l y provided v i d e o t a p i n g f a c i l i t i e s and a s s i s t a n c e . Mr. S. Borden, Ms. Robyn Kardynal, Ms. , Dolores L a u r i e n t e and Mr. B i l l Webb7 provided me with i n v a l u a b l e a s s i s t a n c e i n computer o p e r a t i o n . Thanks are a l s o due to Mr. N. G i l b e r t , and to Drs. J . Bryan, I . E. E f f o r d , C. S. H o l l i n g , N. R. L i l e y and G.G.E. Scudder f o r t h e i r a s s i s t a n c e with v a r i o u s aspects of t h i s study. P a r t i c u l a r thanks are expressed t o my wife Dorothy f o r her t h r e e summers of unpaid a s s i s t a n c e , and f o r her u n r e l e n t i n g support during the course of t h i s t h e s i s . 1 GENERAL INTRODUCTION The competitive displacement p r i n c i p l e states that " d i f f e r e n t species having i d e n t i c a l ecological niches, i e . ecolog i c a l homologues, cannot coexist for long in the same habitat" (DeBach, 1966). Implicit i n t h i s concept, and in i t s cor o l l a r y , the coexistence p r i n c i p l e , i s the understanding that a l l species d i f f e r b i o l o g i c a l l y no matter how closely they are related and how sim i l a r they may appear or act. One species w i l l always be favoured i n any pa r t i c u l a r s i t u a t i o n owing to certai n unique aspects of i t s biology, and a greater number of t h i s species w i l l survive to reproduce. Hence, given enough time, the favoured species w i l l eliminate the other. There are two main problems which must be considered in any study of coexisting species: (a) are the sympatric species a c t u a l l y i n fac t ecological homologues, and i f so, (b) i s coexistence of these ecological homologues in the same habitat r e a l , or i s i t only suggestive and s u p e r f i c i a l . Does i t seemingly occur only because of modifying circumstances such as immigration or crypti c u t i l i z a t i o n of d i f f e r e n t resources. Numerous studies of coexisting species have been reported i n the l i t e r a t u r e , and i n each study, coexistence of ecological homologues was either disproven or could not be' conclusively proven {DeBach, 1966). Cases of actual, or apparent competitive displacement between ecological homologues, however, are too numerous to permit discussion at t h i s point. Character displacement, the 2 evidence of such competition, i s more e a s i l y discernable, as the r e s u l t i s not the elimination of one of the species but adaptive changes between the species. F i e l d studies of competitive displacement are of two types: studies of the process in action, and more commonly, the presumable r e s u l t after i t s completion. An example of the former i s Istock's ( 1965, 1967) studies of sympatric species of the w h i r l i g i g beetle Dineutes, whereas an example of a study of competitive displacement after i t s completion i s Mitchell's (1969) study of two species of sympatric water mites of the genus Arrgnurus which p a r a s i t i z e damselflies. The aim of t h i s study was to investigate the apparent coexistence of f i v e species of the waterstrider Gerris i n southern B r i t i s h Columbia. G. buenoi Kirk., G. incognitus E. 8 H., G. incurvatus D. & H., G. n o t a b i l i s D. S H. and G. remigis Say occur in southwestern B r i t i s h Columbia. It i s f u t i l e to think that consideration of a s i n g l e t r a i t can explain how coexistence i s achieved, so ,a large set of t r a i t s were investigated and,an assessment made of the combinations of interactions that arise from these. This necessitated the construction of a computer model incorporating a l l these t r a i t s . Gerrids are well-suited for modelling, as being r e s t r i c t e d to a two-dimensional environment, the water surface, movement i s greatly s i m p l i f i e d . The basic organization of t h i s thesis i s six sections. Section 1 describes a f i e l d study of the f i v e species, with the intent here being (a) to determine i f the species are ecological \ homologues and (b) to describe the basic biologies of the species for the incorporation of realism into the model. Section 2 describes the voluntary temperature preferences of the d i f f e r e n t species on a gradient to again help define the niches occupied by the five species. Sections 3, 4 and 5 describe food consumption, reactive distance and rate of movement, and predatory c a p a b i l i t i e s respectively for the species. The parameters in Sections 3 and 4 were required as an experimental components analysis type of approach (Helling, 1966) was used i n the construction of a computer model, described in Section 6, wherein the possible situations for coexistence and exclusion are investigated. 4 SECTION 1. FIELD BIOLOGY GROWTH AND FECUNDITY INTRODUCTION Studies on gerrids to date have largely been devoted to rather general descriptions of the i r biology and behaviour (Essenberg, 1915; Bueno, 1917; Ril e y , 1921, 1922; Poisson, 1924; Rensing, 1962; Kaufmann, 1971) and have provided useful background on species, or they have been concerned with the basis and si g n i f i c a n c e of alary polymorphism, which i s p a r t i c u l a r l y prevalent in thi s group (Guthrie, 1959; Brinkhurst, 1959, 1960, 1961, 1963; Vepsalainen, 1971a, 1971b, 1971c). The l a t t e r have helped determine how morphology i s related to microhabitat, but they have not provided data as to what forms are capable of coexisting in the same habitat. Population studies attempted to date (Brinkhurst, 1965, 1966; Vepsalainen, 1971c) have only investigated single species and provide l i t t l e information on how d i f f e r e n t species i n t e r a c t . I t has been claimed that gerrids occupy i d e n t i c a l habitats and yet coexist i n almost equal numbers (Haynard, 1969), but th i s has not been studied in d e t a i l . To be considered in t h i s section are the microhabitats of the species, the temporal features of the l i f e cycles and the responses of the species to d i f f e r e n t temperatures. The main aim of t h i s part of the study was to detect potential differences between the species. A secondary aim was the 5 a c q u i s i t i o n of data required for l a t e r modelling of the species in a coexisting s i t u a t i o n . MATERIALS AND METHODS 1. SPECIES DISTRIBUTIONS Data on the d i s t r i b u t i o n s of the species of Gerris found in B r i t i s h Columbia was compiled from my own c o l l e c t i o n s plus Drake and Harris (1934), Moore (1950), Kuitert (1942), Brooks and Kelton ( 1967), Scudder (1971) and Ashlock (pers. comm. ). 2. THE STUDY AREA The main study areas were on Marion lake, a small muddy bottomed lake ca. 10 ha i n area located in the University of B r i t i s h Columbia Research Forest, 10 km north of Haney, B. C. (latitude 49o90'N; longitude, 122°33«W). In 197 1 six areas (Fig. 1) on the lake were studied commencing May 1 so as to determine the e f f e c t of microhabitat v a r i a t i o n on the di s t r i b u t i o n of the di f f e r e n t species on one body of water. Sampling in Area 6 commenced at the beginning of August, as prior to t h i s date, the Potamogeton natans (L.) had not reached the water surface and hence no cover was available. On the basis of the data acquired i n 1971, other areas Figure 1. Marion Lake showing the pattern of vegetation. The six areas sampled in 1971 are i l l u s t r a t e d . Area 3 was the only area sampled i n 1972. 5B throughout the Research Forest were selected and studied in 1972. At these l a t t e r areas, d i f f e r e n t combinations of the species were present and i t was hoped that t h i s might allow (a) a more precise delimitation of the preferred habitat of each species, and (b) an assessment of the eff e c t s 1 of certa i n i n t e r s p e c i f i c interactions. Figure. 2 i l l u s t r a t e s the areas that were sampled in 1972 and Table 1 l i s t s t heir main c h a r a c t e r i s t i c s . 3. ENVIRONMENTAL TEMPERATURE MEASUREMENTS Fi e l d temperatures i n each of the areas sampled, excluding Reservoir 1, Dorothy* and Placid-*-, were recorded continuously with Ryan Model B -30 submersible temperature recorders (Ryan Instruments, Inc., Seattle). These were suspended from f l o a t s and hung 15 cm below the water surface. Temperatures i n the other areas were recorded with a thermometer on each sampling date. 4. GRID SAMPLING Fiel d sampling was on a weekly basis between mid-April and the end of September. I n i t i a l l y , occurrence of adults i n each of the study areas was recorded. These adults, af t e r i d e n t i f i c a t i o n i n the f i e l d according to Scudder (1971), were released as i t was f e l t that t h e i r removal, p a r t i c u l a r l y i n the spring when they were r e l a t i v e l y few in number, might a f f e c t 8 F i g u r e 2. The mid and south s e c t i o n s of the D.B.C. Research Forest and the areas i n i t that were sampled i n 1972, showing t h e i r e l e v a t i o n s i n f e e t . A = Dorothy Pond; B = R e s e r v o i r 1; C = Jacobs Creek and Jacobs Pond; D = 0 I n l e t ; E = Gate Pond. 9 Table 1. The char a c t e r i s t i c s of the areas sampled i n 1972. A. The eleven areas sampled i n the U.B.C. Research Forest i n 1972 and their c h a r a c t e r i s t i c s . Breeding means that at least three l a r v a l i n s t a r s were collected during the season. * = heavy rain runoff can create a strong current; ** = yes near shore; {) = less than three l a r v a l instars were coll e c t e d during the season; WSV = water surface v e l o c i t y ; HS1 = water surface l i t t e r . Area Gate Pd. Dorothy* Dorothy Pond Placid+ Placid L. Reservoir 1 Marion L. 0- I n l e t Jacobs Ck. Jacobs Pd. Water-body c h a r a c t e r i s t i c s Sunny, man-made pond Sunny, small stream (1 m wide) Sunny, man-made pond; widening of a creek Shaded, small stream Non-draining lake Shaded man-made pond; widening of a small stream Draining lake WSV WSL 0 no** 0 yes slow* no 0 0 0 0 Widening of a small slow stream Creek (3-1 m wide) f a s t Stream-bank pool 0 yes no** no no** no** no no Breeding species*** N R B IV IG X X X ( X ) ( X ) X X X X X X X X X X X X X X ( X ) X X X X ( X ) X 3a Table 1. B. <i) the water temperature differences |°C.) between the below areas and Marion L. at the times given for dif f e r e n t dates during the season. The mean temperature difference (± 1 SE) indicates the r e l a t i v e temperatures of the di f f e r e n t areas to Marion L. * = not included in the cal c u l a t i o n of the mean. ( i i ) The water temperatures (°C.) of the below areas at 12:01 pm for d i f f e r e n t dates during the season and their mean (± 1 SE). B. (i) Date Area Gate Pd. Pl a c i d * C Inlet Dor. + Res. 1 (10:25 am) (11:40 am) (1:50 (11:20 am) (12:01 am) A p r i l 16 5.6* — - - — A p r i l 28 8.6* - - — — May 19 3. 1 -3.5 1.5 -4. 2 -4.5 June 17 - -3.3 -1.0 -2.5 -1.6 July 8 5.5 -1.4 -2.5 -3.6 -July 29 0.4 -3.4 -4.2 -1.5 Aug 24 1.7 -3.3 -5.5 -2.7 0.0 Oct 26 0.2 2.4 1.5 0.5 -5.3 Mean 3.4 ± 1 .1 -1.2 ± 1 .3 -2.6 ± -2. 1 ± 1.0 -2. 1 ± 0.7 (ii ) Date Area Dor. Pd. Mar. Air Jacobs Ck • Placid I. Mar. H 0 Jacobs A p r i l 28 4.0 7.0 5.0 4.0 3.0 4.0 May 19 15.0 13.0 23.0 12.5 9.0 8.0 June 8 14.5 15.0 17.0 13.0 11.5 10.0 July 18 11.5 19.0 9.5 17.0 14.0 10.5 July 29 24.0 22.5 31.0 22.0 19.0 20.0 Aug 24 23,0 21.0 13.5 17.0 14.0 14.5 Mean 15.3 ± 3.0 16.5 ± 3.8 11.8 1 2.2 16.3 ± 2.4 14.3 ± 2.5 11.2 ± 2.3 l a t e r population l e v e l s . Commencing with the appearance of the f i r s t l a r v a l stages, a set number of square-meter samples were taken from each study area on each sampling date. fill the l a r v a l gerrids were preserved either in 70% ethanol or brought back to the laboratory for further study. Larval i d e n t i f i c a t i o n was according to Scudder and Jamieson (1972). Sampling equipment consisted of a square-meter aluminium frame 48 cm deep, with p l a s t i c screening on each of the four sides. Buoyancy in deep water was achieved by f l o a t s on each corner. Sampling consisted of tossing the frame onto the water surface from a distance of ca. 3~4 m and c o l l e c t i n g a l l the enclosed gerrids. The same locations were sampled on each sampling day. As the object of the sampling was not to determine the mean gerrid density over the whole body cf water, but rather to ascertain what species instars were present at any given time and place and their densities i n these locations, the sampling locations chosen were those where gerrids were known to be present. However, fluctuating water leve l s did necessitate a change in sampling location in some,areas. Generally, gerrids tend to be found around the perimeter of the lake and hence the locations sampled tended to be near the water-edge. As the water-edge varied with f l u c t u a t i n g water l e v e l s , especially i n the shallow areas, the sampling locations also varied. Nevertheless, i t was f e l t that the sampling procedure d i d v allow r e a l i s t i c determination of the gerrids present at any given time. In 1971 a l l the adult gerrids collected in the sample areas 11 on Marion L. from May 17 to July 30 in c l u s i v e were marked on the pronotum with a spot of Liquitex a c r y l i c polymer emulsion (Permanent Pigments, Inc., C i n c i n n a t i ) . A d i f f e r e n t colour was used i n each sample area, and capture of these marked gerrids at l a t e r dates was recorded, allowing a measure of the movement of the gerrids between the study areas. Preliminary observations of laboratory gerrids marked i n a s i m i l a r manner suggested that marking did not increase gerrid mortality. During late A p r i l and early May of 1972, a few female gerrids of each of the f i v e species were coll e c t e d from Gate Pd., Marion L. , and the v i c i n i t y of Placid L. These were preserved in 70% ethanol for l a t e r dissection. This allowed the date of commencement of ovariole egg production i n each of the species to be determined. Since the first-generation gerrids of G. buenoi, G. incuryatus and G. inco^nitus vary in the extent of t h e i r wing development, female morphs of these species were col l e c t e d and preserved on July 20 to determine i f alary polymorphism affected commencement of ovariole maturation. 5. LABORATORY STUDIES 12 A. Effect of temperature on i n s t a r duration Eggs or larvae of each of the f i v e species were placed i n d i v i d u a l l y i n open p l a s t i c containers 9.5 cm in diameter and 7 cm deep and maintained at 26°C. in a controlled environmental cabinet (Controlled Environments Inc., Winnipeg): l i g h t i n g was 16 hr l i g h t : 8 hr dark. Eggs were taken cn the f i r s t day following deposition and the number of days to hatching was recorded. S i m i l a r l y , with larvae, the number of days between successive ecdyses were recorded. Feeding with frozen adult J?I2i§°I?hila a n ^ monitoring of development was daily . To ascertain , the e f f e c t of d i f f e r e n t temperatures on development, the above procedure was repeated with larvae at 15°, 18.5°, 22° and 35°C. B. Fecundity Adults c o l l e c t e d early i n the spring before they were sexually mature, were brought back to the laboratory and used to obtain a measure of fecundity. Pairs of G. n p t a b i l i s and of G. remigis were separately placed i n rectangular containers 30 cm by 43 cm with a water depth of 5 cm. The water surface was kept clean by use of an airstone and each container had a 13 cm2, 1 cm thick plywood block for an oviposition s i t e . Eecause of their smaller s i z e , pairs of the other three species were maintained i n p l a s t i c containers 26 cm in diameter and 9 cm deep, each of which had a number of 3 cm2 cork s l i c e s for oviposition. The water in these l a t t e r containers was changed 13 weekly to keep the water surface clean. A l l species were fed frozen adult DrosoDhila once dai l y and room temperature was recorded continuously. F i r s t i n s t a r emergence rather than eggs l a i d was determined daily, as i t was v i r t u a l l y impossible to detect a l l eggs l a i d in the various crevices of the blocks provided. Since G. buenoi, G. incognitus and G. incurvatus are often b i v o l t i n e , the same procedure was repeated with adults of the f i r s t summer generation. These l a t t e r adults were e a s i l y detected in the f i e l d from the overwintered ones, because the l a t t e r had a "weathered" appearance, while the former were "fresh". I t was f e l t preferable to use f i e l d caught gerrids for thi s study as l a r v a l n u t r i t i o n , especially in the l a t e r larvae, can influence adult fecundity (Drooz, 1970). Larval emergence rather than eggs l a i d i s also probably a better measure of fecundity, since at times eggs f a i l e d to hatch. Kaufmann (1971) reported that in Gerris rufoscutellatus there i s , on the average, only an 88.1% egg v i a b i l i t y . Eggs f a i l i n g to hatch and showing no embryonic development were noted. 14 RESULTS 1. FIELD SAMPLING A. Environmental temperature Figure 3 describes the water temperatures of the d i f f e r e n t areas sampled i n 1971 and in 1972. Both mean dai l y temperatures and accumulated degree-days on Marion lake i n 1971 and 1972 above the minimum temperatures required for growth for each of the species (see below) are presented in Fig. 3. Since the minimum temperatures of G. bjjenoi, G. remigis and G. incurvatus are a l l about the same (see below), ca. 12.6°C, a mean temperature value was used for these three species. It can be seen that the t o t a l number cf degree-days accumulated was much greater in 197 1 that in 1972. The conseguences of t h i s difference are described below. B. General biology of the gerrids The continental and B r i t i s h Columbia d i s t r i b u t i o n s of the f i v e species of Gerris studied and two other species of Gerris found i n B r i t i s h Columbia, namely G. jin g r e e n s i s and G. comatus (Scudder, 1971), are shown i n Fig. 4. G. remigis has been collected both east and west of the Rocky Mountains, but has only been recorded from southern B r i t i s h Columbia. G. incognitus and G. incurvatus* have both been collected primarily 1 5 Figure 3- The mean d a i l y temperatures of the water in area 3 on Marion L. 15 cm below the surface of the water, and the accumulative number of degree-days above the three temperature thresholds of the species derived from the data. a. 1971; B-1972. 16 Figure 4. The continental and B r i t i s h Columbia d i s t r i b u t i o n s of the f i v e species of Gerris studied, and of two other species found in B r i t i s h Columbia. 16d 17 west of the Rocky Mountains, although r e c o r d s e x i s t f o r both s p e c i e s i n southern Texas. In B r i t i s h Columbia, they are found only i n the southern r e g i o n s and along the c o a s t . G. buenoi, l i k e G. r e m i g i s , has been c o l l e c t e d across c o n t i n e n t a l north America but u n l i k e G. r e m i g i s , t h i s s p e c i e s has not been c o l l e c t e d below the 40 p a r a l l e l . I t i s found throughout southern and c e n t r a l B r i t i s h Columbia, but has only been c o l l e c t e d along the coast i n the south-west. G. n o t a b i l i s has been p r i m a r i l y found i n the western h a l f of North America, although two r e c o r d s (questionable) e x i s t f o r t h i s s p e c i e s i n the e a s t . T h i s s p e c i e s has been c o l l e c t e d from New Mexico to northern B r i t i s h Columbia. G. comatus and G. J i i n g r e e n s i s have both been c o l l e c t e d a c r o s s North America, but few records e x i s t f o r e i t h e r s p e c i e s south of Kamlcops and west of the Rockies. Both s p e c i e s are v i r t u a l l y i d e n t i c a l to G. i n c u r y a t u s and G. ii}£°3IJiXBS r e s p e c t i v e l y m o r p h o l o g i c a l l y , and can be d i s t i n g u i s h e d from these s p e c i e s only by means of a microscope. I t i s i n t e r e s t i n g to note that the d i s t r i b u t i o n s cf G. egmatus and G. £iMf©SJJSjis e v i d e n t l y do not o v e r l a p with these of G. iUSJJiJJStus and G. i n c o g n i t u s . A l l f i v e s p e c i e s of G e r r i s i n the study area overwintered as a d u l t s , and G. r e m i g i s and G. i n c o g n i t u s t y p i c a l l y emerged e a r l i e s t i n the s p r i n g . By e a r l y May, a d u l t s of a l l f i v e s p e c i e s were c o l l e c t e d and commencing i n l a t e May, l a r v a e began t o appear. O v i p o s i t i o n s i t e s i n a l l the s p e c i e s were beneath the water s u r f a c e , with the edges of the undersides of f l o a t i n g a q u a t i c v e g e t a t i o n (eg. Potamogeton, Nujchar) p r e f e r r e d by G. 18 n o t a b i l i s , G. i n g u r y a t u s and G. buenoi. G. i n c o g n i t u s p r e f e r r e d f l o a t i n g wood and other more s o l i d o b j e c t s , perhaps a r e s u l t of i t s p r e f e r r e d h a b i t a t being c l o s e r to shore (see below) where such d e b r i s accumulates. G. r e m i g i s was o f t e n observed to crawl beneath the water s u r f a c e to a depth of s e v e r a l centimeters i n order t o reach an o v i p o s i t i o n s i t e . Bocks, r e l a t i v e l y l a r g e p i e c e s of f l o a t i n g wood and r o o t s were chosen as o v i p o s i t i o n l o c a t i o n s . Eggs of a l l the s p e c i e s were found to be b a s i c a l l y s i m i l a r i n shape, being c y l i n d r i c a l and ca. 3 times as long as wide (ca. 1.3 by 0.4 mm). The eggs of G. n o t a b i l i s are unigue i n that they are i n d i v i d u a l l y surrounded by a ca. 1 mm t h i c k g e l a t i n o u s sheath; eggs of the other s p e c i e s are without t h i s sheath. The eggs of G. n o t a b i l i s , G. i n c u r y a t u s , G. i n c o g n i t u s and G. buenoi were l a i d i n rows of about 10-15 eggs along l e a f borders, whereas the eggs of G. r e m i g i s were l a i d s e p a r a t e l y . There are c o n s i d e r a b l e s i z e d i f f e r e n c e s between the f i v e s p e c i e s of G e r r i s s t u d i e d , with a d u l t G. r e m i g i s and G. n o t a b i l i s weighing ca. 3 to 6 f o l d more than the other s p e c i e s . F i g u r e 5 presents the r e l a t i o n between l e g length and wet weight f o r a l l the s p e c i e s * i n s t a r s and from t h i s i t can be seen t h a t the a d u l t s of each s p e c i e s are d i s t i n c t i n r e l a t i v e s i z e and leg l e n g t h : G. buenoi i s s m a l l e s t with medium l e n g t h l e g s , G. i n c u r y a t u s i s next h e a v i e s t and possesses r e l a t i v e l y long l e g s f o r i t s weight, G. i i i c o ^ n i t u s i s the h e a v i e s t of the three s m a l l s p e c i e s and has r e l a t i v e l y s h o r t l e g s , G. n o t a b i l i s possesses extremely long l e g s r e l a t i v e t o G. r e m i g i s , although t h e i r 19 Figure 5 . The r e l a t i o n of leg length to wet weight for a l l the inst a r s of the f i v e species studied (± 1 SE). * = l a r v a ; • = adult female; ° = adult male; N = G. n o t a b i l l s ; B = G. remigis; IG = G. incognitus; IV - G. incurvatus; B = G. buenoi. 19a 601 CD z: LLI JV .IV U I °rF L l G -e-48 j IN £ x .36-© larvae o adult male E3 adult female 20 30 AO 50 W E T WEIGHT (mg) Lr 20 average weights are about the same. The females are heavier than males and possess the l o n g e s t l e g s i n a l l the s p e c i e s except G. n o t a b i l i s . In G. n o t a b i l i s the male i s heavier and has longer l e g s than' the female. G. n o t a b i l i s and G. r e m i g i s were u n i v o l t i n e , while the other three s p e c i e s were u s u a l l y b i v o l t i n e . T h e i r success i n having two g e n e r a t i o n s per year v a r i e d and depended on the weather c o n d i t i o n s . Larvae of the b i v o l t i n e s p e c i e s were o f t e n c o l l e c t e d as l a t e as mid-September. Adult G. n o t a b i l i s and G. i££iJIvatus entered o v e r w i n t e r i n g diapause r e l a t i v e l y soon a f t e r they reached the a d u l t stage, s i n c e they disappeared i n l a t e August or e a r l y September. G. buenoi, G. i n c o g n i t u s and G. r e m i g i s remained a c t i v e l o n g e r , although the former two u s u a l l y l e f t the water s u r f a c e by l a t e September. G. r e m i g i s p e r s i s t e d i n l a r g e aggregations u n t i l the water f r o z e over, u s u a l l y i n l a t e October. Winged a d u l t G. buenoi, G. i n c u r v a t u s and G. i n c o g n i t u s flew i n l a t e summer, but d i d not show the same tendency i n the s p r i n g or e a r l y summer. F a l l would thus seem the time of major f l i g h t d i s p e r s a l i n these s p e c i e s although some d i s p e r s a l may occur i n e a r l y s p r i n g . G. n o t a b i l i s , on the other hand, seemed to p r i m a r i l y d i s p e r s e i n the s p r i n g . In l a t e May i n 1972, every pond being s t u d i e d had G. n o t a b i l i s on i t , o f t e n i n l a r g e numbers. None had been observed the week before and w i t h i n a few weeks, the a d u l t G. n o t a b i l i s disappeared o f f those ponds which l a t e r i n the season had no breeding G. n o t a b i l i s p o p u l a t i o n s . Data on d i s p e r s a l i n G. remigis were not o b t a i n e d 21 but s i n c e t h e r e was a very low frequency of winged raorphs ( ca. 5%), f l i g h t would not seem to be the main d i s p e r s a l method f o r t h i s s p e c i e s i n the study area. Alary polymorphism occurred i n the g e r r i d s . Table 2 p r e s e n t s the r a t i o s of the morphs of G. i n c o g n i t u s , G . buenoi, and G. i n c u r v a t u s which were c o l l e c t e d i n 1972. G. n o t a b i l i s always possessed wings extending the f u l l l e n g t h of the abdomen (macropters) , and although a few G. r e m i g i s i n B r i t i s h Columbia are macropterous (Scudder, 1971), not enough a d u l t s were c o l l e c t e d i n the course of sampling to permit i n c l u s i o n of t h i s s p e c i e s i n Table 2. I t can be seen that i n G. buenoi and G. iS£S£y.3iH§ L the macropterous morph was the dominant form in the s p r i n g and f a l l , whereas apterous and shortwinged forms were dominant i n the summer or f i r s t g e n e r a t i o n s . In both the s p e c i e s there were /two generations, with most of the f i r s t g e n e r a t i o n a d u l t s apterous or shortwinged. C. S p a t i a l d i s t r i b u t i o n and m i c r o h a b i t a t s The s p a t i a l d i s t r i b u t i o n of the f i v e s p e c i e s of g e r r i d s c o l l e c t e d on Marion L. i n 1971 i s shown i n F i g . 6. G. not a b i l i s and G. i n c u r v a t u s were each c o l l e c t e d i n approximately equal numbers i n Areas 1 _5. 1 G. buenoi and G. i n c o g n i t u s , although a l s o c o l l e c t e d i n a l l f i v e areas, were most abundant i n Areas 3 and 4. In Area 6, an o f f s h o r e Pqtamogeton bed, o n l y G. i j j c u r y a t u s was abundant. As shown i n F i g . 1, Area 1 i s i n a very shallow r e g i o n of the l a k e . Grasses were present at the water-edge only i n the e a r l y s p r i n g when the lake l e v e l was 22 Tabl e 2- The p r o p o r t i o n s of the v a r i o u s a d u l t morphs of the d i f f e r e n t s p e c i e s observed i n the U.B.C.>Research F o r e s t during 1972. G. n o t a b i l i s had on l y one form, a f u l l y - w i n g e d one, whereas G. r e m i g i s was always apterous. W = macropterous; 1/2w = brachypterous;; 1/4 w = micropterous; nw = apterous. Date Spe c i e s G. buenoi G. : i n c u r y a t u s G« i n c o g n i t u : n w 1w 1w nw n w 1w 1 w nw n w 1 w 1w nw 2 4 2 4 2 4 June 1 0 2 100 6 100 June 6 9 100 1 100 3 100 June 16 5 100 1 100 - 2 100 J u l y 7 16 13 6 81 8 13 88 2 100 J u l y 14 8 13 87 3 33 68 0 J u l y 21 3 33 68 5 80 20 2 50 50 J u l y 28 9 68 33 9 44 11 44 3 33 67 Aug 4 1 100 2 100 0 Aug 9 0 3 68 33 0 Aug 23 12 100 4 100 1 100 Oct 1 15 100 5 100 0 Oct 12 10 100 3 100 2 100 Oct 25 9 100 1 100 5 100 T o t a l 97 69 2 0 26 47 31 3 13 0 26 Percent 70 2 0 27 66 6 28 0 2 0 0 24 8 0 0 92 23 Figure 6. The numbers of the instars of each of the species summed which were collected on Marion 1. i n 1971. The areas shaded represent those numbers of each of the species co l l e c t e d at a distance greater than 3 m from shore. CO CO 24 high; in mid-summer, the lake l e v e l dropped, exposing a large area of lake bottom (mud) which possessed l i t t l e vegetation. G. buenoi and G. incognitus were only collected from this area in the early spring. The same applied, although to a lesser extent, to Areas 2 and 5. Areas 3 and 4, however, had extensive grass and weed beds, and the water depths there were such that the water extended into these beds at a l l times of the year. Extensive cover was thus always available. I t w i l l be noticed that excluding the immediate v i c i n i t y of the i n l e t and outlet streams, G. remigis was only co l l e c t e d i n Area 5. In the s p e c i f i c microhabitat in Area 5 where th i s species was c o l l e c t e d , water from a cold spring flowed into the lake, and i t was only at t h i s one location where the maximum water temperature was ca. 20°C. that G. remigis occurred. i Since i t was noticed that the preferred distance from the shore seemed to vary with the species. Figure 6 also shows the number of the d i f f e r e n t species i n each sampled area collected at a distance greater than 3 m from the water edge. Over 50% of the G. incurva;tus c o l l e c t e d were obtained at a distance greater than 3 m, whereas G. n o t a b i l i s and G. buenoi tended to be found closer to shore. G. incognitus remained closest to shore. Figure 7 and Table 1 present the s p a t i a l d i s t r i b u t i o n s of the species in the areas sampled i n 1972. Gate Pd. and Marion L . both contained four species (lacking only G. remigis), with one of the f i v e species dominating in each of the other areas. Placid I. was somewhat unique among the areas studied in that the i n d i v i d u a l species present there were to a large extent 25 Figure 7. The numbers of the i n s t a r s of each of the f i v e species summed which were coll e c t e d in each of the six main sampling areas in 1972. NUMBER Oi IS o NUMBER M 8 NUMBER Di IS NUMBER ffl IS NUMBER B) IS CD > 3 2 = in 3 u ] a CT m M z n c tn IS) E m m 1 5 s a M ro Cn 05 26 s p a t i a l l y separate. G. n o t a b i l i s was only c o l l e c t e d on the s m a l l pools i n the SjBhaajvum s h e l f which surrounded the l a k e , G. buenoi was only c o l l e c t e d on the edge of the Sphagnum s h e l f where i t abutted the open water of the lake, and G. i n c o g n i t u s was only c o l l e c t e d i n the s m a l l e s t , algae-covered pools c l o s e s t to the a c t u a l l a k e shore. N e i t h e r G. i n c u r v a t u s nor G. r e m i g i s were c o l l e c t e d on t h i s l a k e . Table 1 i d e n t i f i e s the sample s i t e s on the b a s i s of t h e i r p h y s i c a l c h a r a c t e r i s t i c s and from t h i s , i t i s e v i d e n t that each s p e c i e s seems to have a p r e f e r r e d m i c r o h a b i t a t . G. remigis i s unigue i n t h a t i t was u s u a l l y only observed on f a s t f l o w i n g streams, although aggregates d i d occur on the more s h e l t e r e d areas on the streams. N e v e r t h e l e s s , i n d i v i d u a l s of t h i s s p e c i e s are capable of m a i n t a i n i n g t h e i r p o s i t i o n i n the c u r r e n t i f necessary and show an a b i l i t y to d i r e c t t h e i r movements against the water flow. i n c o g n i t u s was found i n areas where the water s u r f a c e v e l o c i t y was zero or s l i g h t , and where f l o a t i n g m a t e r i a l such as algae abounded. These areas were t y p i c a l l y q u i t e shallow, being near the shore, on l a r g e bodies of water and were not much more than s u r f a c e pools i f the water bodies were s m a l l . Ground seepage o f t e n produced enough water i n the form cf s m a l l puddles f o r t h i s s p e c i e s . G * i n c u r v a t u s was only observed on l a r g e bodies of water where the o f f s h o r e r e g i o n s were i n t e r s p e r s e d open water and f l o a t i n g v e g e t a t i o n . The water was thus u s u a l l y not very deep, 27 the v e g e t a t i o n (eg. Nuphar and Potamogeton) being w e l l rooted. 5* i n c u r v a t u s was net found on P l a c i d L. perhaps owing to the water being too deep to permit the development c f the r e g u i r e d o f f s h o r e weeds. G. buenoi was found t y p i c a l l y i n i n s h o r e areas where the water s u r f a c e was not too c l u t t e r e d with f l o a t i n g v e g e t a t i o n , while G. n o t a b i l i s was found on s i m i l a r water but not as f a r o f f s h o r e as G. i n c u r v a t u s and yet f u r t h e r o f f s h o r e than G. buenoi. I t should be emphasized that these m i c r o h a b i t a t preferences are not s t r i c t l y adhered to and thus a g r e a t d e a l of o v e r l a p e x i s t s between the s p e c i e s . Thus on slow moving streams and pools such as 0 I n l e t , a l l the s p e c i e s were c o l l e c t e d , and l i k e w i s e a l l except G. r e m i g i s occur together on Marion L. F u r t h e r , a t d i f f e r e n t times of the day, the degree of o v e r l a p can vary. Thus at mid-day on Marion L., the wind was o f t e n noted t o blow n o r t h , that i s from one end of the lake to the other. The e f f e c t was to f o r c e a l l the g e r r i d s at the north end o f the l a k e to the weed beds where cover was a v a i l a b l e . Here they were f o r c e d i n t o c l o s e c o n t a c t . In the e a r l y morning and evening, when the wind d i e d down, the s p a t i a l s e p a r a t i o n once again became evid e n t . To o b t a i n some measure of the movements of g e r r i d s between d i f f e r e n t h a b i t a t s , a d u l t s on Marion L. i n 1971 were marked between May 17 and J u l y 30 and then r e l e a s e d i n the area of capture, L a t e r sampling gave recapture data from which i t was 28 p o s s i b l e to determine the amount of movement. A t o t a l of 381 g e r r i d s were marked, with 20 r e c a p t u r e s ; 92.3$ of these r e c a p t u r e s were i n the same area that the g e r r i d was marked (Table 3). One r e c a p t u r e was i n an adjacent area, and one was i n an area the next beyond an adjacent area. T h i s suggests that although some movement of g e r r i d s around the lake occurred, they tended to remain i n one area of the l a k e through most of the i season. I t w i l l be noted that a g r e a t e r percentage of the G. buenoi marked were recaptured than i n the other s p e c i e s . This probably r e f l e c t s the g r e a t e r p r o p e n s i t y of i n d i v i d u a l s of t h i s s p e c i e s to move l e s s and to remain c l o s e to shore. Since the l o c a t i o n s w i t h i n an area sampled tended to be those where g e r r i d s were known to be present, cover was u s u a l l y a v a i l a b l e , and hence the sample l o c a t i o n s were u s u a l l y r e l a t i v e l y c l o s e to s h o r e . Recapture of i n d i v i d u a l G. buenoi would thus be more l i k e l y than r e c a p t u r e of G. i n c u r v a t u s , which p r e f e r s an o f f s h o r e m i c r o h a b i t a t . I t would thus seem that although movement of g e r r i d s occurred between adjacent sampled areas, the number which a c t u a l l y moved was q u i t e s m a l l . D. Temporal occurrence and v o l t i n i s m Table 4 shows the temporal occurrence of female g e r r i d s c a r r y i n g c h o r i o n a t e d eggs f o r each of the f i v e s p e c i e s i n 1972. Three sample areas are r e p r e s e n t e d . Gate Pd. at an e l e v a t i o n of 470 1, Harion L. at an e l e v a t i o n of 1002*, and P l a c i d L.-Dorothy Pd. at an e l e v a t i o n of 1650*. G. i n c o g n i t u s and G. r e m i g i s emerged from o v e r w i n t e r i n g e a r l i e s t , and c h o r i o n a t e d eggs were 29 Table 3. The numbers of g e r r i d s marked and the subsequent r e c a p t u r e s i n the d i f f e r e n t areas sampled on Marion 1. i n 1971. Marking terminated on J u l y 30. Date Number marked Number r e c a p t u r e s Same Adj. 2nd adj. Area area area IV N IG R B IV N IG R B May 17 19 1 3 32 May 21 36 3 32 1 2 3 May 24 24 2 22 1 3 4 May 28 14 1 2 15 1 1 1 June 4 6 1 3 20 1 1 June 11 19 2 6 28 4 4 June 18 9 5 23 7 7 J u l y 2 13 1 3 6 1 2 2 J u l y 16 1 3 9 1 1 J u l y 23 3 2 1 3 1 1 J u l y 30 1 2 1 4 Aug 6 T o t a l 145 10 32 0 194 3 0 1 0 22 24 1(B) 1 (IV) 1 1 Percentage 2.1 3.1 11.3 92.3 3.8 3.8 reca ptured Table 4. Occurrence of female gerrids with chorionated eggs. The numbers outside the parentheses indicate the number of gerrids with chorionated eggs and the numbers i n parantheses indicate the sample size. A. Overwintered adults. B. Summer adults. N = G. n o t a b i l i s ; IV = G. incurvatus; B = G. buenoi; IG - G. incognitus; R = G. remigis; w = winged; nw = no wings; 1/2w = brachypterous; x = female gerrids observed but not co l l e c t e d . Date Area A. Gate Pd. Marion L. vie. Placi d L. vie. N IV B IG R N IV B IG R N IV E IG R Apr 16 1 0 X 1 («) (3) Apr 27 X X X 0 0 x 0 3 X 0 0 0 x (2) (5) (1) (3) (3) (3) May 5 X X X X x 0 0 x X X X 0 X (3) (2) (2) May 18 X 1 1 1 1 2 2 2 X 3 3 X X (1) 0 ) (D {1) (4) (2) (2) (<*) CO B. Marion L. IG nw IG w IV w IV Iw B w B nw July 20 6(2) 1(2) 2 15(17) 2(3) 2(2) 1(1) 31 V present i n these species in l a t e A p r i l , before the other three species had emerged. Chorionated eggs were observed in G. AliSoSSiiMS o n Gate Pd. on A p r i l 16, bat they were not found i n i n d i v i d u a l s on Marion L. and Placid L. u n t i l May 18. I t should be noted that Placid L. was s t i l l covered with i c e on A p r i l 16, and the only gerrids observed at t h i s elevation were 3L§migis o n Dorothy Pd. , which was free of ice owing to the swift currents of Gwendoline Ck. Nevertheless, chorionated eggs were present in one G. remigis collected at this time at this elevation. G. n o t a b i l i s , G. buenoi and G. incurvatus a l l emerged at about the same time at the three elevations since a l l except G. incurvatus were present on A p r i l 27 at the highest elevation ( G. incurvatus was not co l l e c t e d on any of the water bodies studied at the highest elevation at any time during the season). Chorionated eggs were not present i n these species u n t i l mid-May. There are thus temporal differences between the f i v e species in attainment of sexual maturity i n the spring. The temporal occurrence of the various i n s t a r s of the four main species of Gerris on Marion L. in 1971, established by guadrate sampling, i s shown i n F i g . 8. Adults of a l l four species f i r s t appeared on the lake i n late A p r i l . The f i r s t larvae appeared on May 24 and larvae of G. buenoi were s t i l l present on the lake when c o l l e c t i n g terminated on September 15. The temperature u n t i l mid-July was unusually low in 1971, with water temperatures averaging ca. 13°C. (Fig. . 3a). Although adults of a l l four species were present on Maricn L. from May 7, G. incognitus may^  have deposited eggs before the 32 Figure 8. The temporal occurrence and numbers of f i r s t instar larvae of the four pond species c o l l e c t e d on Marion L. in 1971. ou = overwintered adults; roman numbers = i n s t a r s ; integer numbers = summer generation adults. 32a 33 other three s p e c i e s . However, a d i s t i n c t sequence of occurrence of f i r s t i n s t a r l a r v a e was found on Marion L. G. i n c o j y i i t u s and G. buenoi appeare'd f i r s t , f o l l o w e d by G. i n c u r vatus and s t i l l l a t e r G. no t a b i l i s . T h i s 'temporal d i f f e r e n c e was evident throughout the year in a l l i n s t a r s and can be seen i n the time of appearance of the f i r s t summer generat i o n a d u l t s . In a d d i t i o n to the temporal d i f f e r e n c e s i n the occurrence of the i n s t a r s of the f o u r s p e c i e s , there were pronounced d i f f e r e n c e s between the times of peak p o p u l a t i o n numbers i n the l a r v a l i n s t a r s . T h i s i s best i l l u s t r a t e d i n the p o p u l a t i o n curves obtained f o r the f i r s t i n s t a r l a r v a e . G. i n c c g n i t u s and G. buenoi showed an e a r l y peak i n numbers, but i t was only when the temperatures i n c r e a s e d i n mid-July that the numbers of f i r s t i n s t a r l a r v a e of G. i n c u r vatus and G. n g t a b i l i s peaked (Figs. 3a and 8). C o i n c i d e n t at t h i s time was a decrease i n f i r s t i n s t a r l a r v a numbers of G. buenoi and G. i n c g g n i t u s , with the l a t t e r d i s a p p e a r i n g o f f the lake completely. When the temperature d e c l i n e d i n the f a l l , the numbers of G. n g t a b i l i s and G. i n c u r v a t u s d e c l i n e d , and then the second generation numbers of G. buenoi i n c r e a s e d . It i s p o s s i b l e t h a t the l a t e peaking of numbers of G. Al3£iJIva_tU£; may not have been e n t i r e l y due to the d i r e c t dependence o f the s p e c i e s on h i g h e r temperatures. Instead, the delay may have been caused by the l a t e emergence of the v e g e t a t i o n i n the deeper water. The v e g e t a t i o n f i n a l l y reached the water s u r f a c e i n l a t e J u l y and then provided cover away from the water-edge. As has been noted e a r l i e r , G. i n c u r v a t u g 34 appears to p r e f e r a m i c r o h a b i t a t o f f s h o r e , and i t i s p o s s i b l e t h a t the l a c k of o f f s h o r e Potamocjeton and Nuphar beds e a r l y i n the year s i g n i f i c a n t l y reduced the numbers of t h i s s p e c i e s . 1 The temporal occurrence of the i n s t a r s of the f o u r main s p e c i e s on the i n s h o r e Area 3 of Marion t . i n 1972 i s shown i n F i g . 9. The weather t h i s year ( F i g . 3b) was very d i f f e r e n t from that of 1971, the temperatures being g u i t e high i n e a r l y June, low i n l a t e June, and then g e n e r a l l y remaining moderate u n t i l e a r l y i n September, except f o r one n o t i c e a b l e c o l d s p e l l i n mid-July. The temporal sequence i n 1972 was not as e v i d e n t as i n 1971 with the f i r s t i n s t a r s of G. i n c o g r i i t u s and G. buenoi were present e a r l i e s t , but a l l s p e c i e s showed a somewhat e a r l i e r development, no doubt because of the higher temperatures present i n e a r l y June. The numbers of f i r s t i n s t a r G. i n c g g n i t u s and G. buenoi peaked i n June. However, G. buenoi d i d not show a pronounced second g e n e r a t i o n i n the f a l l as i n 1971, although G. i n c g g n i t u s , i n c o n t r a s t to 1971, d i d produce a second generation on Marion L. Gen e r a l l y the numbers of G. i n c u r v a t u s i n 1972 were much gr e a t e r than i n 1971, with the numbers i n Area 3 i n 1972 being g r e a t e r than the t o t a l number i n a l l the areas i n 1971. The f i r s t i n s t a r numbers of G. i n c u r v a t u s d e c l i n e d i n Area 3 on Marion L. i n l a t e July., as i n 1971. However, at t h i s time the o f f s h o r e Potamogeton beds developed and G. i n c u r v a t u s moved to t h e s e . Hence the disappearance of t h i s s p e c i e s i n l a t e J u l y 35 Figure 9. The temporal occurrence and numbers of f i r s t instar larvae of the four pond species c o l l e c t e d on Marion L. in 1972. (see Fig. 8 for legend). 35a MARION L A K E OW . i i — G. n o t a b i l i s m - IV-v — 1--o w I — i i -in G. i n c u r v a t u s iv v 1 -- I : i I III.--IV-v  2  o w -II — i i i - : — t v -G. i n c o g n i t u s i - - « = = II — 75 50 25 0 i n IV-55" i n • IV-MA JN JU 1972 AU SE 36 from Area 3 was not a lake-wide phenomenon. A l l the a d u l t s , except apterous G. i n c o g n i t u s c o l l e c t e d on Marion L. on J u l y 20, 1972 c o n t a i n e d c h o r i o n a t e d eggs. The two female G. i n c o g n i t u s c o l l e c t e d at t h i s time were g u i t e t e n e r a l and s e x u a l l y immature. Whether they matured l a t e r was not determined. However, s i n c e the three G. i n c o g n i t u s which d i d produce young i n the f e c u n d i t y experiments i n the l a b o r a t o r y were a l l apterous, t h i s morph can at times breed the same year i t matures. F i g . 10 presents the temporal occurrence of the various i n s t a r s of the f o u r pond s p e c i e s on Gate Pd. i n 1972. Owing to the lower a l t i t u d e and sunny exposure, temperatures here averaged c o n s i d e r a b l y higher dur i n g the e a r l y s p r i n g than on Marion L. T h i s r e s u l t e d i n the development being more advanced than on Marion L. Comparison of G. buenoi i n the two l o c a l i t i e s c l e a r l y demonstrated that the i n s t a r s on Gate Pd. were two weeks ahead of those on Marion L. The peaks of the f i r s t i n s t a r l a r v a e on Gate Pd. a l s o d i f f e r e d from those on Marion L. Large numbers of G. buenoi and i n c u r v a t u s l a r v a e were c o l l e c t e d i n l a t e J u l y and e a r l y August on Gate Pd., whereas on Marion L. the l a r g e r peaks occurred i n June. Thus the s i z e of the two g e n e r a t i o n s d i f f e r e d markedly i n these two l o c a l i t i e s . From the study of Marion L. i n 1971 and 1972, and Gate Pd. i n 1972, i t i s c l e a r t h at G. n o t a b i l i s , l i k e G. r e m i g i s on Jacobs Ck., Jacobs Ck. Pd., Dorothy Pd. and R e s e r v o i r 1 i n 37 F i g u r e 10. The temporal occurrence and numbers of f i r s t i n s t a r l a r v a e of the f o u r s p e c i e s c o l l e c t e d on Gate Pd. i n 1972. (see F i g . 8 f o r legend). 37a 38 1972, was u n i v o l t i n e ( F i g . 11). G. i n c u r v a t u s on the other hand b i v o l t i n e . Since the peaks of abundance of the f i r s t i n s t a r s of the pond s p e c i e s that o c c u r r e d together seemed t c be temporally spaced, i t i s p o s s i b l e t h a t i n t e r s p e c i f i c i n t e r a c t i o n s might i n f l u e n c e the temporal occurrence of the i n s t a r s , t h e i r abundance and hence the number of g e n e r a t i o n s . Observations of the s p e c i e s i n l o c a l i t i e s where only one s p e c i e s was dominant ( F i g . 10) suggests t h a t i n t e r s p e c i f i c i n t e r a c t i o n s do not g r e a t l y a f f e c t the temporal occurrence of the i n s t a r s and do not a f f e c t the number of g e n e r a t i o n s . However, i n t e r s p e c i f i c i n t e r a c t i o n s may have some i n f l u e n c e on the t i m i n g of i n s t a r peaks of abundance. Thus the dominant s p e c i e s on P l a c i d L. was G- n g t a b i l i s , and here the numbers cf f i r s t i n s t a r l a r v a e peaked i n e a r l y June ( F i g . 11), although they were a l s o c o l l e c t e d u n t i l mid-August. In c o n t r a s t , i n other l o c a l i t i e s where t h i s s p e c i e s o c c u r r e d at lower p o p u l a t i o n l e v e l s with other pond s p e c i e s , the peaks occurred i n l a t e June, J u l y or August. On 0 I n l e t , G. i n c g g n i t u s was dominant and here the numbers of f i r s t i n s t a r l a r v a e peaked i n l a t e May-early June (Fig. 11). Young were c o l l e c t e d u n t i l e a r l y August f o l l o w i n g t h i s peak p r o d u c t i o n , but i n g r e a t l y reduced numbers. The absence of a second g e n e r a t i o n or f a l l peak i n f i r s t i n s t a r numbers o f t h i s s p e c i e s a t t h i s l o c a l i t y would thus not seem to be the r e s u l t of i n t e r a c t i o n with other s p e c i e s . i££2S£.ii3S, G. buenoi and G. were e i t h e r u n i v o l t i n e or 39 Figure 1 1 . The temporal occurrence and numbers of f i r s t instar larvae of the dominant species c o l l e c t e d on Jacobs Ck., Pl a c i d L. and 0 Inlet. (see Fig. 8 for legend). 39a JACOB'S CREEK HOW . i i - -6. r e m i g i s in. PLACID L. o w G . n o t a b i l i s i i i - -0. INLET o w -G . i n c o g n i t u s jjj — i v - -v -i — • 75 50 25 0 i n CO o IV-v 2 - r II - -III-IV-MY JN JU 1972 AU SE 40 I t would thus seem that whether or not G. incurvatus, G. iB°il3£iiu£> and G. buenoi are univoltine or b i v o l t i n e does not depend on i n t e r s p e c i f i c interactions. Instead, i t evidently depends on the weather that p a r t i c u l a r year. As shown i n Figs. 3a and 3b, the t o t a l accumulated number of degree-days on Marion L. i n 1972 was only two-thirds that of 1971, and i t would seem that t h i s difference was s u f f i c i e n t to prevent a s i g n i f i c a n t second generation i n the b i v o l t i n e species i n 1972. Gate Pd., however, at a lower elevation and less enclosed by mountains and with less cloud, had enough degree-days to allow two generations. The univoltine species, l i k e G. n g t a b i l i s , w i l l breed i n the spring i f the temperatures are high enough, but may be delayed u n t i l early summer i f cooler conditions p r e v a i l , as in 1971. The above f i e l d r e s u l t s suggest that for breeding to commence i n G. n o t a b i l i s , either a c r i t i c a l temperature threshold must be exceeded, or a set number of degree-days must be accumulated. The other four species produced young i n both 1971 and i n 1972 soon after the adults emerged from overwintering, whereas with G. n g t a b i l i s , p a r t i c u l a r l y i n 1971 when early June was unusually ecol, the appearance of f i r s t i n star larvae of t h i s species was delayed and appeared to coincide with the onset of warmer weather. As the temperature threshold below which growth of d. n o t a b i l i s ceases i s known (see below), the c a l c u l a t i o n of the accumulated degree-days for both 1971 and 1972 i s f e a s i b l e and hence i t i s possible to determine which of the above two mechanisms i s l i k e l y to be in 41 operation in t h i s species. I t can be seen (Figs. 3a and 3b) that accumulated degree-days seems to have l i t t l e e f f e c t on the time of i n i t i a t i o n of the main breeding period i n th i s species. In 1971, breeding commenced after ca. 300 degree-days had accumulated, whereas in 1972 at the i n i t i a t i o n cf breeding, only ca. 125 degree-days had been accumulated. In both years, however,- breeding seemed to commence after periods of a few days' duration where the temperatures averaged ca. 15°C. (Figs. 3a and 3b). Thus there seems to be a c r i t i c a l temperature threshold for development in G. n o t a b i l i s . Later drops of the average temperature below t h i s "threshold" did not result i n the cessation of breeding, but rather resulted simply i n a decrease in the number of young produced., 2. LABORATORY STUDIES A. E f f e c t of temperature on i n s t a r duration Figure 12 describes the mean stadium length of the eggs and each in s t a r of the f i v e species of Gerris at 26°C. In G. remigis and G. incognitus, the egg stadium i s the longest, whereas i n the other three species, the f i f t h instar larvae stadium i s longest. In a l l f i v e species, the second and t h i r d instar larva stadia were shortest, with increasing times between moults required by both the larger and smaller instars. Similar observations have been reported for G. oJc£togaster (Vepsalainen, 1971c) and G. rufoscutellatus (Kaufmann, 1971). 42 ( F i g u r e 12. The mean stadium l e n g t h (± 1 SE) of each of the g e r r i d i n s t a r s of the f i v e g e r r i d s p e c i e s s t u d i e d . '42a .51 cn £ CD -4 l-JZ i n X> t— co tn x> D c CM (0 > l _ Z3 U c CD x: 4-» m 4-* x> i— x> c CM + tn cn cn LU cn cn LU JZ i n JZ x> co X> c C M 4-* O c ID c o o c CD i n sz I— CO XI c CM i n X> co a: < t— cn x> z c — CM O C a> XI CD Cn CXi LU cn cn LU • 0 1 + cn LU CM V cn CD co o cn co co o CJ) co o (SAVQ)H19N3"1 WniQViS 43 Fi g u r e 13 d e s c r i b e s the e f f e c t of temperature on the mean i n s t a r stadium l e n g t h . G. n o t a b i l i s , G. i n c u r v a t u s and G. Aj}£°3!!i.£us show a g e n e r a l l y l i n e a r decrease i n the l e n g t h of the stadium with i n c r e a s i n g temperature. G. remigis and G. buenoi, however, o n l y show a decrease i n stadium length up to ca. 22°C. and a t temperatures above t h i s , stadium length i n c r e a s e s i n d u r a t i o n . The optimum temperature f o r minimizing generation time i n these two l a t t e r s p e c i e s was lower than f o r the other s p e c i e s . / T h i s temperature e f f e c t , e s t a b l i s h e d f o r the f i r s t i n s t a r l a r v a e , a l s o seems to apply i n the other f o u r l a r v a l i n s t a r s , f o r i n experiments with the other l a r v a l i n s t a r s a t 15°, 18.5° and 22°C. a s i m i l a r e f f e c t of temperature was noted. The average percent d e v i a t i o n of such measurements, when compared to the stadium length a t 26°C. and the r a t i o e x h i b i t e d by the f i r s t i n s t a r l a r v a e at the same temperatures, was +1% (S.E. = 6%) (with an average sample s i z e of 8.6 g e r r i d s ) . Thus, even a s l i g h t change i n temperature can s i g n i f i c a n t l y a f f e c t the r e l a t i v e g eneration length of the d i f f e r e n t s p e c i e s . I f i t i s assumed t h a t the e f f e c t of temperature on stadium le n g t h i s l i n e a r (up to 22°C. f o r G. re m i g i s and G. buenoi), then i t i s p o s s i b l e to o b t a i n an estimate of the temperature at which development f u n c t i o n a l l y ceases f o r each c f the f i v e s p e c i e s . F i g u r e 14 d e s c r i b e s t h i s f o r each of the s p e c i e s . In order to b e t t e r i l l u s t r a t e the e f f e c t of temperature, the r e c i p r o c a l of the stadium l e n g t h (days) i s p l o t t e d a g a i n s t 44 F i g u r e 13. The mean d u r a t i o n i n degree-days (± 1 SE) above 0°C. o f t h e f i r s t i n s t a r l a r v a l s tadium a t d i f f e r e n t c o n s t a n t t e m p e r a t u r e s . \ 250 6. buenoi 200 150-i • 100-50 co $ °-D l LU £ 200-CD LU O ~ 150 X r— CD Lu 100 2 50-< co o-200-150-100 50-0 12 • 6. notab i I is G- incognitus 18 G. remi ais G. incurvatus J 24 30112 18 T E M P E R A T U R E ' (°C) 24 30 45 F i g u r e 14. The r e c i p r o c a l s o f the mean d u r a t i o n i n days of the f i r s t i n s t a r l a r v a l stadium of each of the f i v e s p e c i e s at d i f f e r e n t constant temperatures. The i n t e r c e p t of r e g r e s s i o n s f o r each s p e c i e s p l o t t e d through the above values with the base l i n e , s e t a t an a r b i t r a r y maximum stadium length c f 30 days, allows determination on the X - a x i s of the minimum temperature at which growth occurs., ° = G. n o t a b i l i s ; p = G. r e m i g i s ; v = G. buenoi ; • = G. i n c u r v a t u s ; • = G . i n c o g n i t u s . 46 temperature. I f i t i s a r b i t r a r i l y assumed that 30 days i s the maximum stadium l e n g t h that i s l i k e l y to e x i s t , then the r e c i p r o c a l of t h i s value, 0.033, can be thought of as a base~ l i n e . The i n t e r c e p t of the l i n e a r r e g r e s s i o n d e s c r i b i n g the e f f e c t of temperature on stadium l e n g t h with the base length i s then the p o i n t at which development ceases. Even i f 30 days i s not a r e a l i s t i c value, i t s t i l l a l l o w s the r e l a t i v e temperatures a t which development ceases to be determined f o r the f i v e s p e c i e s . Table 5 d e s c r i b e s the temperatures (X~values of the i n t e r c e p t ) at which development ceases as d e r i v e d i n the above manner f o r the f i v e s p e c i e s s t u d i e d : G. i n c c g n i t u s and G. £5i§bilis cease development at the lowest temperatures whereas the other three s p e c i e s cease development a t r e l a t i v e l y higher tempera t u r e s . B. F e c u n d i t y F i g u r e 15 d e s c r i b e s the accumulative number of f i r s t i n s t a r l a r v a e emerging from eggs deposited by females of the f i v e s p e c i e s of G e r r i s i n the l a b o r a t o r y . The l o g i s t i c curve developed by P e a r l and Reed (1920) and improved by Davidson (1942, 1944), namely YH = 1.Q + ete/-ED<TDW. was used to d e s c r i b e the data^, where YH i n d i c a t e s the number c f young hatched, kS r e p r e s e n t s the maximum number of eggs l a i d by each i n d i v i d u a l female, TD r e p r e s e n t s the accumulative number of 47 F i g u r e 15. The accumulative mean number of young hatched <± 1 SE) from eggs l a i d by females of the f i v e s p e c i e s of G e r r i s . h The l o g i s t i c curve d e s c r i b e d i n the t e x t was used i n each case t o d e s c r i b e the data, which i s p l o t t e d a g a i n s t accumulated degree-days above each s p e c i e s 1 ' temperature t h r e s h o l d , as d e r i v e d i n F i g . 14. 47b 0 100 200 300 400 500 D E G R E E - D A Y S 47c 47a 100 80+ Q LU I TC 60--< X CD UN 4 0 -o > 20--G. incoqnitus ( S U M M E R ) _^ ©—e c i ED=00160 100 2.00 300 D E G R E E - D A Y S 400 500 48 Table 5. The temperature t h r e s h o l d v a l u e s f o r each of the f i v e s p e c i e s below e h i c h growth ceases. Species Temperature (°C.) G. i n c o g j i i t u s 9.3 G. n o t a b i l i s 10.3 G. r e m i g i s 12.4 G. i n c u r v a t u s 12.6 G. buenoi 12.9 49 degree-days above the t h r e s h o l d f o r that s p e c i e s , EY i n d i c a t e s the r e l a t i v e p o s i t i o n of the o r i g i n of the curve on the a b s c i s s a , and ED determines the slope of the curve. The u s e f u l n e s s of t h i s curve i n q u a n t a t i v e b i o l o g y has been demonstrated by s e v e r a l i n v e s t i g a t o r s (Davidson, 1944; Lamb, 1961; T r p i s , 1972). Ta b l e 6 presents the values of the unknowns i n the e g u a t i o n . G. r e m i g i s and G. i n c o g n i t u s have a t o t a l f e c u n d i t y about h a l f that of the other s p e c i e s . Whether the i n d i v i d u a l s i n the b i v o l t i n e s p e c i e s had j u s t emerged from o v e r w i n t e r i n g or had j u s t matured as the f i r s t g e n e r a t i o n females, a f f e c t e d the f e c u n d i t y only s l i g h t l y , although i n a l l s p e c i e s , the f i r s t g e n e r a t i o n females produced on the average fewer young i f those i n d i v i d u a l s which d i d not breed are i n c l u d e d . There was c o n s i d e r a b l e v a r i a t i o n i n the f e c u n d i t y c f the f i r s t g e n e r a t i o n females, as shown by the r e l a t i v e l y l a r g e standard e r r o r s . T h i s v a r i a b i l i t y was most pronounced i n G, i££°3£iiJ2S where o n l y three of the seven females produced eggs. Thus not a l l of the i n d i v i d u a l s of G. i n c o g n i t u s breed the year they emerge. F u r t h e r , the f e c u n d i t y of those G. i n c o g n i t u s females that d i d breed was about twice that of overwintered females. Hence the net production of young by the summer pop u l a t i o n may be i d e n t i c a l to t h a t of the s p r i n g p o p u l a t i o n . Greater numbers of young from the summer ge n e r a t i o n a d u l t s were a l s o evident i n G. buenoi and G. i n c u r v a t u s . In these two s p e c i e s the summer morph was apterous or shortwinged, whereas the o v e r w i n t e r i n g females were macropterous. It w i l l be noted i n Table 6 t h a t the i t e r a t i v e number of 50 Table 6. The values of the unknowns i n the eguation YH = 1.0 + e< £V - £o (. tot > f used to describe fecundity i n F i g . 19, where YH = the accumulative number of young hatched, AS = the maximum number of eggs a l a i d by a female gerrid i n a season, TD = the accumulative number of degree-days above that species 1 minimum temperature threshold for growth, and EY and EE are constants. Species Over-wintered females: n Equation constants AS EY ED TD Observed mean AS G. no t a b i l i s 18 200.7 3. 30 0.0187 294.8 203. 3 ± 23. 2 G- remigis 21 95. 5 2. 42 0.0205 225. 5 98. 0 ± 13.6 G. buenoi 21 152. 2 2. 47 0.0237 197. 9 152. 7 ± 10.6 G. incurvatus 21 * 120.7 2. 41 0.0298 155. 4 122. 8 ± 7.0 G. incognitus 21 70. 6 2. 58 0.0228 211. 4 72. 6 ± 9.7 Summer females: G. buenoi 14 145. 4 3. 00 0.0182 285. 6 143. 1 ± 15. 1 G. incurvatus 9 110. 3> 2. 86 0.0279 180. 3 112. 7 ± 18.0 G. incognitus 7 48.4 1. 93 0.0160 , 258. 6 49. 1 ± 30.6 51 degree-days r e q u i r e d f o r the h a t c h i n g of 90% of the average t o t a l number of young produced per female i s g r e a t e r f o r the f i r s t g e n e r a t i o n females ' than i t i s f o r the overwintered females. T h i s may i n d i c a t e t hat the former are a c c l i m a t e d to the h i g h e r temperatures t h a t p r e v a i l i n the summer. A few f i r s t g e n e r a t i o n females r a i s e d i n the l a b o r a t o r y provided some i n d i c a t i o n of the time r e q u i r e d f o r these i n s e c t s to produce young f o l l o w i n g the moult to the a d u l t . Young hatched from eggs l a i d by one female G. i n c u r v a t u s and two G. buenoi r e g u i r e d on average 258.7 (S.E. = 1.3) degree-days f o l l o w i n g t h e i r moult to the a d u l t . DISCUSSION Pre v i o u s t o t h i s study, there have very few d e t a i l e d p u b l i s h e d papers on g e r r i d s p e c i e s s t u d i e d over the whole season. Kaufmann (1971) s t a t e d t h a t G. £u|oscutellatus i n Alaska i s u n i v o l t i n e , with the o v e r w i n t e r i n g a d u l t s emerging i n l a t e May, and the new a d u l t s appearing i n mid-July and then d i s a p p e a r i n g i n l a t e September - mid-October. E r i n k h u r s t (1966) s t u d i e d G. najas i n B r i t a i n and observed b a s i c a l l y a s i m i l a r l i f e c y c l e of t h i s un^ivoltine s p e c i e s . To date, the most d e t a i l e d study of l i f e c y c l e s i n G e r r i s has been by Vepsalainen (1971c) on G. 2j°Bi2a§Siei i n southern F i n l a n d . Although p r i m a r i l y concerned with the f a c t o r s i n f l u e n c i n g wing polymorphism, he d e s c r i b e d the l i f e c y c l e of 52 t h i s b i v o l t i n e s p e c i e s i n d e t a i l and showed that i t i s r a t h e r s i m i l a r to that of G. buenoi s t u d i e d h e r e i n . However, as G. °l|ojQi53a..§ter was the only g e r r i d t y p i c a l l y present i n the study area, i n t e r s p e c i f i c i n t e r a c t i o n s were not considered by Vepsalainen (1971c). No study of g e r r i d l i f e c y c l e s to date has i n v o l v e d a comparative study of s e v e r a l s p e c i e s c o e x i s t i n g on the same body of water. The r e s u l t s show that numerous d i f f e r e n c e s e x i s t among the f i v e s p e c i e s of G e r r i s s t u d i e d . T a b l e 7 summarizes these d i f f e r e n c e s and demonstrates c l e a r l y that they cannot be c o n s i d e r e d as e c o l o g i c a l homologues. There are d i f f e r e n c e s i n almost every aspect s t u d i e d . Each s p e c i e s has i t s own m i c r o h a b i t a t preference and although the s p e c i e s o v e r l a p i n d i s t r i b u t i o n , even to the extent that they can be a l l c o l l e c t e d i n the same sample, they do not always occur together. Each s p e c i e s was a l s o shown to respond d i f f e r e n t l y t o temperature. The combination of these m i c r o h a b i t a t d i f f e r e n c e s and the temperature d i f f e r e n c e s provide a c l e a r b a s i s f o r e c o l o g i c a l s e p a r a t i o n of the s p e c i e s . They do not occupy the same niche and thus do not prove an e x c e p t i o n to the Coexistence P r i n c i p l e . A r e l a t i o n between morphology and m i c r o h a b i t a t i s suggested i n the present study. G. i n c u r v a t u s , f o r example, i s t y p i c a l l y found f u r t h e s t o f f s h o r e on open water and has r e l a t i v e l y long l e g s , and t h i s would seem an a d a p t a t i o n to rowing. As d i s c u s s e d by B r i n k h u r s t (1960), an i n c r e a s e i n the s u r f a c e area of the 53 Table 7. A summary of the biologies of each of the f i v e species of Gerris studied. 53a Microhabitat Size Leg length ( r e l a t i v e to weight) Dominant morphs Generations Spring emergence Chorionated eggs f i r s t present G_. notabi l i s Ponds ,lakes; inshore;clean water surface G_. remigis Streams G_. incognitus G. incurvatus buenoi Large .. Large Long Normal Macropters Apters Univoltine Average generation 1045 duration to.end of egg-laying (degree-days) Late A p r i l Mid-May F a l l disappearance Ear l y Sept. Average number of 203 eggs hatched per female Minimum temperature 10.3 f o r growth (°C.) Optimum temperature 26 + f o r growth ( C.) Relative e f f e c t of Moderate temperature on •.. growth (slope) Univoltine 900 February A p r i l Late October 98 12.4 22 Pronounced Ponds,lakes ; inshore; c l u t t e r e d water surface Small Short Apters B i v o l t i n e 924 March A p r i l Late Sept. 67 9.3 26 + S l i g h t Ponds,lakes; Ponds,lakes; offshore;clean inshore;clean water surface water surface Small Long S p r i n g , F a l l : macropters; Summer: micropters B i v o l t i n e 789 Late A p r i l Mid-May Early Sept. 120 12.6 26 + Moderate Small Normal S p r i n g , F a l l : macropters; Summer:apters B i v o l t i n e 844 Late A p r i l Mid-May Late Sept. 149 12.9 22 Pronounced 54 t a r s u s (the o n l y p o r t i o n of the l e g to a c t u a l l y ccme in c o n t a c t with the water) compensates f o r the i n c r e a s e d weight of the l a r g e r s p e c i e s , with an i n c r e a s e i n length cf the femur and t i b i a having no e f f e c t . T h i s i n c r e a s e w i l l have an e f f e c t on the maximum speed with which the g e r r i d can move, however, as the main r e t r a c t o r muscle i n s e r t s on the fulcrum of the limb (the t r o c h a n t e r ) and hence i t s c o n t r a c t i o n i s magnified by the l e n g t h of the l e g . A longer s t r i d e i s thus p o s s i b l e , thereby a l l o w i n g the g e r r i d to move f a s t e r . T h i s would appear to d i s a g r e e with the o b s e r v a t i o n of no s i g n i f i c a n t d i f f e r e n c e i n s t r i d e l e n g t h between the s m a l l s p e c i e s ( S e c t i o n 3) , but under the c o n d i t i o n s by which s t r i d e length was measured, a l l the g e r r i d s were moving s l o w l y and i t i s f e l t that any d i f f e r e n c e would not n e c e s s a r i l y be apparent. A n o t i c e a b l e d i f f e r e n c e was observed between the l a r g e and s m a l l s p e c i e s , with s t r i d e l e n g t h d i r e c t l y p r o p o r t i o n a l to l e g l e n g t h . G. buenoi i n c o n t r a s t possesses short l e g s and i s t y p i c a l l y found inshore amongst the reeds and g r a s s e s . In t h i s m i c r o h a b i t a t , long l e g s would hinder movement r a t h e r than help owing to the o f t e n dense v e g e t a t i o n . G. n o t a b i l i s would appear to be the e x c e p t i o n to the above, as i t possesses r e l a t i v e l y l o n g l e g s f o r i t s s i z e and yet o f t e n i n h a b i t s areas of dense v e g e t a t i o n . Here, though, i t seems t h a t s e l e c t i o n has favoured long l e g s f o r mating behaviour. During the mating behaviour, the sexes of G. n o t a b i l i s , l i k e the s p e c i e s of Rhagadgtarsus s t u d i e d by Wilcox (1972) i n A u s t r a l i a , communicate with s u r f a c e waves ( s i g n a l s ) . The males of G. 55 n o t a b i l i s i n p a r t i c u l a r produce waves, a p p a r e n t l y by v i b r a t i n g t h e i r body v e r t i c a l l y while maintaining t h e i r l e g s s t a t i o n a r y , the r e s u l t being s c o r e s of t i n y waves which r a d i a t e outwards. V e r t i c a l movements of the f o r e l e g s a l s o on o c c a s s i o n seem t o be used to produce s i g n a l s . Since the males t y p i c a l l y i n i t i a t e the mating behaviour, v i b r a t i n g to any g e r r i d which approaches, and s i n c e t h i s s p e c i e s i s unique among those s t u d i e d i n that the l e g s of the male are longer than those of the female, i t seems that the production and perhaps r e c e p t i o n of these waves has been the main determining f a c t o r regarding l e g l e n g t h . The exact m i c r o h a b i t a t which t h i s s p e c i e s p r e f e r s seems to be s l i g h t l y f u r t h e r o f f s h o r e than t h a t p r e f e r r e d by G. buenoi, and t h i s may be a r e s u l t of i t s g r e a t e r l e g l e n g t h . G. remi_gis and G. i n c o g n i t u s are both t y p i c a l l y apterous and do not possess long legs f o r t h e i r s i z e s . G. r e m i g i s i s a r i v e r i n e s p e c i e s , p r e f e r r i n g an environment with c u r r e n t s , whereas G. i n c o g n i t u s seems to p r e f e r the extreme inshore areas and c l u t t e r e d water s u r f a c e s . G. r e m i g i s has o f t e n strong c u r r e n t s to contend with and G. i n c o g n i t u s o f t e n cannot s l i d e on the water s u r f a c e owing t c i t s c l u t t e r e d nature and hence must jump, which r e q u i r e s the f l i n g i n g of i t s body i n t o the a i r . B r i n k h u r s t (1960) has shown t h a t i n short-wing G. l a c u s t r i s , there was a g r e a t e r than normal development of the mesothoracic limb r e t r a c t o r muscles f i l l i n g the space normally occupied by the f l i g h t muscles. In G. remijgis and G. i n c o g n i t u s , i t seems a rea s o n a b l e assumption t h a t here too l o s s of the f l i g h t muscles has r e s u l t e d i n an i n c r e a s e i n s i z e of the mesothoracic limb 56 r e t r a c t o r s . The temperature d i f f e r e n c e s noted with the f i v e s p e c i e s , together with apparent s p e c i e s i n t e r a c t i o n r e s u l t i n a temporal s e p a r a t i o n of the s p e c i e s with r e s p e c t to t h e i r p o p u l a t i o n peaks. G. i n c o g n i t u s breeds i n mid-spring, as temperature has r e l a t i v e l y l i t t l e e f f e c t on i t s r a t e of development. Whether or not a second generation i s present seems to depend on the accumulated number of degree-days. This s p e c i e s has the lowest temperature t h r e s h o l d (9.3°C.) f o r development of the s p e c i e s s t u d i e d . However, the average gener a t i o n length i n t h i s s p e c i e s i s c a . 924 degree-days, c a . 80-125 more than i s r e q u i r e d by G. buenoi and G. i n c u r v a t u s r e s p e c t i v e l y . G. buenoi and G. r e m i g i s a l s o breed i n mid-spring, and t h i s probably i s a r e s u l t o f a low optimum temperature f o r growth (22 ° C ) . Both s p e c i e s have a r e l a t i v e l y high minimum temperature f o r growth (12.9°C. and 12.4°C. r e s p e c t i v e l y ) , which i n d i c a t e s t h a t these two s p e c i e s are r e l a t i v e l y more stenothermal than the other s p e c i e s . T h i s i s supported i n the case of G. r e m i g i s by data on the e f f e c t of temperature on both food consumption and p r e f e r r e d m i c r o h a b i t a t . The maximum food consumption per day was observed to be at a temperature of ca. 30°C. (see S e c t i o n 2) and on a water temperature g r a d i e n t , a temperature of ca. 20°C. was p r e f e r r e d (see S e c t i o n 2). G. r e m i g i s i s u n i v o l t i n e , but being the only s p e c i e s s t u d i e d which can s u c c e s s f u l l y i n h a b i t streams, i t i s i s o l a t e d from the other s p e c i e s s p a t i a l l y . In t h i s s p e c i e s there i s thus l i t t l e i n t e r s p e c i f i c i n t e r a c t i o n and the l i f e c y c l e i s spread over the summer. G. buenoi, however. 57 i s b i v o l t i n e and i n h a b i t i n g ponds, i s i n f l u e n c e d by the other s p e c i e s present. Since i t can t o l e r a t e low temperatures, i t breeds e a r l y i n the year and again l a t e r i n the f a l l and thus avoids much i n t e r a c t i o n with the other s p e c i e s . The peak breeding p e r i o d s of G. i n c u r v a t u s and G. n o t a b i l i s are not i n the s p r i n g but i n e a r l y summer. In G. n o t a b i l i s t h i s seems to be the r e s u l t of a t e m p e r a t u r e - a c t i v a t e d switch mechanism which must be turned on before breeding can commence. Temperatures averaging >15°C. f o r a few days seem to be r e q u i r e d to i n i t i a t e breeding and as these weather c o n d i t i o n s are not normally present i n the study area u n t i l mid-June, breeding of t h i s u n i v o l t i n e s p e c i e s t y p i c a l l y peaks i n l a t e June or J u l y . Both G. i n c u r v a t u s and G. n o t a b i l i s have r e l a t i v e l y high optimum temperatures f o r growth (>26°C.) and t h i s a l s o seems to be r e l a t e d to when they breed. G. i n c u r v a t u s , although i t commences breeding when G. buenoi does, i s not so t o l e r a n t to the low s p r i n g temperatures, and t h i s appears t c delay the p e r i o d of peak breeding i n G. i n c u r v a t u s s u f f i c i e n t l y to e x p l a i n the d i f f e r e n c e between them. ' Thus, the b i v o l t i n e pond s p e c i e s breed i n the s p r i n g and f a l l and the u n i v o l t i n e G. n o t a b i l i s breeds i n the summer, although c o n s i d e r a b l e o v e r l a p occurs between the s p e c i e s . Whether these d i f f e r e n c e s i n v o l t i n i s m are a r e s u l t of i n t e r a c t i v e processes or not i s not c l e a r as the e v o l u t i o n a r y h i s t o r y of the s p e c i e s i n unknown. However, i t i s of i n t e r e s t to note that such temporal d i f f e r e n c e s do e x i s t . 58 Both s p a t i a l and temporal s e p a r a t i o n of the f i v e s p e c i e s of G e r r i s thus e x i s t , i n d i c a t i n g t h a t the water s u r f a c e h a b i t a t i s perhaps not g u i t e as simple as p r e v i o u s l y thought. A number of m i c r o h a b i t a t s are d i s c e r n a b l e and i n t h i s study area a t l e a s t , perhaps the maximum number of s p e c i e s that can c o e x i s t do so. 59 SECTION 2. MOVEMENT ON A TEMPERATURE GRADIENT INTRODUCTION P o i k i l o t h e r m i c animals such as i n s e c t s by d e f i n i t i o n have body temperatures l a r g e l y determined by the e x t e r n a l environment. A few l a r g e i n s e c t s such as the bumble bee Eombus vagans ( H e i n r i c h , 1972), the sphinx moth Mandusa sex t a ( H e i n r i c h and Casey, 1973) and the g i a n t silkmoth Hjjalophora c e c r o p i g (Heath e t a l , 1971) can a d j u s t heat production and heat l o s s . G e r r i d s , l i k e most i n s e c t s , however, cannot s i g n i f i c a n t l y a l t e r t h e i r body temperatures e n d o t h e r m i c a l l y and must r e l y on adjustments i n l o c a t i o n to r e g u l a t e body temperatures. To maximize r e p r o d u c t i o n , a s p e c i e s should attempt to l o c a t e and occupy those p a r t s of i t s h a b i t a t that are at an optimum temperature, s i n c e any d e v i a t i o n away from t h i s c o u l d a d v e r s e l y i n f l u e n c e metabolism and hence r e p r o d u c t i v e c a p a b i l i t i e s . Only through m a i n t a i n i n g themselves i n such a m i c r o h a b i t a t can they maximize t h e i r r e p r o d u c t i v e p o t e n t i a l . C l o s e l y r e l a t e d s p e c i e s l i v i n g s y m p a t r i c a l l y o f t e n s p e c i a l i z e d i f f e r e n t l y so as to minimize competitive i n t e r a c t i o n s . T h i s s e p a r a t i o n may be s p a t i a l , temporal, or i n the manner i n which they feed or reproduce. Temperature responsiveness i n d i r e c t l y or d i r e c t l y can i n f l u e n c e a l l these parameters. D i f f e r e n t optimal temperatures have been demonstrated to e f f e c t i v e l y s e p a r a t e c l o s e l y r e l a t e d s p e c i e s . The 17-year p e r i o d i c a l c i c a d a s Magicicada c a s s i n i , M. 60 §®EtenJejcium and M. sejtejng^cula (Heath e t a l , 1971) and the wolf s p i d e r s Pardosa s i e r r a and P. ramulosa (Sevacherian and Lowrie, 1972) are both examples of c o e x i s t i n g , c l o s e l y r e l a t e d s p e c i e s separated by d i f f e r e n t temperature t o l e r a n c e s . Since temperature seems to a f f e c t d i f f e r e n t l y the f i v e s p e c i e s of G e r r i s s t u d i e d (see S e c t i o n 1 ) , the aim here was (a) to i n v e s t i g a t e the temperature t o l e r a n c e s of each g e r r i d s p e c i e s , (b) see i f the d i f f e r e n t s p e c i e s a c t i v e l y s e l e c t d i f f e r e n t temperatures and i f so, (c) to determine i f temperature p r e f e r e n c e s of the s p e c i e s c o r r e l a t e with those m i c r o h a b i t a t temperatures t y p i c a l l y encountered by each s p e c i e s i n the f i e l d . MATERIALS AND METHODS Two troughs 122 cm long, 19 cm wide and 10 cm deep made of aluminium s h e e t i n g contained water to a depth of 1 cm. Eoth troughs r e s t e d on p a r t i t i o n s i n a l a r g e r tank ( 216 cm long , 43 cm wide and 15 cm deep) f i l l e d with water to the height of the p a r t i t i o n s . The aluminium troughs were thus i n c o n t a c t with the water s u r f a c e of the l a r g e r tank f o r t h e i r e n t i r e length. One end compartment of the l a r g e r tank contained a copper heating c o i l c a r r y i n g hot water and the other end had a s i m i l a r c o i l c a r r y i n g a r e f r i g e r a n t : the c o l d c o i l was connected t c a Haake c i r c u l a t i n g and thermostated pump. Model K41 (Haake Inc. L t d . , K a r s t r u l e , West Germany) and the hot c o i l was connected to a hot water tap. By t h i s arrangement a non-ccntinucus water 61 temperature g r a d i e n t was e s t a b l i s h e d i n the l a r g e r tank (owing to the p a r t i t i o n s ) and a continuous water temperature g r a d i e n t was s e t up i n both the aluminium troughs, which lacked p a r t i t i o n s . Owing to water s t r a t i f i c a t i o n , g r a d i e n t s could o n l y be e s t a b l i s h e d i n water a few centimeters deep, but t h i s was s u f f i c i e n t s i n c e g e r r i d s are s u r f a c e water bugs and o f t e n occur on very shallow water. The troughs were i l l u m i n a t e d by two, t h r e e - f o o t , c o o l - w h i t e f l u o r e s e c n t tubes placed end-to-end. The whole apparatus was surrounded by black p l a s t i c s h e e t i n g (2 mil) with vents to allow a i r c i r c u l a t i o n . One-way m i r r o r s allowed o b s e r v a t i o n of the g e r r i d s and prevented sudden movements by the observer from d i s t u r b i n g them. Room temperature was 20-22°C. throughout the study. Each trough was marked out i n t o s e c t i o n s and at the commencement of each s e t of o b s e r v a t i o n s , the water temperature i n each was measured with a t h e r m i s t e r temperature r e c o r d e r (Yellow Springs telethermometer, Model 4 4 T B ). At the t e r m i n a t i o n of a l l the o b s e r v a t i o n s , the water temperature i n each s e c t i o n was rechecked and the a i r temperature 3 mm above the water s u r f a c e , the temperature a t the l e v e l of the bodies of the g e r r i d s , were measured with a microthermocouple (see below). These data allowed d e t e r m i n a t i o n of both the a i r and water s u r f a c e temperatures experienced by the g e r r i d s i n each s e c t i o n . As the temperature range which was observed i n the f i e l d d u r i n g the summer exceeded t h a t which c o u l d be e s t a b l i s h e d at any one time i n the troughs, two temperature g r a d i e n t s were used: a 62 " c o l d " g r a d i e n t ( c a . 10-25°C.) and a "hot" g r a d i e n t ( ca. 18-35 ° C . ) . Adult g e r r i d s of each of the f i v e s p e c i e s f o r study were c o l l e c t e d i n the f i e l d i n l a t e June and i n d i v i d u a l s were taken randomly f o r t e s t i n g . G e r r i d s of each s p e c i e s were observed twice on each g r a d i e n t and twice on a c o n t r o l g r a d i e n t which had a uniform temperature i n a l l s e c t i o n s . G e r r i d i n t r o d u c t i o n s onto the g r a d i e n t were at op p o s i t e ends of the g r a d i e n t i n each p a i r o f o b s e r v a t i o n s . F o l l o w i n g i n t r o d u c t i o n , the g e r r i d s were l e f t 15 minutes to a c c l i m a t e . The l o c a t i o n of each g e r r i d i n the g r a d i e n t was then recorded. Recordings were repeated every minute f o r s i x , eight-minute o b s e r v a t i o n p e r i o d s , with f i v e -minute time-out p e r i o d s s e p a r a t i n g each o b s e r v a t i o n p e r i o d . A t o t a l o f 288 r e c o r d i n g s were thus obtained f o r each s p e c i e s i n each of the g r a d i e n t s . For measurement of body temperatures of a l l f i v e s p e c i e s i n the f i e l d , a d u l t g e r r i d s were taken and a copper-constantan thermocouple made from 0.0508 mm diameter wire (Esch, 1960) was i n s e r t e d i n t o the body through the anus. The thermocouple wires, attached to a s t i f f e r wire bent at r i g h t angles to the i n s e c t , were connected to a minithermocouple potentiometer (Doran Instrument Co. L t d . Stroud, England) f o r measurement of body temperatures. The s t i f f wire was a l s o used to r e s t r a i n the i n s e c t i n one l o c a t i o n and p o s i t i o n i t j u s t above the water s u r f a c e , the exact h e i g h t being c o n t r o l l e d by use of a micromanipulator. Measurements were taken 3 minutes a f t e r i n s e r t i o n of the thermocouple or movement of the g e r r i d to a new l o c a t i o n . 63 The f i e l d data of a d u l t s was c o l l e c t e d a t Marion Lake on J u l y 5 and J u l y 21, 1972 between 1:00 and 2:30 p.m. Measurements were taken on i n s e c t s placed both i n the sun and i n the shade. RESULTS When f i r s t i n t r o d u c e d onto the g r a d i e n t , some of the g e r r i d s explored the g r a d i e n t , moving back and f o r t h i n the trough. Within f i f t e e n minutes, most had s e t t l e d down and t h e i r behaviour c o n s i s t e d of remaining s t a t i o n a r y f o r a few ninutes f o l l o w e d by r e l a t i v e l y short p e r i o d s of a c t i v i t y . In g e n e r a l , ' G. r e m i g i s moved c o n s i d e r a b l y more than the other s p e c i e s . The number of g e r r i d s of each s p e c i e s i n each s e c t i o n i n each of the three g r a d i e n t s ( c o n t r o l , hot and cold) i s shown i n F i g . 16. Observations where g e r r i d s were i n t r o d u c e d i n t o o p p o s i t e ends of the same temperature g r a d i e n t have been summed. I n d i v i d u a l s of each of the s p e c i e s demonstrated a marked preference f o r the ends of the troughs as shown by t h e i r p r e f e r e n c e s i n the c o n t r o l g r a d i e n t . T h i s f a c t was used as an i n d i c a t o r on the experimental g r a d i e n t s to d e t e c t what temperatures were avoided, and hence which were t o l e r a t e d by each s p e c i e s . I f the g e r r i d s were predominantly observed i n the middle r e g i o n s of the g r a d i e n t , t h i s was taken to i n d i c a t e avoidance of the temperatures a t the ends of the g r a d i e n t , and i f both ends of the g r a d i e n t were e g u a l l y p r e f e r r e d , then 64 F i g u r e 16. The number of g e r r i d s of each of the s p e c i e s i n each s e c t i o n on the water g r a d i e n t s s t u d i e d . • = temperature of each s e c t i o n . ( 64a 100 75-, CONTROL 50-25-0 G, notabilis © 100-1 a: 75-LiJ DD § 50H zz. 25-1 0 9 e -30 -20 -10 0 r-40 — i m -30 i m zo -20 > c: HO rn 0 o O 100-1 75-50-25-0 9 • 9 ® ~9~ © © r40 -30 o 20 M0 0 0 10 20 30 40 50 60 70 80 90 100 110 120 GRADIENT (CM.) 64b 100-75-CQNTROL G. incurvatus 50-25-0 o © a • o e • 9 i — © -100n a: 75-UJ CD §50--z. 25-] 0 HOT © 9 9 9 1 0 0 _ COLD 75-50-25-i 9 • 0 r-40 -30 -20 -10 0 r40_ m -30 -o m -20 5; cz -10 H o O 0 r-40 -30 -20 -10 0 0 10 20 30 40 50 60 70 80 90 100 110 120 GRADIENT (CM.) 64c 6. buenoi 100-1 75-CONTROL 50H 25-0 100n U J 75-oo 5 50--9- O • © • O 9 9 25-0 HOT e 9 100-i 75 50-25-0 COLD • e -40 -30 • -20 •10 0 r 4 0 m -30 -o m -20 5; cz X) -10 rn '"o O 0 w  r 4 0 30 20 HO 0 0 10 20 30 40 50 60 70 80 90 100 110 120 GRADIENT (CM.) 64d 100H 75-50-25-0 CONTROL G. incognitus 100-i 75-50 25-1 0 HOT 9 100-j 75-50-25-0 COLD © -30 -20 -10 0 r40_ m -30 1 m -20 _5 c -10 rn o O 0 r-40 -30 -20 -10 0 0 10 20 30 40 50 60 70 80 90 100 110 120 GRADIENT (CM.) 64e 100-1 75-50-CONTROL G. remigis 25-1 0 100- H 0 T • © 9 © © © or 75. DD § 5 0 -25-0 & 9 9 © 100-1 75-50-25-0 COLD © © © © © © -40 -30 20 HO 1 F 0 -40 _ m -301 m -20 5 XI -10 ™ o O 0 ~ -40 -30 -20 -10 0 0 10 20 30 40 50 60 70 80 90 100 110 120 GRADIENT (CM.) 65 temperatures i n the g r a d i e n t were assumed to have l i t t l e e f f e c t on d i s t r i b u t i o n . G. n o t a b i l i s p r e f e r r e d the warm end of the c o l d g r a d i e n t and on the hot g r a d i e n t avoided both ends. The s e c t i o n s cn the hot g r a d i e n t with the g r e a t e s t number of g e r r i d s present had a temperature of from ca. 24°C. to 30°C. G. r e m i g i s avoided the c o l d e s t end on the c o l d g r a d i e n t and the h o t t e s t end on the hot g r a d i e n t , p r e f e r r i n g temperatures from ca. 16°C. to 22°C. G. iS£23I!i*. uJ> avoided o n l y the hot end of the hot g r a d i e n t , and so would seem to be a r a t h e r eurythermal s p e c i e s , t o l e r a n t of temperatures from ca. 14°C. to 29°C. G. i n c u r v a t u s avoided the c o l d end of both the c o l d and hot g r a d i e n t s and p r e f e r r e d temperatures from ca. 25°C. to 31°C. F i n a l l y , G. buenoi avoided the c o l d end of the c o l d g r a d i e n t and the hot end of the hot g r a d i e n t , i n d i c a t i n g a preference f o r temperatures between ca. 24°C. and 30°C. Thus, the d i f f e r e n t s p e c i e s have d i f f e r e n t temperature p r e f e r e n c e s . The measurement of a i r temperatures i n each s e c t i o n 3 mm above the water s u r f a c e were found to be very s i m i l a r to water temperatures ( F i g . 17). Thus, i f a i r temperature i s s e l e c t e d by the g e r r i d r a t h e r than water temperature, the r e s u l t s w i l l not be g r e a t l y d i f f e r e n t . O b servations i n the f i e l d (Table 8) i n d i c a t e d t h a t the body temperature of a g e r r i d i s i n f l u e n c e d by s o l a r r a d i a t i o n . Measurements demonstrated that i n the shade, the body temperature was v i r t u a l l y i d e n t i c a l to t h a t of the surrounding 66 Figure 17. The a i r and water temperatures of each section on the hot and cold gradients studied. A i r temperatures are 3 mm above the water surface. ° = water temperature; • = a i r temperature. 30 HOT o ° o o 2 4 LU cr ? 18 < cr U J a. 12 o © o 9 C O L D U J 0 1 1 1 1 1 1 1 0 20 40 60 80 100 120 GRADIENT L E N G T H CD 05 V 67 T a b l e 8. F i e l d temperature measurements of g e r r i d s on a sunny day with a microthermocouple i n s e r t e d i n t o the body. A i r temperatures were 3 mm above the water s u r f a c e and water temperatures were at the water s u r f a c e . S p e c i e s Time A i r Water G e r r i d (pm) temperature temperature temperature < ° C ) (OC.) (OC. ) shade sun J u l y 5,1972 G. i n c o g n i t u s 1:00 27.0 G. r e m i g i s - 27.6 G. i n c u r v a t u s 28.0 G. i n c o g n i t u s 2:30 28.3 29.4 29.4 30.5 31.3 27.2 27.6 28.1 28.4 30.5 31. 1 31.8 32.3 J u l y 20, 1972 G. i n c o g n i t u s 2:30 23.2 G. r e m i g i s - 24.0 G. i n c u r v a t u s 4:00 22.7 26.6 27.7 27.2 23.5 27.8 24.2 29.9 22.9 27.2 68 a i r , i e . the a i r 3 mm abcve the water s u r f a c e . In the sun on J u l y 5, 1972, however, body temperatures averaged 3.9°C. warmer than the a i r 3 mm above the water s u r f a c e , and on J u l y 20, 1972, body temperatures averaged 5°C. warmer than the a i r at t h i s l e v e l . DISCUSSION i One of the g r e a t e s t problems i n experiments with g r a d i e n t s i s s e t t i n g up and determining the i n i t i a l d i s t r i b u t i o n of animals on the g r a d i e n t . The method d e s c r i b e d i n t h i s study of using the c o n t r o l s to determine i n i t i a l d i s t r i b u t i o n s has been found a p p r o p r i a t e i n previous s t u d i e s (Sevacherian and Lowrie, 1972; Yinon and Shulov, 1970). The s e l e c t i o n then of areas i n experimental temperature g r a d i e n t s other than those p r e f e r r e d i n the c o n t r o l s i s taken to i n d i c a t e that t h i s s e l e c t i o n i s p r i m a r i l y a r e a c t i o n to the temperature i n the g r a d i e n t . In s t u d i e s on t e r r e s t r i a l i n s e c t s i n temperature g r a d i e n t s , r e l a t i v e humidity i n the d i f f e r e n t s e c t i o n s must be considered or responses on the g r a d i e n t cannot be c o n c l u s i v e l y r e l a t e d to temperature. Since g e r r i d s are found on the water s u r f a c e , and s i n c e the water s u r f a c e was used i n t h i s study, i t i s assumed that there i s l i t t l e or no humidity v a r i a t i o n . H u m i d i t i e s were assumed high i n every s e c t i o n of the g r a d i e n t . It should be noted that the g e r r i d s used i n t h i s study were overwintered a d u l t s a c c l i m a t e d to l a t e June temperatures. 69 Sevacherian and Lowrie (1972) noted that s p e c i e s of wolf s p i d e r of d i f f e r i n g sex and age p r e f e r r e d d i f f e r e n t temperatures. In both wolf s p i d e r s s t u d i e d , the immatures s e l e c t e d lower temperatures than did the males, which i n turn s e l e c t e d lower temperatures than d i d the females. Since only unsexed a d u l t g e r r i d s o f one age c l a s s were observed i n t h i s study, the temperature p r e f e r e n c e s suggested may not n e c e s s a r i l y be r e p r e s e n t a t i v e of a l l g e r r i d i n s t a r s and breeding s t a t e s . However, Sevacherian and Lowrie (1972) d i d observe s i g n i f i c a n t d i f f e r e n c e s between the two s p e c i e s i n a l l the age c l a s s e s s t u d i e d , which suggests that the same r e l a t i v e d i f f e r e n c e s observed here between the s p e c i e s may apply r e g a r d l e s s of i n s t a r and stage of development. That g e r r i d s i n s u n l i g h t had higher temperatures than those i n shade i s as expected. Body temperatures of c a . 5°C. above t h a t of the ambient a i r when i n the sun were a l s o r e p o r t e d by Edney (1953) f o r the woodlouse L i g i a oceanica. A f t e r d i s c u s s i n g the e f f e c t s of i n s o l a t i o n on i n s e c t s i n g e n e r a l , Edney concluded t h a t most i n s e c t s i n s u n l i g h t can be expected to undergo very c o n s i d e r a b l e and r a p i d i n c r e a s e s i n temperature, as owing t o t h e i r r e l a t i v e l y water impermeable integument, e v a p o r a t i v e water l o s s would be s l i g h t . The a c t u a l temperature a t t a i n e d by an i n s e c t depends on many f a c t o r s . Body c o l o r a t i o n , the i n t e n s i t y of r a d i a t i o n , the temperature and humidity of the a i r and i t s v e l o c i t y a l l determine temperature. That each g e r r i d s p e c i e s p r e f e r s a s p e c i f i c temperature range i s not unexpected. That the range p r e f e r r e d by each 70 s p e c i e s can be c o r r e l a t e d with the temperature that each s p e c i e s t y p i c a l l y encounters i n the f i e l d i s of g r e a t e r i n t e r e s t . S e c t i o n 1 shows t h a t the s p e c i e s have s l i g h t l y d i f f e r e n t m i c r o h a b i t a t s : G. r e m i g i s p r e f e r s streams; G. n o t a b i l i s , G. iH^urvatus and G. buenoi p r e f e r open water on l a k e s and pcnds; and G. i n c o g n i t u s p r e f e r s algae-covered water by the edge of ponds and on slow-moving streams. Each of these t h r e e types of m i c r o h a b i t a t has c e r t a i n temperature c h a r a c t e r i s t i c s . Water movement i n streams prevents thermal s t r a t i f i c a t i o n and as a r e s u l t , s u r f a c e water temperatures on streams are t y p i c a l l y c o o l e r than those on s t i l l water where thermal s t r a t i f i c a t i o n can occur. That G. r e m i g i s p r e f e r r e d the c o o l e s t temperatures of a l l the s p e c i e s would thus seem an a d a p t a t i o n to stream e x i s t e n c e . The open water areas on ponds and s m a l l l a k e s show thermal s t r a t i f i c a t i o n , with the s u r f a c e waters o f t e n becoming q u i t e warm. T h i s i s t y p i c a l l y most e v i d e n t i n the shallow areas near the water-edge and i n the areas s t u d i e d , the temperatures of s t i l l s u r f a c e water o f t e n approached 30°C. on a sunny day i n mid-summer. The t h r e e open water pond s p e c i e s p r e f e r temperatures i n t h i s range, i n agreement with the temperatures encountered i n t h e i r m i c r o h a b i t a t . F i n a l l y , the algae-covered water m i c r o h a b i t a t i s t y p i c a l l y found c l o s e to shore i n the shallows, and i n pools with only a few centimeters of water. Such h a b i t a t s , l i k e the t e r r e s t r i a l h a b i t a t , t y p i c a l l y experience a wide range of temperatures i n c l e a r weather, as u n l i k e l a r g e r bodies of water, there i s no 71 great volume of water to heat up during the day and c o o l o f f at n i g h t . Temperatures thus f l u c t u a t e g r e a t l y , making the temperature of t h i s m i c r o h a b i t a t the most v a r i a b l e cf a l l . That J § « iSS25J3liJi§ seems to t o l e r a t e the widest range of temperatures i s thus perhaps not s u r p r i s i n g . On the b a s i s of the r e s u l t s obtained h e r e i n and p r e v i o u s l y (see S e c t i o n 1), i t i s now p o s s i b l e to e s t a b l i s h some of the temperature zone c h a r a c t e r i s t i c s of the f i v e s p e c i e s f o l l o w i n g the scheme i n Imms (1937). T a b l e 9 presents these comparative data. I t i s obvious t h a t the s p e c i e s of G e r r i s s t u d i e d d i f f e r i n t h e i r responses to temperature. The responses are seen to r e l a t e c l o s e l y to the m i c r o h a b i t a t p r e f e r r e d by each s p e c i e s . The temperature preference d i f f e r e n c e s may be i n v o l v e d i n m i c r o h a b i t a t r e c o g n i t i o n and s e l e c t i o n by the g e r r i d s and may thus be important i n t h e i r a b i l i t y t c c o e x i s t . A p r e ference f o r c o o l water might e x p l a i n p a r t l y why G. r e m i g i s avoids ponds and l a k e s i n mid-summer, when the temperatures there are r e l a t i v e l y h i g h . T h i s i s supported by o b s e r v a t i o n s of G. r e n d g i s on Marion L. i n 1971, where i n e a r l y s p r i n g the s p e c i e s ranged over more of the l a k e . In mid-summer however, i t was c o n f i n e d to a c o l d s p r i n g upwelling area where the water temperature averaged ca. 20°C.: the g e r r i d s a p p a r e n t l y became c u t - o f f and i s o l a t e d there as the lake water warmed during the summer to around 25°C. The f a c t t h at G. r e m i g i s a l s o p r e f e r s moving water f u r t h e r serves to i s o l a t e t h i s s p e c i e s from the others that occur i n south-western B r i t i s h Columbia. 72 Table 9. The temperature zone c h a r a c t e r i s t i c s of the f i v e s p e c i e s of G e r r i s s t u d i e d . S p e c i e s H a b i t a t Zone of p r e f e r r e d Threshold temperature temperature , f o r growth ( ° C . ) (OC.) G. r e m i g i s Stream ' 16-22 G. n o t a b i l i s Open water ,25-36 G. i n c u r v a t u s Open water 25-31 5 - buenoi Open water 24-30 G. i n c o g n i t u s Inshore 14-29 12.4 10.3 12.6 12.9 9.3 i 73 Where the g e r r i d s p e c i e s i n h a b i t a t the same m i c r o h a b i t a t and the zone of p r e f e r r e d temperature i s the same, the t h r e s h o l d s of development would be expected to be somewhat d i f f e r e n t . T h i s was observed to be the case between G. n o t a b i l i s and G. buenoi, two s p e c i e s which p r e f e r the same temperature range and have o v e r l a p p i n g d i s t r i b u t i o n s i n the f i e l d s i t u a t i o n . T h i s may be important i n t h e i r a b i l i t y to c o e x i s t as i n other s t u d i e s such as those of Heath et a l (1971) and Sevacherian and Lowrie (1972), i t was found t h a t temperature responses d i f f e r e d between c l o s e l y r e l a t e d symaptric s p e c i e s . 74 SECTION 3. FOOD CONSUMPTION INTRODUCTION To date, q u a n t i t a t i v e s t u d i e s on the food consumption of predatory Hemiptera have been r e s t r i c t e d to a few s p e c i e s with the pentatomid Podisus m a c u l i v e n t r i s perhaps being most e x t e n s i v e l y s t u d i e d ( M o r r i s , 1963; Mukerji and LeEoux, 1969a, 1969b; G a l l o p i n and K i t c h i n g , 1972). Although Podisus i s an important predator of crop pests (Morris, 1963; LeRoux, 1964), no attempt has been made to i n c o r p o r a t e l a b o r a t o r y data i n t o the cont e x t of the r o l e Podisus p l a y s i n i t s h a b i t a t . Instead, the r e l a t i o n s h i p between food consumption and growth has been emphasized. S i m i l a r l y , prey consumption behaviour of the mantid ( H o l l i n g , (1966) and the predatory mite Imbljseius l a r q o e n s i s (Sandness and McMurtry, 1972) have been s t u d i e d , but to date no attempt has been made to i n c o r p o r a t e these data i n t o the context of how the animal i s adapted to i t s environment i n the f i e l d s i t u a t i o n . Indeed, owing to the complexity of t h e i r t e r r e s t r i a l h a b i t a t s , such a task i s extremely d i f f i c u l t . In an attempt to b e t t e r understand the i n t e r a c t i o n s between pre d a t o r s , and i n p a r t i c u l a r perhaps the s i g n i f i c a n c e of ca n n i b a l i s m i n i n s e c t p o p u l a t i o n r e g u l a t i o n , an experimental components a n a l y s i s type of approach ( H o l l i n g , 1966) has been adopted to i n v e s t i g a t e i n t e r a c t i o n s between s g e r r i d s p e c i e s i n 75 the f i e l d . An e s s e n t i a l s e t of parameters i n t h i s approach are the f e e d i n g c h a r a c t e r i s t i c s of the animal being i n v e s t i g a t e d . The r e s u l t s of t h i s s e c t i o n , t h e r e f o r e , besides d e s c r i b i n g f o r the f i r s t time i n a q u a n t i t a t i v e manner food consumption i n t h i s f a m i l y , are p r i m a r i l y intended as a c o n t r i b u t i o n to the model. MATERIAL AND METHODS 1. GENERAL METHODS The f i v e g e r r i d s p e c i e s d e s c r i b e d p r e v i o u s l y which occur together on Marion Lake i n the U n i v e r s i t y of B r i t i s h Columbia Research F o r e s t at Haney, B.C. were used i n t h i s study. Adults were determined using the key i n Scudder (1971), and l a r v a e were i d e n t i f i e d a c c o r d i n g to data i n Scudder and Jamiescn (1972). The v a r i o u s i n s t a r s of the f i v e s p e c i e s were e i t h e r r a i s e d i n the l a b o r a t o r y or obtained from the f i e l d . G e n e r a l l y , the f i r s t f o u r l a r v a l i n s t a r s were r a i s e d i n the l a b o r a t o r y . Owing tc the r a t h e r high m o r t a l i t y at the f o u r t h and f i f t h moults, i t was o f t e n necessary to c o l l e c t the f i f t h l a r v a l i n s t a r s and the a d u l t s i n the f i e l d . To some extent t h i s was an advantage i n th a t i t probably reduced any p o s s i b l e e f f e c t s of n u t r i t i o n a l d e f i c i e n c i e s on body s i z e and hence gut s i z e . When c o l l e c t e d i n the f i e l d , i f the temperatures d i f f e r e d from that i n the l a b o r a t o r y , a three day a c c l i m a t i o n p e r i o d was 76 provided before t e s t i n g commenced. I n s e c t s were held i n constant temperature c a b i n e t s or c o n t r o l l e d environment rooms at the r e g u i r e d temperature. Adult v e s t i g i a l winged Drosojghila melanogaster r a i s e d on, s t a n d a r d D r o s o p h i l a c u l t u r e media i n the l a b o r a t o r y were used throughout t h i s study both as an experimental fcod and a standard food. The experimental fcod c o n s i s t e d of f l i e s r a i s e d under r e l a t i v e l y uncrowded c o n d i t i o n s so as to s t a n d a r d i z e f l y s i z e as much as p o s s i b l e . S i z e v a r i a b i l i t y d i d e x i s t , as the sex of the f l i e s was not determined p r i o r to f e e d i n g and the male f l i e s were u s u a l l y n o t i c e a b l y s m a l l e r than the female. However, as i t was not the number of f l i e s but r a t h e r the t o t a l biomass eaten which was measured, these d i f f e r e n c e s were not considered important. In a l l f e e d i n g experiments, an excess of food was p r o v i d e d . T e s t i n g , unless otherwise noted, took place i n round p l a s t i c c o n t a i n e r s 9.5 cm i n diameter and 7 cm deep. However, f i f t h i n s t a r and a d u l t G. n o t a b i l i s and G. remigis , owing to t h e i r l a r g e s i z e , were t e s t e d i n c o n t a i n e r s 26 cm i n diameter and 9 cm deep. Regardless of c o n t a i n e r s i z e , water depth was 1 to 3 cm and the water s u r f a c e was kept c l e a n . Temperature, again unless otherwise noted, was maintained at 26°C. and r e l a t i v e humidity at 20% during both s t a r v a t i o n and t e s t i n g . The photoperiod schedule was 16 hr l i g h t : 8 hr dark throughout the study. G e r r i d f e e d i n g behaviour, once the animal has captured a food item, c o n s i s t s of p i e r c i n g the prey with i t s p r o b o s c i s , and 77 then sucking up the f l u i d s of the prey. S i n c e feeding u s u a l l y occurs on the water s u r f a c e , the p o s s i b i l i t y of water e n t r y i n t o the prey f o l l o w i n g t e r m i n a t i o n of feeding e x i s t s ; simply weighing the prey to determine biomass i n g e s t e d was not a c c e p t a b l e , In order to overcome t h i s , the f o l l o w i n g procedure was f o l l o w e d . I n i t i a l measurements on the experimental f l i e s showed that t h e i r water content was c o n s i s t e n t l y 74$ (S.E. = ±0.16), the dry weight of the f l i e s thus being 26%. T h i s dry weight f i g u r e t h e r e f o r e was assumed to apply to a l l f l i e s u t i l i z e d , and i s the t h e o r e t i c a l v a l u e c i t e d below. Thus, i n a l l s t u d i e s t h e r e a f t e r , the wet weight of CO z-asphyxiated f l i e s was measured, and these i n s e c t s were then f r o z e n u n t i l r e q u i r e d . F o l l o w i n g p r e s e n t a t i o n to the g e r r i d , the f l i e s were c o l l e c t e d and d r i e d (24 - 48 hours) i n an oven at 95°C. to a c o n s t a n t weight. A l l weighing was done on a M e t t l e r Grammatic Balance, Model 758 ( M e t t l e r Instrument Corp., Hightstown, N.J. ) a c c u r a t e to ± 0.002 mg. A f t e r d r y i n g , the f l i e s were reweighed, and any d i f f e r e n c e s between t h i s weight and 26% of the i n i t i a l wet weight of the f l i e s ( t h e o r e t i c a l dry weight) was c o n s i d e r e d to be the amount eaten. However, i t was found t h a t t h i s c o u l d only be a p p l i e d i f the f l i e s were removed immediately a f t e r f e e d i n g . I f the f l i e s were l e f t with the g e r r i d f o r more than a few minutes, e r r o r s a r o s e . These e r r o r s o c c u r r e d both as a r e s u l t of l o s s of body con t e n t s i n t o the water i n the c o n t a i n e r and perhaps as a r e s u l t 78 o f breakdown of body contents i n t o more v o l a t i l e compounds as a r e s u l t of decomposition p r o c e s s e s . These would then be l o s t on d r y i n g . S i n c e the experimental procedure o f t e n i n v o l v e d l e a v i n g f l i e s on the water s u r f a c e f o r some time, i t was important to o b t a i n a measure of p o s s i b l e e r r o r s over known p e r i o d s of time. Table 10 presents data r e l e v a n t t o the percentage l o s s e s over time. From these data modified t h e o r e t i c a l dry weights were c a l c u l a t e d and the d i f f e r e n c e between t h i s and the f i n a l dry weight was c o n s i d e r e d as the amount eaten. I t f o l l o w s then that the amount eaten was the l e s s i n dry weight of the f l i e s . Since i t i s i m p o s s i b l e to use t h i s measure to determine the maximum gut c a p a c i t y of the g e r r i d s , then some convers i o n i s r e g u i r e d . I f i t i s assumed that f u n c t i o n a l l y the f l u i d s of the i n g e s t e d f l y are homologous, then knowing the water content of the f l y , i t i s p o s s i b l e to use a c o r r e c t i o n c o e f f i c i e n t . For the r e l a t i v e values used i n t h i s study, such a c o e f f i c i e n t i s unnecessary and i s not used. Nevertheless, to permit c a l c u l a t i o n of maximum gut c a p a c i t y f o r comparison with other s t u d i e s , i f i t i s assumed a r b i t r a r i l y t h a t 25% of the water i n the f l y i s u n a v a i l a b l e to the g e r r i d , then (0.740 -0.185) = 55.5% of the t o t a l weight of the f l y i s i n g e s t a b l e water. Experiments i n which 20 starved g e r r i d s were each fed s e p a r a t e l y one weighed f l y i n d i c a t e d t hat 24.5% (S. E. = ±1.81) of the t h e o r e t i c a l dry weight of the f l y may not be u t i l i z e d by the g e r r i d . Thus, (0.26 - 0.064) = 19.6% of the dry weight of the f l y may be i n g e s t e d . The c o e f f i c i e n t t h e r e f o r e i s (19.6 • 79 Table 10. T h e o r e t i c a l dry weight c o e f f i c i e n t s of D r o s o p h i l a wet weights f o l l o w i n g v a r i o u s time periods with g e r r i d s . * = environmental c a b i n e t (no humidity c o n t r o l ) ; ** = environmental room (B.H. = 20%) ; C = the t h e o r e t i c a l dry weight c o e f f i c i e n t ( i t can be c a l c u l a t e d from the l i n e a r r e g r e s s i o n , C = 25.4 -0.11(temp.) f o r any temperature). No. Temp. Time Average % d i f f e r e n c e T h e o r e t i c a l f l i e s (°C.) p e r i o d water content from expected dry weight (hr) {%) 26% dry matter c o e f f i c i e n t 24 - - 74. 0 ± 0.16 0. 0.26 (groups of 4 f l i e s ) 0.24** 40 26 1 n 20 26 2 •i 20 26 3 II 40 26 24 » 20 10 24 II 20 20 24 II 20 32 24 II 0.215** C* 80 55.5)/19.6 = 3.83 ; 75.1% of the o r i g i n a l wet weight of the f l y i s thus p o t e n t i a l l y i n g e s t a b l e . 2. SATIATION TIME A. E f f e c t of g e r r i d s i z e To determine the e f f e c t of g e r r i d s i z e on the d u r a t i o n of f e e d i n g , immature and a d u l t G. r e m i g i s and G. i n c o g n i t u s taken one day a f t e r e c d y s i s were each s t a r v e d 48 h r . These s p e c i e s i were chosen s i n c e they r e p r e s e n t a l l the g e r r i d s i z e s found i n the study area. F o r t y specimens i n each i n s t a r were d i v i d e d i n t o f o u r groups of ten g e r r i d s . Each of these f o u r groups was allowed to feed f o r d i f f e r e n t p e r i o d s of time w i t h i n a f o u r hour i n t e r v a l . P r e l i m i n a r y o b s e r v a t i o n s i n d i c a t e d t h a t no matter how long a g e r r i d was deprived of food, s a t i a t i o n i n the f i r s t f e e d i n g c o u l d be achieved i n l e s s than four hours (see below). B. E f f e c t of s t a r v a t i o n S i m i l a r t e s t s on f i r s t i n s t a r , t h i r d i n s t a r and male G. r e m i g i s and on male G. i n c g g n i t u s were undertaken to a s c e r t a i n the e f f e c t of l e n g t h of food d e p r i v a t i o n time on the d u r a t i o n of the f e e d i n g p e r i o d . Three s i z e s of G. r e m i g i s were s t u d i e d so as to determine the e f f e c t of s i z e w i t h i n a s p e c i e s . The p r e v i o u s t e s t would not be a p p l i c a b l e i f , f o r example, d i g e s t i v e 81 r a t e s v a r i e d with g e r r i d s i z e . F i g u r e 18 i l l u s t r a t e s the c r i t e r i o n f o r s a t i a t i o n of a t h i r d i n s t a r G. r e m i g i s s t a r v e d f o r 20 hr. In t h i s example, 80 g e r r i d s d i v i d e d i n t o e i g h t groups of ten were used to demonstrate the e f f e c t of f e e d i n g on amount eaten. In t h i s case, s a t i a t i o n o c c u r r e d a f t e r 2 hr of f e e d i n g , the nearest p e r i o d to when the amount eaten l e v e l e d o f f . The high value a f t e r 4 hr of f e e d i n g i s thought to r e p r e s e n t another fe e d i n g bout. In most t e s t s , four time periods were s u f f i c i e n t to e s t a b l i s h t h i s curve f o r an i n s t a r a t any l e v e l of s t a r v a t i o n . 3. DIGESTIVE PAUSE Four male and s i x female a d u l t G. r e m i g i s were taken i n d i v i d u a l l y and s t a r v e d f o r 48 hr. They were then observed c o n t i n u o u s l y f o r 270 minutes i n the presence of an excess number of l i v e D r o s o p h i l a . At each minute i n t e r v a l , i t was recorded whether the g e r r i d had fed (impaled a D r o s o p h i l a on i t s p r o b o s c i s f o r at l e a s t 20 seconds) or simply k i l l e d the f l y . No d i f f i c u l t y was encountered o p e r a t i o n a l l y i n d e c i d i n g whether f e e d i n g d i d indeed occur, as u s u a l l y , f e e d i n g on any p a r t i c u l a r f l y was of s e v e r a l minutes 1 d u r a t i o n . Figure 18. The e f f e c t of feeding time on the amount eaten by a t h i r d i n s t a r G. remigis starved for 20 hours. 750 H O U R S F E D 83 4. FOOD DEPRIVATION The e f f e c t of length of food d e p r i v a t i o n p e r i o d cn the amount eaten was determined f o r f i r s t i n s t a r and t h i r d i n s t a r G. r e m i g i s , and f o r the males of a l l f i v e s p e c i e s . Three s i z e s of I S J i S i J ? were s t u d i e d sc as to determine the e f f e c t of s i z e within a s p e c i e s and the a d u l t s of a l l the s p e c i e s were s t u d i e d to determine i f s p e c i e s - c h a r a c t e r i s t i c d i f f e r e n c e s e x i s t e d . Since t h i s study was done i n the spring and e a r l y summer, only males were used, as i t was f e l t that oocyte maturation and egg l a y i n g by the female might r e s u l t i n feeding v a r i a b i l i t y during the course of the study. I n d i v i d u a l male g e r r i d s were not used f o r longer than three weeks, as i t was f e l t that at the non-f l u c t u a t i n g and r e l a t i v e l y high temperature under which the g e r r i d s were maintained, aging might begin to i n f l u e n c e f e e d i n g behaviour: i n the study area, these g e r r i d s normally l i v e d u n t i l mid-July. S t a r v a t i o n i n the f i r s t and t h i r d i n s t a r s commenced one day f o l l o w i n g e c l o s i o n or e c d y s i s , and s i n c e the mean stadium length fo r the f i r s t and t h i r d i n s t a r s was approximately f i v e days, depending on the food d e p r i v a t i o n p e r i o d , the same g e r r i d could be used i n two separate t e s t s . The male g e r r i d s were used r e p e a t e d l y , but at l e a s t one complete day i n the presence of excess food separated t e s t s . Feeding p e r i o d s (time i n the presence of food) were determined on the t a s i s of the s t a r v a t i o n t e s t s (see above). Food d e p r i v a t i o n i n t e r v a l s used were: 1, 2, 4, 6, 8, 12, 16, 20, 24, 32, 40, 48 and 72 hours. 84 5. MAXIMUM GUT CAPACITY On t h e b a s i s o f t h e ab o v e s t u d i e s w h i c h d e t e r m i n e d t h e t i m e r e q u i r e d t o s a t i a t e g e r r i d s d e p e n d i n g on hody s i z e and l e n g t h o f f o o d d e p r i v a t i o n p e r i o d , t h e amount c f f o o d t h a t g e r r i d s would v o l u n t a r i l y e a t f o l l o w i n g maximum s t a r v a t i o n was m e a s u r e d . The gut c a p a c i t y o f e a c h i n s t a r o f e a c h of t h e f i v e s p e c i e s was m e a s u r e d , w i t h a s a m p l e s i z e o f n o t l e s s t h a n 18 f o r e a c h s p e c i e s - i n s t a r . A f o o d d e p r i v a t i o n i n t e r v a l o f 24 hr f o r l a r v a e and 48 h r f o r a d u l t s was s u f f i c i e n t t o a t t a i n maximum amounts i n g e s t e d . Owing t c t h e m o r p h o l o g y and b e h a v i o u r o f g e r r i d s , p r o c e d u r e s t o d e t e r m i n e s a t i a t i o n s u c h as t o u c h i n g t h e m o u t h p a r t s w i t h j u i c e s o f t h e p r e y and t h e n n o t i n g t h e r e s p o n s e ( H o l l i n g , 1966) were f o u n d i m p r a c t i c a l . They were a l s c n o t f e l t t o s i m u l a t e r e a l i s t i c a l l y t h e c o n d i t i o n s a g e r r i d w c u l d e n c o u n t e r i n t h e f i e l d . L a r v a e were d e p r i v e d o f f o o d commencing one day f o l l o w i n g e c l o s i o n o r e c d y s i s , w h e r e a s a d u l t s ( h a v i n g m o u l t e d t h e p r e v i o u s F a l l ) were u s e d as t h e y became a v a i l a b l e . 6. EFFECT OF TEMPERATURE ON FEEDING The amount o f f c o d i n g e s t e d p e r day by t e n male g e r r i d s o f e a c h o f t h e f i v e s p e c i e s u n d e r d i f f e r e n t t e m p e r a t u r e r e g i m e s was m e a s u r e d . Each g e r r i d was m a i n t a i n e d i n a s e p a r a t e c o n t a i n e r and was f e d o n c e d a i l y . T e s t i n g t o o k p l a c e i n e n v i r o n m e n t a l c a b i n e t s w i t h o u t h u m i d i t y c o n t r o l . T h r e e d a y s o f a c c l i m a t i o n f o l l o w e d by two c o n s e c u t i v e d a y s c f t e s t i n g were p e r m i t t e d a t e a c h t e m p e r a t u r e , w i t h t h e t e m p e r a t u r e b e i n g c h a n g e d ( i n c r e a s e d ) 85 t o the next temperature immediately f o l l o w i n g the second day of t e s t i n g . T e s t i n g was undertaken at 3°C. i n t e r v a l s between 5°C. and 3 2 ° C , but as two environmental c a b i n e t s were a v a i l a b l e , the temperatures i n each were staggered so that the temperature i n c r e a s e a t one time f o r any group of i n s e c t s was 6°C. G. re m i g i s and G. i n c g g n i t u s were t e s t e d f i r s t s t a r t i n g a t 5°C., as they emerge e a r l i e s t from o v e r w i n t e r i n g . Since the mean temperature seldom i s below 8°C. i n our study area when the other t h r e e s p e c i e s emerge, t e s t i n g of those s p e c i e s commenced at 8°C. Regardless of s p e c i e s , no i n d i v i d u a l g e r r i d was t e s t e d a t more than three temperature regimes so as to minimize aging e f f e c t s which might i n t r o d u c e v a r i a b i l i t y , e s p e c i a l l y at the highe r temperatures. Thus, a complete change of animals occurred f o r the experimental run at 17°C. or 20°C. 7. VARIATION OF FEEDING DURING DEVELOPMENT The amount of food i n g e s t e d per day i n l a b o r a t o r y reared g e r r i d s was determined. Laboratory reared g e r r i d s were used throughout so as to s t a n d a r d i z e f e e d i n g as much as p o s s i b l e . The f i v e l a r v a l i n s t a r s and a d u l t of G. n o t a b i l i s were s t u d i e d . Feeding was once d a i l y and r e g a r d l e s s of g e r r i d s i z e , the g e r r i d s were maintained i n the s m a l l e r p l a s t i c c o n t a i n e r s . Owing to the r e l a t i v e l y h i g h . m o r t a l i t y i n the l a t e r moults and the d u r a t i o n of the experiment (53 days), t h i s experiment i n v o l v e d a l a r g e number of g e r r i d s . The amount eaten f o r each day i s the mean o f 10 o b s e r v a t i o n s . In the a d u l t , f e e d i n g s were 86 continued for 25 days following the f i n a l moult. RESULTS 1. SATIATION TIME The effect of gerrid size on the duration of the feeding period following starvation for 48 hr i s shown in Fig. 19b. Since the lengths of the feeding periods were measured at half hour i n t e r v a l s , and the time at which s a t i a t i o n occurred was rounded off to the nearest half hour, the data in Fig. 19a represent the approximate values at which s a t i a t i o n i s achieved. Nevertheless, i t appears that for gerrids above about 10 mg wet weight, feeding time reguired to achieve s a t i a t i o n was between 2 and 2.5 hr. For gerrids below t h i s weight, feeding time declined at a rate adeguately described by a logarithmic function, with one hour reguired by the smallest gerrids tested. Figure 19a shows the e f f e c t of food deprivation period on the duration of feeding period. Feeding periods of less than one hour duration were measured at guarter hour i n t e r v a l s , whereas those of longer duration were measured at half hour i n t e r v a l s . Logarithmic functions again seem to adeguately describe the data. No noticeable difference in " b e s t - f i t " function existed between the regression for t h i r d instar G. remigis and the adult gerrids. Third instar gerrids, which have 87 Fi g u r e 19. The e f f e c t of (A) s t a r v a t i o n and (B) g e r r i d s i z e on the maximum d u r a t i o n of the f e e d i n g p e r i o d . A. A - f i r s t i n s t a r l a r v a G. r e m i g i s ; • = t h i r d i n s t a r l a r v a G. r e m i g i s ; ° = a d u l t male G. r e m i g i s and a d u l t male G. i n c o g n i t u s . E. S t a r v a t i o n was f o r 24 hr (larva) and 48 hr ( a d u l t ) . • = G. o g n i t u s ; «a = G. r e m i g i s . A WET WEIGHT (MG.) 88 a wet weight of 3.5 mg, respond to d e p r i v a t i o n p e r i o d s i n a manner very s i m i l a r to t h a t of a d u l t male G. remicjis, which weigh 38.6 mg. A r e g r e s s i o n based on the pooled data can thus be used to d e s c r i b e the f e e d i n g c h a r a c t e r i s t i c s . The r e s u l t i s b a s i c a l l y i n agreement with t h a t observed i n F i g . 19b, where i t i s only with the f i r s t and second i n s t a r l a r v a e that a s i g n i f i c a n t d i f f e r e n c e i n f e e d i n g behaviour appears to e x i s t . 2. DIGESTIVE PAD SE In order to decide whether or not a s i g n i f i c a n t amount of f e e d i n g d i d indeed occur, f i v e c o n s e c u t i v e minutes of e i t h e r f e e d i n g or not f e e d i n g were used a r b i t r a r i l y to d e l i n e a t e f e e d i n g bouts. F i g . 20 presents the recorded o b s e r v a t i o n s of G. £Sffli£iS and Table 11 the l e n g t h of c o n s e c u t i v e f e e d i n g and non-f e e d i n g p e r i o d s . The i n i t i a l f e e d i n g bout l a s t e d on the average 147.4 minutes: t h i s compares f a v o u r a b l y with 2.5 hr p r e d i c t e d on the b a s i s of the t o t a l amount i n g e s t e d ( F i g . 19b). I t thus appears t h a t , although the r a t e of i n g e s t i o n may vary, continued i n g e s t i o n occurs as long as the f l y i s impaled. Feeding bouts f o l l o w i n g t h i s i n i t i a l lengthy one were t y p i c a l l y of s h o r t d u r a t i o n o n l y , v a r y i n g from 5 to 16.5 minutes, with a mean of 10.4 minutes. However, the p e r i o d s between f e e d i n g bouts ( d i g e s t i v e pauses) v a r i e d i n l e n g t h from 9 to 51.5 minutes. Whether or not t h i s i s a consequence of the experimental design, with the g e r r i d s being completely s t a r v e d before the study 89 F i g u r e 20. The p a t t e r n of f e e d i n g a c t i v i t y over 4.5 hr as observed i n ten i n d i v i d u a l G. r e m i g i s f o l l o w i n g s t a r v a t i o n f o r 48 hr. Each v e r t i c a l bar i n d i c a t e s t h a t f e e d i n g occurred f o r at l e a s t 20 seconds i n t h a t minute p e r i o d (the t h i c k bars re p r e s e n t two, a d j a c e n t one-minute p e r i o d s ; owing to the s c a l e , the bars may appear j o i n e d ) . m = male; f = female. 90 T a b l e 11. Mean lengths i n minutes of c o n s e c u t i v e f e e d i n g and non-feeding p e r i o d s ( f o l l o w i n g 48 hr s t a r v a t i o n ) of G. r e m i g i s i n the presence of an excess amount of food. + = time elasped i n p e r i o d when o b s e r v a t i o n s were terminated. These values were not used i n the c a l c u l a t i o n of the mean. G e r r i d no. Consecutive f e e d i n g bouts 1 2 3 4 5 6 7 1 39 7 7 2 92 11 21 18 8 6 3 148 6 6 4 237 5 235 6 270+ 7 144 7 20 8 92 6 6 15 6 13 5 9 249 9 10 91 10 21 Mean 147.4 8.0 13.5 16. 5 7.0 9.5 5.0 Consecutive non- f e e d i n g bouts 1 2 3 4 5 6 7 1 70 7 140 + 2 7 20 53 19 14 1 + 3 50 15 45 + 4 33 + 5 35+ 6 — 7 16 8 75 + 8 9 10 48 22 13 7 18 + 9 6 6 + 10 71 27 50 + - — _ — — mmm — — ^ — , I I I I Mean 32.7 14.5 51.5 20.5 13.5 7.0 91 commenced, i s u n c e r t a i n . Nevertheless, under these c o n d i t i o n s , which one might expect would maximize i n g e s t i o n , the average d i g e s t i v e pause was 24.6 minutes. I t should be noted that i n the experiments the g e r r i d s seldom " a t e " one f l y completely, but r a t h e r every few minutes or so, s h i f t e d to a new f l y . In c o n t r a s t , when 20 s t a r v e d g e r r i d s were presented with only one f l y each, the mean time cf f e e d i n g from the f l y was 14.8 minutes (S.E.= ±1.54 minutes). as a r e s u l t , the t o t a l number of f l i e s k i l l e d here was f a r i n excess of t h a t simply r e q u i r e d to s a t i a t e the g e r r i d . However, f i e l d o b s e r v a t i o n s suggest t h a t p o t e n t i a l prey items i n the wild are not l i k e l y ever to be as numerous and as r e a d i l y a v a i l a b l e as the v e s t i g i a l - w i n g e d D r o s o p h i l a were i n t h i s study. Under normal c o n d i t i o n s , g e r r i d u t i l i z a t i o n of each prey would probably be r e l a t i v e l y complete, i e . u n t i l e i t h e r s a t i a t i o n of the g e r r i d was reached or u n t i l the prey was sucked dry. 3. FOOD DEPRIVATION The e f f e c t of the d u r a t i o n of food d e p r i v a t i o n (TF) on the weight of food r e g u i r e d to s a t i a t e (or hunger) (H) f o r the g e r r i d s t e s t e d i s shown i n F i g . 21. Table 12 summarizes the c o n s t a n t s f o r the " b e s t - f i t " r e g r e s s i o n which f i t s the equation H = HK - BYe<W0 CTT) > where HK = the maximum amount of food the gut can hold, BY = HK 92 F i g u r e 21. The e f f e c t o f s t a r v a t i o n on the amount eaten by the f o l l o w i n g g e r r i d s ; (a) f i r s t i n s t a r G. r e m i g i s ; (b) t h i r d i n s t a r G. r e m i g i s ; (c) a d u l t male G. r e m i g i s ; (d) a d u l t male G. iU£23i } i i J S; (e) a d u l t male G. n o t a b i l i s ; (f) a d u l t male G. i n c u r v a t u s ; and (g) a d u l t male G. buenoi. AD i s the value of the s l o p e , which f u n c t i o n a l l y r e p r e s e n t s the r a t e of d i g e s t i o n i n the eguation H = HK - (BY) et^trfyy which was used to d e s c r i b e the data. The middle r e g r e s s i o n r e p r e s e n t s the " b e s t - f i t " , and the other two p l u s or minus one standard e r r o r of HK and AD. 2000 r M A L E ' GERRIS REMIGIS 10 20 30 40 50 60 HOURS OF FOOD DEPRIVATION 70 80 D 750 M A L E GERRIS INCOGNITUS 20 30 40 50 60 HOURS OF FOOD DEPRIVATION 70 80 93 Table 12. Values of con s t a n t s i n the equation, H = HK -(BY) eX^° <ff) t used to d e s c r i b e the r a t e of d i g e s t i o n i n G e r r i s , and the maximum gut c a p a c i t i e s as determined by the r e g r e s s i o n s with 24 hr (larva) and 48 hr (adult) of food d e p r i v a t i o n (mean ± SE). G e r r i d HK BY AD Maximum gut (ug) (ug) c a p a c i t y (ug) Obser. I t e r . G. r e m i g i s 80.5 ± 3.8 75.0 0.170 ± 0.040 89 ± 5 79 ( f i r s t i n s t a r l a r v a ) G. r e m i g i s 471.6 ± 29.4 429.3 0.194 ± 0.060 456 ± 25 468 ( t h i r d i n s t a r l a r v a ) G. r e m i g i s 1272.6 ± 48.8 959.3 0.063 ± 0.013 1335 ± 110 1226 (ad u l t male) G. i n c o g n i t u s 732.1 ± 14.9 579.5 0.029 ± 0.003 598 ± 20 588 (adult male) G. buenoi 297.5 ± 10.6 290.8 0.054 ± 0.007 315 ± 42 276 (adult male) G. i n c o g n i t u s 440.2 ± 14.9 431.0 0.043 ± 0.006 422 ± 45 386 (adult male) G. n o t a b i l i s 1892 .5 ± 48.5 1827.2 0.047 ± 0.005 1823 ± 63 1701 (adult male) 94 minus the Y-axis i n t e r c e p t , and AD = a c o n s t a n t , the r a t e of food disappearance. The c e n t r e r e g r e s s i o n i n each graph r e p r e s e n t s the " b e s t - f i t " r e g r e s s i o n , with those on e i t h e r s i d e r e p r e s e n t i n g p l u s and minus one standard e r r o r of HK and AC. As the time of food d e p r i v a t i o n i n c r e a s e d , hunger, cr the amount of food r e g u i r e d to s a t i a t e , a l s o i n c r e a s e d , but as i n the mantid ( H o l l i n g , 1966) , a t a p r o g r e s s i v e l y decreasing r a t e . Thus, the same r e g r e s s i o n as used by H o l l i n g , which assumes t h a t the r a t e at which hunger changes with time i s d i r e c t l y p r o p o r t i o n a l to the amount of food i n the gut, was used to d e s c r i b e the data. The r a t e of change of hunger would be g r e a t e r when the gut was very f u l l than when the gut was nea r l y empty. As i t was f e l t t h a t not enough values were l o c a t e d on the pla t e a u to allow averaging of these values f o r the de t e r m i n a t i o n of maximum gut c a p a c i t y , t h i s l a t t e r value was determined i t e r a t i v e l y with a d e p r i v a t i o n time of 24 hr f o r the l a r v a l i n s t a r s and 48 hr f o r the a d u l t s . These d e p r i v a t i o n times were chosen as i t was f e l t t h a t they provided reasonably r e a l i s t i c v a l u e s ( F i g . 21). I n f i v e of the seven r e g r e s s i o n s , the i t e r a t i v e values are wit h i n one standard e r r o r of the observed values and i n the other two r e g r e s s i o n s the i t e r a t i v e values are w i t h i n two standard e r r o r s . I t w i l l be noted i n F i g . 21 that the curves do net gc through the o r i g i n but i n s t e a d , i n t e r c e p t the Y~axis at some poi n t above i t . The values of t h i s i n t e r c e p t , the d i f f e r e n c e between HK and BY i n Tab l e 12, are f e l t to be r e a l and a r e s u l t of the experimental d e s i g n . H o l l i n g (1966) , i n determining the hunger curve of the mantid, used as h i s c r i t e r i o n c f s a t i a t i o n 95 t h r e e c o n s e c u t i v e r e f u s a l s to eat crushed f l i e s t h a t were touched to the mantid's mouthparts. Besides hardly being a measure of v o l u n t a r y f e e d i n g i n an animal, such a procedure i s i m p r a c t i c a l with G e r r i s , as the p r o b o s c i s i s held c l o s e to the body beneath the animal when i t i s not f e e d i n g . The only c r i t e r i o n f o r s a t i a t i o n used i n t h i s study was a lack of s i g n i f i c a n t i n g e s t i o n over a h a l f hour time peri o d while i n the presence of excess food. S a t i a t i o n i n g e r r i d s preceeding food d e p r i v a t i o n was achieved by p l a c i n g them i n a h o l d i n g c o n t a i n e r with excess food f o r a time p e r i o d known to allow s a t i a t i o n to be achieved (see above). As the " s a t i a t e d " g e r r i d s used here were not chosen f o r t e s t i n g immediately a f t e r they had fed i n the holding c o n t a i n e r , i t was g u i t e p o s s i b l e t h a t v o l u n t a r i l y , the g e r r i d had not eaten f o r a while and was i n a d i g e s t i v e pause f o l l o w i n g i t s previous f e e d i n g . The no v e l t y of being placed i n a new c o n t a i n e r coupled with the dropping i n of new food could e a s i l y be enough sti m u l u s to e l i c i t an immediate a t t a c k . Hence, f e e d i n g would occur even i n " s a t i a t e d " g e r r i d s , and the r e g r e s s i o n thus would not pass through the o r i g i n . No s i g n i f i c a n t d i f f e r e n c e i n r a t e of d i g e s t i o n (the AE val u e , or s l o p e , i n the equation) i s evident between the a d u l t s of the f i v e s p e c i e s , with the p r o b a b i l i t y of a common s l o p e being 69%. S i m i l a r l y , f o r the d i g e s t i v e r a t e s o f the two i n s t a r s s t u d i e d , no s i g n i f i c a n t d i f f e r e n c e i s evident and the p r o b a b i l i t y of a common s l o p e i s 67% ( f o r sample s i z e s , see F i g . 21) . 96 4. MAXIMUM GUT CAPACITY Fi g 22, a and b, d e s c r i b e the r e l a t i o n between stomach c a p a c i t y and wet weight when on a d a i l y s a t i a t i o n r a t i o n o f D r o s p p h i l a . I f a l l weights of the g e r r i d s (expressed as weight r e l a t i v e to the weight o f the female o f t h e i r species) are p l o t t e d , the maximum stomach c a p a c i t y i s seen as an e x p o n e n t i a l f u n c t i o n of the body weight, with a maximum gut c a p a c i t y of 23.6$ of the wet weight one day a f t e r moult f o r an average s i z e f i r s t i n s t a r . I t w i l l be n o t i c e d i n F i g . 22 that v a r i a b i l i t y i n the amount of food i n a • • f u l l " stomach i s much grea t e r with the s m a l l e r than the l a r g e r g e r r i d s . I t should be noted again, however, t h a t the value o f maximum gut c a p a c i t y o n l y r e f l e c t s the dry weight i n g e s t e d and excludes the f l u i d s . Thus, f o r comparison with other s t u d i e s , i f i t i s assumed that a r b i t r a r i l y 75% of the prey's f l u i d are in g e s t e d , then the maximum gut c a p a c i t y f o r an average s i z e f i r s t i n s t a r i s 90.25? of the wet weight one day a f t e r e c d y s i s . F i g . 23 r e l a t e s the amount eaten on a d a i l y b a s i s at 26°C. to maximum gut c a p a c i t y f o r both l a r v a l i n s t a r s and a d u l t g e r r i d s . T r e a t i n g the l a r v a e and a d u l t s s e p a r a t e l y , i t i s shown that the r e l a t i v e d i f f e r e n c e s between the amount consumed on a d a i l y b a s i s and the maximum amount consumed at a s i n g l e f e e d i n g are much g r e a t e r f o r the l a r v a e than f o r the a d u l t s . However, perhaps owing to the l a r g e standard e r r o r and s m a l l sample s i z e f o r the s m a l l e r g e r r i d s , the r a t i o of i t e r a t i v e v a l u e s f o r the d a i l y amount eaten over the maximum consumed a t a s i n g l e f e e d i n g f o r the s m a l l e s t i n s t a r s (1.20) i s s m a l l e r than t h a t f o r the 97 F i g u r e 22. The r e l a t i o n between maximum stomach c a p a c i t y and g e r r i d s i z e . A. The r e l a t i o n between maximum stomach c a p a c i t y , i e . stomach co n t e n t s expressed as a percent of wet body weight, of each of the i n s t a r s of the f i v e s p e c i e s and t h e i r wet weight, expressed as r e l a t i v e to the wet weight of the female of t h e i r s p e c i e s . • = l a r v a or a d u l t . 8. The r e l a t i o n between maximum stomach c a p a c i t y (defined as above) of each of the i n s t a r s of the f i v e s p e c i e s and t h e i r wet weight. D i f f e r e n t r e g r e s s i o n s d e s c r i b e t h i s r e l a t i o n s h i p depending on whether the g e r r i d i s a l a r v a or a d u l t . • = l a r v a ; A = a d u l t . (It should be noted that i n both (A) and (B), stomach c a p a c i t y r e f e r s only to the dry weight of food i n g e s t e d ; t o c a l c u l a t e t o t a l stomach c a p a c i t y f o r comparison with other s t u d i e s , i f i t i s assumed t h a t 75% of the prey's f l u i d s are a l s o i n g e s t e d , m u l t i p l y the values o f the o r d i n a t e by 3.83.) 98 F i g u r e 23. The r e l a t i o n between maximum amount consumed at a s i n g l e f e e d i n g (S), expressed as a percent of wet body weight, to the average amount consumed per day (D) f o r l a r v a l G. U S i a b i l i s and the males of a l l f i v e s p e c i e s . For c a l c u l a t i o n of the a c t u a l volumes consumed f o r comparison with other s t u d i e s (amount consumed here only r e f e r s to the dry weight of food i n g e s t e d , r e f e r to the legend of F i g . 6 ) . • = l a r v a ; <4 = a d u l t . 98a WET WEIGHT (MG.) 99 s m a l l a d u l t s (1.63). The b a s i c d i f f e r e n c e between the l a r v a e and a d u l t s i s i n agreement with the f a s t e r d i g e s t i o n observed i n l a r v a l g e r r i d s over a d u l t g e r r i d s . Thus, an enhanced a b i l i t y to consume more food i s shown f o r the s m a l l e r g e r r i d s over the l a r g e r g e r r i d s , not o n l y at a s i n g l e feeding, but to a g r e a t e r extent on a d a i l y b a s i s as w e l l . Owing to the o v e r l a p i n s p e c i e s s i z e (the f i f t h i n s t a r s of £§Si3i.§ an<* G. n o t a b i l i s both being l a r g e r than the a d u l t s of the three s m a l l e r s p e c i e s (G. buenoi, G. i n c o g n i t u s , G. i n c u r v a t u s ) ) , two r e g r e s s i o n s were r e q u i r e d ( F i g . 22b) to d e s c r i b e the maximum stomach c a p a c i t y on the b a s i s of body weight i r r e s p e c t i v e of s p e c i e s . Both f i f t h i n s t a r s possessed stomach c a p a c i t i e s , as a percent of body weight, l a r g e r than the a d u l t g e r r i d s , and t h i s i s thought to be a r e a l phenomenon. An e x p o n e n t i a l f u n c t i o n best d e s c r i b e s stomach c a p a c i t y among the l a r v a l i n s t a r s , whereas the r e l a t i o n s h i p among the a d u l t s i s l i n e a r. 5. EFFECT OF TEMPERATU BE ON FEEDING The e f f e c t s of temperature on the amount eaten i n G e r r i s i s shown i n F i g . 24. The g e n e r a l t r e n d i n those s p e c i e s s t u d i e d , excepting G. r e m i g i s , i s f o r a g r e a t e r amount to be eaten a t higher temperatures than at lower temperatures. In G. r e m i g i s , however, the amount eaten i n c r e a s e s with i n c r e a s i n g temperature up to ca. 19°C., beyond which the amount eaten decreases with i n c r e a s i n g temperature. In a l l s p e c i e s , the r a t e of change of 100 Figure 24. The e f f e c t of temperature on the amount consumed by males of each of the f i v e species of Gerris. (a) males of each of the f i v e species of G e r r i s . (A). N = G. n o t a b i l i s ; E = G. remigis. (B) . B = G. buenoi; IG - G. incognitus; IV = G. ipguryatus. =3. U J 3 O 3000 2400 1800 1200 < 600 0 N y=-85-20 + 95-22 X R y= 44-60 +70-76 X (7-19) R y= 1916- - 27-38X (19-30) 14 21 TEMPERATURE (°C> L. 28 i 35 B 1500 IV Y = 302-7 • 26-63X B Y = 207-4 + 28-03X IG Y= 32-3 2 7-29X 1200 CD U J ZD O 900 600 < 300 • IV 14 21 TEMPERATURE (°C) 28 L 35 101 amount eaten can be adequately d e s c r i b e d by means of l i n e a r r e g r e s s i o n s , although with G. r e m i g i s , two r e g r e s s i o n s , one f o r above and one f o r below 19°C., are r e g u i r e d . i 6.' VARIATION OF FEEDING DURING DEVELOPMENT Fig u r e 25b d e s c r i b e s the r e l a t i v e amounts i n g e s t e d per day throughout the l i f e c y c l e of G. n g t a b i l i s . Each stadium i s c h a r a c t e r i z e d by an i n i t i a l r a p i d i n c r e a s e i n the amount of food ingested over the amount eaten i n the pre v i o u s stadium, f o l l o w e d by a gradual decrease i n amount eaten. The amount eaten peaks approximately 403 of the way through the stadium i r r e s p e c t i v e o f the d u r a t i o n of the stadium i f a l l the values i n each of the s t a d i a are c o n s i d e r e d together ( F i g . 25a). In F i g . 25a the r e l a t i v e p o s i t i o n i n the stadium c f the r e l a t i v e amount eaten, expressed as a percent of the maximum amount eaten i n the stadium, i s p l o t t e d f o r a l l the l a r v a e and the a d u l t s . Thus, i f the stadium i s of two da y s 1 d u r a t i o n , the r e l a t i v e amount eaten on the l a s t day of the previous stadium would be at 0, the r e l a t i v e amount eaten on day 1 would be at 33, the r e l a t i v e amount eaten on day 2 would be a t 67, and the r e l a t i v e amount eaten on day 1 of the next stadium would be at 100. The l a s t day of the p r e v i o u s stadium and the f i r s t day of the f o l l o w i n g are i n c l u d e d s i n c e e c d y s i s occurred somewhere between these two peri o d s and the f i r s t and l a s t day of the stadium being c o n s i d e r e d r e s p e c t i v e l y . Assuming the period of development i n the a d u l t f o l l o w i n g the f i n a l e c d y s i s to be s i m i l a r to that of 102 F i g u r e 25. The e f f e c t of developmental s t a t e on food consumption, (A). The r e l a t i v e p o s i t i o n i n the stadium of the r e l a t i v e d a i l y amount eaten, expressed as a percent of the maximum amount eaten i n the stadium, f o r each day i n the s i x s t a d i a o f G. n o t a b i l i s . (B). The observed amounts eaten per day by the f i v e l a r v a e and the a d u l t of G. n o t a b i l i s during the course of t h e i r development. The p l o t t e d r e g r e s s i o n s f o r each i n s t a r are the two " b e s t - f i t " r e g r e s s i o n s d e r i v e d on the b a s i s of the data i n (A). 102a 100 r ZD zz. — UJ X h- < < S UJ I— s 3 Q O < 2 *-< CO < 2 _ j LU LU O cr cr: LU 20 Y=32-3 + 1-5X Y= 102 -4 -0 -25X 0 20 40 60 80 R E L A T I V E POSITION IN STADIUM 100 103 the immature stages, on the b a s i s of when amount eaten peaked, the d u r a t i o n of t h i s developmental p e r i o d was c a l c u l a t e d t c be i n the v i c i n i t y of 8 days. Only the f i r s t 8 values of the a d u l t "stadium" were thus i n c o r p o r a t e d i n t o F i g . 25a. The i t e r a t i v e v alues f o r amounts eaten throughout the l i f e c y c l e of G. n o t a b i l i g were c a l c u l a t e d on the b a s i s of l i n e a r r e g r e s s i o n s d e r i v e d from t h i s f i g u r e which i n v o l v e the s t a d i a d u r a t i o n s and the maximum d a i l y amounts eaten f o r each stadium. These values are d e s c r i b e d i n F i g . 25b. In most i n s t a n c e s , the d i f f e r e n c e from a c t u a l amount eaten i s within one standard e r r o r , which suggests that t h i s technigue of determining the amount eaten i s reasonably r e a l i s t i c . DISCUSSION In a l l s t u d i e s of f e e d i n g , the data must be considered i n the context of the value of the prey as a food item. Numerous s t u d i e s (eg. Hsiao and F r a e n k e l , 1968; Latheef and H a r c c u r t , 1972) have shown t h a t animals have fcod p r e f e r e n c e s , and so i f a p r e f e r r e d prey item i s not chosen by the experimenter, one can expect the t e s t animal's behaviour to d i f f e r from t h a t where the p r e f e r r e d food i s a v a i l a b l e . Food q u a l i t y may not immediately be obvious on the b a s i s of behaviour. Thus, f o r example, the chrysomelid L e p t i n o t a r s a decernlineata reared on tomato, fed longer and consumed more f o l i a g e than when re a r e d on potato, i t s p r i n c i p a l host, yet i t had a lower s u r v i v a l r a t e ( l a t h e e f and Harcourt, 1972). In a d i f f e r e n t manner, i t was found that e a r l y 104 i n s t a r l a r v a e d i g e s t e d tomato b e t t e r , but l i t t l e u t i l i z a t i o n of the food f o r growth r e s u l t e d . L a t e r i n s t a r s , although showing lower d i g e s t a b i l i t y on tomato, had a higher r a t e c f food u t i l i z a t i o n f o r growth. S i m i l a r o b s e r v a t i o n s have been r e p o r t e d by Mukerji and Guppy (1970) f o r the l a r v a e of P s e u d a l e t i a unipuncta. The above are a l l h e r b i v o r e s , however, and the same e f f e c t may not apply i n c a r n i v o r e s , such as G e r r i s . G e r r i d s are o p p o r t u n i s t i c predators that u t i l i z e whatever prey items become trapped on the s u r f a c e f i l m of the water. There i s no recorded evidence t h a t g e r r i d s feed on p l a n t j u i c e s . However, th e r e i s evidence that g e r r i d s do have p r e f e r r e d foods (see S e c t i o n 5), but t h i s i s a mechanical p r e f e r e n c e , not a n u t r i t i o n a l one. As mentioned e a r l i e r , g e r r i d s , l i k e mantids, are r a p t o r i a l , and t h i s means t h a t depending on the s i z e of the f o r e l e g s r e l a t i v e to the prey, h a n d l i n g of the prey may or may not be e f f i c i e n t . F u n c t i o n a l l y , there i s no upper l i m i t with dead or motionless prey (assuming the integument i s not t h i c k e r than the p r o b o s c i s i s l o n g ) , as there i s always some p a r t o f the body, such as a l e g , which may be grasped. The g e r r i d has only to g r i p the prey and steady i t s e l f . However, t h e r e can be an upper l i m i t with l i v e prey, as the l a t t e r may be too a c t i v e and may r e q u i r e subduing. F u r t h e r , there i s o f t e n a lower s i z e l i m i t , as very small prey, dead or a l i v e , cannot be grasped e a s i l y . Even i f s m a l l items are u l t i m a t e l y grasped and impaled on the p r o b o s c i s , they may not provide enough n u t r i t i o n to make the e f f o r t worthwhile. There i s a s i g n i f i c a n t d i f f e r e n c e i n s i z e between a f i r s t i n s t a r G. buenoi (0.109 mg.) and an a d u l t 105 female G. r e m i g i s (50.170 mg.), and experiments with g e r r i d s as prey and predator i n d i c a t e t h a t food items as s m a l l as D r o s o p h i l a are not p r e f e r r e d by the l a r g e r g e r r i d s . I f t h i s d i f f e r e n c e i n p r e f e r e n c e does indeed r e s u l t from s m a l l e r s i z e and not b e h a v i o u r a l d i f f e r e n c e s on the p a r t of the prey, then i t may be that D r o s o p h i l a are not the most s u i t a b l e prey f o r the l a r g e r g e r r i d s . However, i n order to maintain n u t r i t i o n a l constancy throughout the study, such preference d i f f e r e n c e s were con s i d e r e d of l e s s e r importance, as no prey other than ^£2§2£]}il a M a s r e a d i l y a v a i l a b l e throughout the year, easy to -c u l t u r e , of a s i z e a c c e p t a b l e to both the l a r g e s t and s m a l l e s t g e r r i d s , and with an a b i l i t y to f l o a t on the water s u r f a c e . The prey used a l s o had to be s m a l l so that the r e l a t i v e l y s m a l l amounts i n g e s t e d by the s m a l l g e r r i d s would not be hidden i n the normal weight v a r i a b i l i t y o f the prey. The r e s u l t s of t h i s study should t h e r e f o r e be c o n s i d e r e d with these l i m i t a t i o n s i n mind. In the present study, the time to reach s a t i a t i o n i s r a t h e r long. Long e a t i n g p e r i o d s r e q u i r e d to reach s a t i a t i o n are not unique to G e r r i s , as M o r r i s (1963) r e p o r t s that Podisus r e q u i r e s on the average 4.5 hours. They are perhaps c h a r a c t e r i s t i c of hemipterans, however, which owing to t h e i r modified mouthparts, have to suck t h e i r food. Other i n s e c t s which chew, and hence eat t h e i r prey i n d i s c r e t e amounts, would presumably r e g u i r e c o n s i d e r a b l y l e s s time to achieve s a t i a t i o n . Sandness and McMurtry (1972) observed i n the mite AJS^Alseius l a r g g e n s i s d u r i n g 24~hr o b s e r v a t i o n p e r i o d s that the 106 l e n g t h o f the d i g e s t i v e pause w a s - c y c l i c i n nature, with the l o n g e s t pause g e n e r a l l y o c c u r r i n g a f t e r the f o u r t h or f i f t h prey was eaten, 3 -9 hr a f t e r the beginning of the o b s e r v a t i o n . Owing to the r e l a t i v e l y s h o r t length of the o b s e r v a t i o n p e r i o d i n t h i s study, t h i s phenomenon remains u n c e r t a i n . I f the e x i s t e n c e of a d i g e s t i v e pause i n d i c a t e s , as suggested by H o l l i n g (1966), the presence of a t h r e s h o l d , where a t t a c k only r e s u l t s i f the animal's hunger exceeds t h i s t h r e s h o l d , then the c y c l i c nature of d i g e s t i v e pause length as reported by Sandness and McMurtry would suggest the presence o f two t h r e s h o l d s . More c o n f i r m i n g evidence of these c y c l e s i s r e g u i r e d , before the exact nature and g e n e r a l i t y of t h e i r e x i s t e n c e may be determined. The r e l a t i o n between hours of food d e p r i v a t i o n and amount eaten suggests that two r a t e s of d i g e s t i o n e x i s t i n G e r r i s , cne c h a r a c t e r i s t i c of the immature stages with AD = 0.182 and the other c h a r a c t e r i s t i c of the a d u l t s i r r e s p e c t i v e of s p e c i e s , with AD = 0.047. Since AD i n d i c a t e s the slope of the r e g r e s s i o n , f u n c t i o n a l l y t h i s means t h a t the d i g e s t i v e r a t e i s f a s t e r i n immature g e r r i d s than i n a d u l t g e r r i d s , at l e a s t i n G. r e m i g i s , and as the a d u l t s are a l l somewhat s i m i l a r , i t would seem a reasonable assumption t h a t t h i s would apply to a l l immature stages i r r e s p e c t i v e of s p e c i e s . In t h i s study with G e r r i s , there was c o n s i d e r a b l e v a r i a b i l i t y i n the amount of fcod i n a " f u l l " stomach, the v a r i a b i l i t y being g r e a t e r i n the s m a l l e r g e r r i d s and l e s s i n the l a r g e r g e r r i d s . As d i s c u s s e d by B r e t t (1971), such v a r i a b i l i t y can be a t t r i b u t e d to three s o u r c e s : d i f f e r e n c e s r e s u l t i n g from 107 r e l a t i v e predator-prey s i z e s , and from d i f f e r e n t stomach c a p a c i t i e s (morphological and p h y s i o l o g i c a l d i f f e r e n c e s ) ; r e l u c t a n c e o f some animals to f i l l t h e i r stomach at every f e e d i n g ( b e h a v i o u r a l d i f f e r e n c e s ) ; and d i f f i c u l t i e s i n handling the minute stomach contents of the s m a l l animals ( t e c h n i c a l d i f f i c u l t i e s ) . A l l three sources c o u l d i n t r o d u c e d i s t o r t i o n which would tend to depress the mean values below the p o t e n t i a l stomach c a p a c i t y . Being p o i k i l o t h e r m i c animals, the i n c r e a s e i n amount eaten by the g e r r i d s with i n c r e a s i n g temperature presumably r e f l e c t s the i n c r e a s e d food requirements a r i s i n g from an inc r e a s e d r a t e of metabolism. Since r e p r o d u c t i o n r e g u i r e s a r e l a t i v e l y high r a t e of metabolism, i e . above t h a t normally r e q u i r e d f o r the b a s a l metabolism of the animal, the temperature at which the amount of food eaten i s maximized may r e p r e s e n t t h a t at which r e p r o d u c t i o n i s a l s o maximized. T h i s assumes that no d e l i t e r i o u s e f f e c t s r e s u l t from the high temperature and t h a t the i n t r i n s i c f a c t o r s maximizing r e p r o d u c t i o n have not alre a d y been o p t i m i z e d . N e v e r t h e l e s s , up to a t h r e s h o l d at l e a s t , h i g h e r temperatures should i n c r e a s e r e p r o d u c t i v e p o t e n t i a l and so i f female g e r r i d s behave as the males of t h e i r s p e c i e s do, the o p t i m a l temperature f o r maximizing r e p r o d u c t i o n may be i n the v i c i n i t y of that maximizing food i n t a k e . Hence, a l l the s p e c i e s except f o r G. r e m i g i s would be expected to occur i n warm h a b i t a t s , whereas G. r e m i g i s would be expected to occur i n h a b i t a t s with a temperature approximating 19°C. t h i s was observed i n the study area (see S e c t i o n 1) and i n the s t u d i e s on 108 temperature preference (see S e c t i o n 2). In F i g . 24, i t w i l l be n o t i c e d that f o r G. n o t a b i l i s , G. buenoi and G. i n c u r v a t u s , there i s a sudden i n c r e a s e i n the amount eaten between 14°C. and 17°C. No such sudden i n c r e a s e i s as evident f o r G. i n c o g n i t u s and G. r e m i g i s . Since the data above t h i s temperature range are l i n e a r , t h i s sudden i n c r e a s e i n food i n g e s t e d i s thought to be a r e a l phenomenon, r e f l e c t i n g a change i n metabolism as the temperature exceeds that r e q u i r e d to terminate o v e r w i n t e r i n g . As, mentioned e a r l i e r , G. i n c o g n i t u s and G. r e m i g i s emerge r e l a t i v e l y e a r l y i n the s p r i n g , as socn as the snow and i c e melt ( l a t e February - e a r l y March) and when temperatures are r e l a t i v e l y low. The other three s p e c i e s i n the study area emerge t y p i c a l l y i n l a t e A p r i l , when higher temperatures are r e c o r d e d . This suggests that i n e a r l y s p r i n g , the two groups of s p e c i e s behave d i f f e r e n t l y to r i s i n g temperatures. In the three l a t e r emerging s p e c i e s , s i n c e they were c o l l e c t e d i n the f i e l d and hence had a l r e a d y come out of o v e r w i n t e r i n g , f e e d i n g i s minimized from 8-14°C. and i s at a r e l a t i v e l y constant l e v e l . Above 14°C. these s p e c i e s become more a c t i v e and respond to i n c r e a s i n g temperature. The very l a r g e amounts of food eaten i n the v i c i n i t y of t h i s t h r e s h o l d may r e p r e s e n t r e s t o r a t i o n of the body's s t o r e d food l o s t o v e r w i n t e r i n g . I f t h i s r e s t o r a t i o n i n v o l v e d a s h o r t time p e r i o d , such as only a few days, owing to the design of t h i s experiment, i t would o n l y a f f e c t the amount eaten a t the temperature j u s t exceeding the t h r e s h o l d . With r e s p e c t to the e f f e c t of growth and development on 109 f e e d i n g , i n the study area, G. n o t a b i l i s i s u n i v o l t i n e , which may e x p l a i n the r a p i d d e c l i n e i n the amount eaten f o l l o w i n g the adu l t developmental p e r i o d . F i e l d o b s e r v a t i o n s suggest that the a d u l t s of t h i s s p e c i e s prepare f o r o v e r w i n t e r i n g r e l a t i v e l y scon a f t e r t h e i r f i n a l moult; no l a r g e f a l l a g g regations (as are found i n G. remigis) were observed i n G. n o t a b i l i s . The d e c l i n e i n amount eaten by G. n o t a b i l i s i s a t a r a t e s i m i l a r to the d e c l i n e w i t h i n a stadium f o l l o w i n g peak amount eaten, as shown i n F i g . 25a. As mentioned above, the time r e g u i r e d by a 48 hr s t a r v e d , a d u l t G. r e m i g i s to eat a measured amount of food was recorded. T h i s allowed c a l c u l a t i o n of the maximum r a t e of i n g e s t i o n (assuming that 75% of the prey's f l u i d s are ingested) f o r t h i s s p e c i e s , ca. 0.062 mg/min/individual. Over the time r e g u i r e d to reach s a t i a t i o n (2 .5 hr i f s t a r v e d f o r 48 hr) , the average r a t e of i n g e s t i o n was ca. 0.031 mg/min. T h i s i s l e s s than the 0.160 mg/min c a l c u l a t e d f o r a d u l t Podisus m a c u l i v e n t r i s (from M o r r i s , 1963) over the time r e g u i r e d to reach s a t i a t i o n . G a l l o p i n and K i t c h i n g (1972) r e p o r t a f e e d i n g r a t e of 0.03 mg/min f o r Podisus, but t h e i r technigue i n v o l v e d i n t e r r u p t i o n of f e e d i n g at hourly i n t e r v a l s t c permit weighing of the prey. Adult mantids, on the other hand, which chew r a t h e r than suck t h e i r food, eat c o n s i d e r a b l y f a s t e r , with a r a t e of 5.6 mg/min f o r Mantis r e l i g i o s a and 20.6 mg/min f o r the l a r g e r Hierodula c r a s s a (from H o l l i n g , 1966)... Since the average r a t e of i n g e s t i o n over the 2.5 hr p e r i o d r e q u i r e d to reach s a t i a t i o n i n G. re m i g i s was s i g n i f i c a n t l y l e s s than that when i n g e s t i o n commenced, i t 110 appears t h a t as amount of food i n the gut approachs the maximum, the r a t e of i n g e s t i o n decreases. T h i s i s supported by data f o r the length of the f e e d i n g p e r i o d s and d i g e s t i v e pauses. If the p a t t e r n of f e e d i n g bouts and d i g e s t i v e pauses i n d i c a t e d e a r l i e r i s assumed to be not simply a r e s u l t of the previous 48 _hr s t a r v a t i o n i n t e r v a l , the amount consumed per day can be c a l c u l a t e d to be 5.125 mg at 26°C. (assuming again t h a t 15% of the prey's f l u i d s are i n g e s t e d ) . T h i s i s c l o s e to the observed amount of 5.002 mg eaten per day and the i t e r a t i v e amount a d j u s t i n g f o r temperature of 4.611 mg per day. With the above f e e d i n g p a t t e r n , the average r a t e of i n g e s t i o n i s 0.013 mg/min, i n agreement with the lower r a t e of d i g e s t i o n expected when the gut i s f i l l e d with food. However, that t h i s may be c l o s e to, but not the r e a l s t o r y i s i n d i c a t e d by data which showed that with dead D r o s o p h i l a as prey, 24 g e r r i d s on a 12 hr l i g h t : 1 2 hr dark photoperiod over three c o n s e c u t i v e days ate on the average 3.807 mg i n the dark and o n l y 1.025 mg i n the l i g h t . Why t h i s b i a s towards the dark i s not c l e a r , unless being predominantly responsive to moving prey (see S e c t i o n 4 ) , they do not r e a c t p r o p o r t i o n a t e l y to s t a t i o n a r y prey while they can see; only i n the dark do t h e i r t a c t i l e senses assume importance. The s i g n i f i c a n c e of t h i s lower d i g e s t i v e r a t e i s t h a t f o r models op e r a t i n g with a s h o r t time s c a l e , i e . s i g n i f i c a n t l y l e s s than the time r e g u i r e d to d i g e s t a f u l l gut, e r r o r may a r i s e i f the mean rate of consumption i s simply c a l c u l a t e d by d i v i d i n g the , maximum gut c a p a c i t y by the time r e g u i r e d by a s t a r v e d g e r r i d to reach s a t i a t i o n . I f t h i s value i s used f o r G. r e m i g i s , the mean d a i l y food consumption with the observed p a t t e r n of f e e d i n g and 111 non-feeding bouts would be c a . 13.251 mg, g r e a t l y i n excess of that observed. S i m i l a r l y , s i n c e d a i l y food consumption v a r i e s s i g n i f i c a n t l y through the course of a stadium, i t i s not accurate to simply d i v i d e the t o t a l food consumed i n each stage by the number of days spent f e e d i n g i n the stage to determine mean d a i l y food consumption i f a sh o r t time s c a l e i s used. This i s of p a r t i c u l a r importance here, as i n the o v e r a l l study of co e x i s t e n c e i n G e r r i s , the r e l a t i v e l y s h o r t time s c a l e u n i t of an hour i s used. With r e s p e c t to fe e d i n g i n G e r r i s , then, given the: 1. s a t i a t i o n time '2. length of fe e d i n g and non-feeding p e r i o d s 3. e f f e c t o f food d e p r i v a t i o n on amount consumed 4. maximum gut c a p a c i t y 5. e f f e c t of temperature on food consumption 6. e f f e c t of developmental s t a t e on food consumption, i t i s p o s s i b l e to determine f o r any p a r t i c u l a r g e r r i d a value f o r the maximum amount c f food i t co u l d eat at any time as shown i n F i g . 26, i f the hours o f food d e p r i v a t i o n f o l l o w i n g i t s pre v i o u s f e e d i n g are known. The b a s i c parameters f o r modelling food consumption i n the f i v e s p e c i e s of G e r r i s s t u d i e d are thus a v a i l a b l e . 112 i F i g u r e 26. A flow diagram to i l l u s t r a t e bow the parameters d i s c u s s e d i n t h i s study may be used. The t o t a l amount of food i n g e s t e d over a given time p e r i o d f o r any g e r r i d i n the presence of excess food may be determined i f the hours of food d e p r i v a t i o n f o l l o w i n g the previous f e e d i n g are known. 112a Number of hours of food d e p r i v a t i o n Allows | c a l c u l a t i o n of | the amount eaten^ i n the time | period T = Y J I C o r r e c t Y f o r temperature i Maximum gut c a p a c i t y Allows c a l c u l a t i o n of the maximum amount of food t h a t can be •>consumed a t 26°C. = X (modify d i g e s t i v e r a t e as to whether l a r v a or adult) P a t t e r n of f e e d i n g and non-feeding p e r i o d s ; rate of food i n t a k e T o t a l time minus time r e q u i r e d to achieve ^ s a t i a t i o n = T i 1 1 4^  C o r r e c t Y f o r | | day of stadium | I I \1/ | T o t a l amount j | i n g e s t e d i n the j I t o t a l time | | p e r i o d (X + Y) | I 1 | C o r r e c t X f o r | -f-\> temperature | I I | C o r r e c t X f o r | | day of stadium J I \ T o t a l time | No| p e r i o d l e s s than) | t h a t r e g u i r e d | | t o achieve | | s a t i a t i o n | r~ I Yes Y = 0 113 SECTION 2. REACTIVE DISTANCE AND RATE OF MOVEMENT INTRODUCTION G e r r i d s are o p p o r t u n i s t i c p r e d a t o r s which feed en i n s e c t s trapped on the water s u r f a c e , and are capable of c a p t u r i n g l i v e prey, i n c l u d i n g other g e r r i d s , i f the need a r i s e s . G e r r i d s have been shown to be capable of l o c a t i n g t h e i r prey by responding to d i s t u r b a n c e s of the water s u r f a c e through r e c e p t o r s on the l e g s (Rensing, 1961; L i c h e , 1936; Murphey, 1971a). Murphey (1971a) has shown t h a t o r i e n t a t i o n i n G. r e m i g i s c o n s i s t s of a s e r i e s of d i s c r e t e t u r n i n g movements l e a d i n g to capture of the source of the d i s t u r b a n c e . He a l s o analyzed (Murphey, 1971b) the system c o n t r o l l i n g o r i e n t a t i o n to prey by l o c a l i z i n g those r e c e p t o r s which mediate the response through s e l e c t i v e a b l a t i o n of d i f f e r e n t p a r t s of the l e g s . v These experiments suggested that the system may f u n c t i o n by determining the r e c e p t o r nearest the s o u r c e of the r i p p l e s , as a number of f a i r l y accurate p r e d i c t i o n s may be made by assuming t h i s i s the case. In a c l o s e l y r e l a t e d f a m i l y , the V e l i i d a e , Meyer (1971a, 1971b) has i n v e s t i g a t e d the v i s u a l s i g n s t i m u l i r e s u l t i n g i n attack by V§2i§. c a p r a i Tam. and has demonstrated that t h i s s p e c i e s o,nly r e a c t s to v i s u a l prey s t i m u l i when they are accompanied by v i b r a t i o n s t i m u l i from the same d i r e c t i o n . Whether g e r r i d s a l s o r e g u i r e s u r f a c e v i b r a t i o n s i n a s s o c i a t i o n with v i s u a l s t i m u l i b efore an a t t a c k i s e l i c i t e d has not yet been determined. Regardless of the mechanism r e q u i r e d to e l i c i t an a t t a c k . 114 however, the number of k i l l s made by a predator i s the product of two parameters: the ra t e of encounter of the prey and the k i l l success. T h i s s e c t i o n d i s c u s s e s the former, the parameters which determine the encounter r a t e . These are: the d i s t a n c e a t which a predator w i l l respond to a prey item, i e . the r e a c t i v e d i s t a n c e ; and the r a t e of movement of the predator and the prey through the environment. Combining the two al l o w s c a l c u l a t i o n of the swath the predator would cover as i t moves across the water s u r f a c e . Thus, depending on the d i s t r i b u t i o n of the prey (random, contagious, e t c . ) , the number of encounters may be determined. The d i s t r i b u t i o n of the d i f f e r e n t s p e c i e s w i t h i n the study area i s d i s c u s s e d i n S e c t i o n 1 and the k i l l success w i l l be con s i d e r e d i n S e c t i o n 5. MATERIALS AND METHODS 1. REACTIVE FIELD Recording was accomplished by v i d e o t a p i n g on one-inch v i d e o t a p e the i n t e r a c t i o n s of g e r r i d s with prey ( e i t h e r SlosojDhila or other g e r r i d s ) . The g e r r i d s were placed i n two arenas, and two General E l e c t r i c V i d i s o n TV cameras were used, one f o r each arena. As i t was i m p o s s i b l e to o r i e n t the cameras v e r t i c a l l y above the arenas and f i l m the i n t e r a c t i o n s d i r e c t l y , an apparatus i n v o l v i n g the use of a mir r o r s e t a t 45° was r e q u i r e d . The arenas, 30 by 30 cm, were opaque, white, p l a s t i c 115 b a s i n s 15 cm deep f i l l e d with 26°C. water to a depth o f ca. 3 cm. L i g h t i n g was f i v e , c o o l -white, 2-foot f l u o r e s c e n t tubes l o c a t e d beneath the b a s i n s . B a c k - l i g h t i n g was r e q u i r e d to avoid r e f l e c t i o n o f f the water s u r f a c e . Recording of only one arena at a time was p o s s i b l e , so the recorded a c t i o n was s h i f t e d from camera to camera, depending on the arena i n which the a t t a c k s were o c c u r r i n g a t the time. Replay was on a 23" sc r e e n , and i t was p o s s i b l e to f r e e z e the at t a c k at the moment i t commenced when the a t t a c k i n g g e r r i d f i r s t began to b l u r i n i t s rush towards the prey. a t t a c k s were so f a s t , much,less than a second i n d u r a t i o n , t h a t at 18 frames per second, the a c t i o n was b l u r r e d . Replay was of a g u a l i t y such that i t was j u s t p o s s i b l e to d i s c e r n the l e g s of a f r u i t f l y - no d i f f i c u l t y was encountered i n o b s e r v i n g the g e r r i d s . The parameters measured were the d i s t a n c e between predator and prey, the angle of the prey r e l a t i v e to the a n t e r i o r -p o s t e r i o r a x i s of the predator, and the angle of the prey r e l a t i v e to the s t r a i g h t l i n e c o n necting predator and prey. The l a t t e r was r e g u i r e d when g e r r i d s were used as prey, as along with the l e n g t h and width of the prey, i t allowed c a l c u l a t i o n of the r e l a t i v e v i s u a l angle subtended by the prey t c the predator. O b v i o u s l y , i f the prey were f a c i n g the predator, i t would subtend a much s m a l l e r angle than i f i t were a t r i g h t - a n g l e s to the predator. With D r o s o p h i l a as prey, i t was found that the d i f f e r e n c e between i t s l e n g t h and width was so smal l as to make a c o r r e c t i o n unnecessary and so they were c o n s i d e r e d as being s p h e r i c a l . Whether or not the a t t a c k was s u c c e s s f u l was a l s o 116 recorded, and i n the case of s u c c e s s f u l a t t a c k s , the prey was removed immediately from the predator to prevent f e e d i n g . I t should be noted that the c r i t e r i a f o r i n i t i a t i o n of an a t t a c k was the commencement of movement by the predator g e r r i d . As i n most i n s t a n c e s the prey g e r r i d was a l r e a d y moving, t h e r e was no way of a s c e r t a i n i n g the a c t u a l d i s t a n c e at which the predator g e r r i d f i r s t became aware of the prey g e r r i d and decided to a t t a c k . I t could not be assumed that the p r o b a b i l i t y of the prey g e r r i d moving toward the predator was the same as the p r o b a b i l i t y of i t s moving away, as o f t e n , the prey g e r r i d , i f i n i t i a l l y s t a t i o n a r y , was only moving i n response to the predator g e r r i d ' s c l o s e p r o x i m i t y , and hence was moving away. Thus from a morphological and p h y s i o l o g i c a l view, t h i s c r i t e r i a w i l l not allow determination of the exact d i s t a n c e at which a g e r r i d w i l l respond to a prey g e r r i d . However, from an o p e r a t i o n a l view, t h i s approach does d e s c r i b e the predator's response. With prey items unable to respond to the approach of a g e r r i d , such as D r o s o p h i l a , the exact d i s t a n c e at which a g e r r i d responded to a prey was d e s c r i b e d . I t was f o r t h i s reason and to provide a constant prey s i z e that E r o s g p h i l a were used as prey i n the de t e r m i n a t i o n of the e f f e c t of g e r r i d s i z e and hunger on r e a c t i v e d i s t a n c e (see below) . To determine the o v e r a l l shape of the r e a c t i v e f i e l d of G e r r i s , o b s e r v a t i o n s of the a t t a c k s of 24 hr s t a r v e d a d u l t G. r e m i g i s on a d u l t G. i n c o g n i t u s were recorded. To assess the e f f e c t of hunger on the mean r e a c t i v e 117 d i s t a n c e , o b s e r v a t i o n s of a d u l t G. remicjis at d i f f e r e n t hunger l e v e l s a t t a c k i n g l i v e D r o s o p h i l a were recorded. Four hunger l e v e l s were e s t a b l i s h e d (0 hr, 6 hr, 18 hr and 48 hr starved) and 20 o b s e r v a t i o n s were recorded a t each. The e f f e c t of g e r r i d s i z e on the mean r e a c t i v e d i s t a n c e of G e r r i s was measured by determining the d i s t a n c e at which ten s i z e c l a s s e s of G e r r i s attacked l i v e D r o s o p h i l a . Table 13 summarizes the s p e c i e s - i n s t a r s , t h e i r s i z e , and the s t a t e of hunger of the g e r r i d s which were used i n t h i s study. The ten s i z e c l a s s e s used were e s t a b l i s h e d on the b a s i s of l e g length (mesothoracic femur + t i b i a plus metathoracic femur + t i b i a ) , as t h i s i s one of the few parameters to remain constant throughout the d u r a t i o n of an i n s t a r . Leg l e n g t h was used i n p a r t i c u l a r as i n other r e l a t e d s t u d i e s c f p r e d a t i o n i n G e r r i s (Section 5), i t was noted that l e g length seemed c r i t i c a l i n determining the success of an a t t a c k . In s i t u a t i o n s where the "pre d a t o r " and "prey" g e r r i d s were s i m i l a r i n s i z e , s u c c e s s f u l a t t a c k s u s u a l l y r e g u i r e d the f l i p p i n g o f the "prey" g e r r i d onto i t s back. The longer i t s l e g s , i r r e s p e c t i v e of body s i z e , the more s t a b l e i t was on the s u r f a c e f i l m . These s i z e c l a s s e s are used to provide constancy i n terminology i n the o v e r a l l study: the r e l a t i o n between s p e c i e s - i n s t a r s and s i z e c l a s s i s shown i n F i g . 27. In determining the angle subtended by an animal, body s i z e , not l e g l e n g t h , i s the c r i t i c a l parameter. Thus mean body s i z e was used to c h a r a c t e r i z e the s i z e c l a s s e s used i n t h i s aspect of the study. The mean body s i z e o f each g e r r i d s i z e c l a s s i n alTable 13 i s the mean of the (width + len g t h ) / 2 of randomly 118 Figure 27. The r e l a t i o n between species-instars and leg length (± 1 SE), and their grouping into ten s i z e classes used in t h i s study. Leg length i s the suns of the femurs and t i b i a s of the meso- and metathoracic legs. • = larvae; A = adult female ; 4 = adult male. LEG LENGTH (MM) ro ro co "D m — o < m co © © 9 © 0> © @ o © o 3D © CO cn —> r o c o *N 01 c n c o c o SIZE GLASS CO o H t-r1 m 119 Table 13. The g e r r i d s p e c i e s - i n s t a r s , the mean body s i z e , and the s t a t e of hunger used i n determining the e f f e c t of body s i z e on mean r e a c t i v e d i s t a n c e . S i z e c l a s s S p e c i e s - i n s t a r Bean body s i z e Hours s t a r v e d (mm) 1 G« i n c u r v a t u s 1st 5 1.2 25 2 G . r e m i g i s 1st 5 1.5 25 3 G . r e m i g i s 2nd 5 2.1 25 4 4 G . i n c o g n i t u s 4 th 5 3.1 25 f(3. n o t a b i l i s 3rd 5 "| 5 < ^ 2 . 8 25 ( G . i n c u r v a t u s 4th 5 ) 6 5 - incoqn i t u s 5th 5 3.8 25 4.7 25 n G . i n c u r v a t u s 1st 5 G. re m i g i s 1st 5 G. r e m i g i s 2nd 5 G. i n c o g n i t u s 4 th 5 n o t a b i l i s 3rd 5 G. i n c u r v a t u s 4th 5 G. g  i t th G. i n c u r v a t u s 5th 5 G. remi_gis 4th 5 G. n o t a b i l i s 4th 5 G. buenoi a d u l t 5 G. i n c u r v a t u s a d u l t 5 G . i n c o g n i t u s adu It 5 G. rem i g i s 5th 5 G. n o t a b i l i s 5 th 5 §. r e m i g i s a d u l t 5 G . n o t a b i l i s a d u l t 5 8 < w \ 5.5 48 7.0 25 10 ^ V 9.6 48 120 chosen i n s e c t s c o l l e c t e d i n the summer from the f i e l d and preserved i n 70% e t h a n o l . As d i f f e r e n c e s i n l e g l e n g t h do not always r e f l e c t corresponding d i f f e r e n c e s i n body s i z e , two s i z e c l a s s e s with only a s m a l l d i f f e r e n c e i n l e g l ength may vary such th a t the " l a r g e r " s i z e c l a s s has a smaller mean body s i z e . T h i s i s why s i z e c l a s s 5 i n Table 13 has a s m a l l e r body s i z e than s i z e c l a s s 4. 2. MOVEMENT The r a t e of movement of the i n s t a r s of the f i v e g e r r i d s p e c i e s s t u d i e d were measured as two components: the mean s t r i d e l e n g t h and the number of s t r i d e s per u n i t time. The mean s t r i d e l e ngth was measured by t r a c i n g on t r a n s p a r e n t , a c r y l i c s h e e t i n g the d i s t a n c e the i n d i v i d u a l c o n s e c u t i v e s t r i d e s moved the g e r r i d on the v i d e o t a p e system d e s c r i b e d above. Measurements o f t h i s d i s t a n c e (= s t r i d e length) of a d u l t s of each s p e c i e s were made to determine s p e c i e s d i f f e r e n c e s . A d d i t i o n a l t r a c i n g s from the l a r v a l i n s t a r s r e p r e s e n t i n g the d i f f e r e n t s i z e c l a s s e s i n Table 13 were used to a s sess the e f f e c t of l e g l ength on s t r i d e l e n g t h . The number of s t r i d e s per u n i t time were measured by o b s e r v i n g i n d i v i d u a l g e r r i d s f o r 15 c o n s e c u t i v e minutes each. G e r r i d s of s i z e c l a s s e s 4 to 10 were s t u d i e d i n a c i r c u l a r tank 1.22 m i n diameter and 11 cm deep, with 20 g e r r i d s a t each time being observed i n the tank. S i z e c l a s s e s 1 to 3 were s t u d i e d i n a p l a s t i c c o n t a i n e r 31 cm by 25 cm and 9.5 cm deep, with 10 121 g e r r i d s being p r e s e n t . Regardless of c o n t a i n e r s i z e , water depth was 3-4 cm and water temperature was ca. 25°C. An a i r stone was placed i n the l a r g e r tank to keep the water s u r f a c e c l e a n . Each g e r r i d was s a t i a t e d before t e s t i n g commenced and the v a r i o u s a c t i v i t i e s of each i n s e c t were recorded on magnetic tape. A panel which emitted sounds at d i f f e r e n t f r e q u e n c i e s was connected to a tape r e c o r d e r , and by i d e n t i f y i n g a p a r t i c u l a r b e h a v i o u r a l movement with a p a r t i c u l a r sound, i t was p o s s i b l e to r e c o r d as a s e r i e s of "beeps" at d i f f e r e n t f r e q u e n c i e s , the a c t i v i t i e s of the g e r r i d s (Dawkins, 1972). Replay of the tape i n t o a computer allowed c a l c u l a t i o n not only of the number of times each b e h a v i o u r a l movement occurred, but the temporal sequence of the d i f f e r e n t movements and t o t a l time spent at each movement. To determine i f s p e c i e s d i f f e r e n c e s were present, a d u l t s of each of the f i v e s p e c i e s were observed as d e s c r i b e d above. Then to a s s e s s the e f f e c t of s i z e , s a t i a t e d i n d i v i d u a l s of each of the s i z e c l a s s e s 1 to 7 and 9 were observed, the l a r v a e i n v o l v e d being as noted i n Table 14. The e f f e c t of hunger was measured by observing i n d i v i d u a l a d u l t G. r e m i g i s a f t e r 6, 26 cr 48 hr of s t a r v a t i o n . F i n a l l y , g e r r i d s r e p r e s e n t i n g a l l f i v e s p e c i e s were a l s o observed i n the f i e l d f o r 15 minutes each to determine i f the data obtained i n the l a b o r a t o r y a c c u r a t e l y r e f l e c t e d t h a t found i n the f i e l d . On the day when o b s e r v a t i o n s were made (July 21 , 1 972, 10: 00 a.m. To 3:00 p.m. ), the weather was sunny and the ambient water temperature was 23 _25°C. The s t a t e of hunger of the g e r r i d s observed i n the f i e l d i s unknown. 122 Table 11. The s p e c i e s - i n s t a r s r e p r e s e n t i n g the v a r i o u s s i z e c l a s s e s t e s t e d i n determining the e f f e c t of s i z e on frequency of movement, and the mean number of s t r i d e s (± 1 SE) observed i n the 15 minute t e s t p e r i o d . c l a s s S p e c i e s - i n s t a r n S t r i d e s / 1 5 min, 1 G. i n c u r v a t u s 1st 10 87.a ± 35.9 2 G. i n c u r v a t u s 2nd 10 17.a ± a. a 3 G. i n c u r v a t u s 3rd 10 9.6 ± 1.2 ti £. buenoi ath 10 a2.2 ± 9. 1 5 G. i n c u r v a t u s ath 10 113.6 ± 27.6 6 G. buenoi 5th 10 2a. 1 8. a 7 G. i n c u r v a t u s 5th 10 100.6 ± 15.8 9 G. rem i g i s 5th 10 319.a ± 52. 2 123 RESULTS 1. REACTIVE FIELD The o v e r a l l shape of the r e a c t i v e f i e l d of G. re m i g i s i s i l l u s t r a t e d i n F i g . 28. A d e f i n i t e p r e f e r e n c e f o r moving prey ( F i g . 28b) was e x h i b i t e d , with o n l y 12.9% of the 178 observed a t t a c k s being a g a i n s t s t a t i o n a r y prey ( F i g . 28a). Only 10.7% of the t o t a l observed a t t a c k s were d i r e c t e d behind the g e r r i d , i e . a t an angle g r e a t e r than 100° when the 0-180° a x i s i s the body a x i s of the g e r r i d and 0° the a n t e r i o r end. The t o t a l number of s u c c e s s f u l a t t a c k s (9) i s too smal l to i n d i c a t e i f t h i s observed p r e f e r e n c e to a t t a c k a n t e r i o r l y t r u l y r e f l e c t s the p r o b a b i l i t y of making a s u c c e s s f u l a t t a c k . However, i f the p r o b a b i l i t y of making a s u c c e s s f u l a t t a c k a t an angle of l e s s than 100° ( a n t e r o l a t e r a l region) i s weighted a g a i n s t t h a t at an angle g r e a t e r than 100° ( p o s t e r o l a t e r a l r e g i o n ) , the p r o b a b i l i t y of making a s u c c e s s f u l a t t a c k (5%) i s the same f o r both r e g i o n s . That the preference f o r a t t a c k i n g i n the a n t e r o l a t e r a l d i r e c t i o n i s not simply a r e s u l t of the g e r r i d ' s s being unable to detec t prey behind i t , e i t h e r v i s u a l l y or by s u r f a c e waves, can be seen i n escape responses when the g e r r i d i s prey ( F i g . 28c). Of 97 s u c c e s s f u l escapes by the prey g e r r i d observed, 55.6% were escapes i n which the predator attacked the prey g e r r i d from behind, i e . a t an angle g r e a t e r than 100° to the prey g e r r i d * s body a x i s . G e r r i d s can thus respond to a t t a c k s i r r e s p e c t i v e of the angle from which they are launched. 124 F i g u r e 28. The e f f e c t of prey l o c a t i o n r e l a t i v e to the predator on the p r o p e n s i t y of a d u l t G. r e m i g i s to a t t a c k (A) s t a t i o n a r y prey and (B) moving prey (prey = a d u l t G. i n c o g n i t u s ) , and (C) the e f f e c t of predator l o c a t i o n r e l a t i v e to the prey on the a b i l i t y o f the prey to s u c c e s s f u l l y a v o i d capture, i e . escape, from the predator. The histograms on the outer c o n c e n t r i c c i r c l e r e p r e s e n t the number of a t t a c k s made i n the 5 degrees on e i t h e r s i d e ; the numbers at the top i n d i c a t e the number of a t t a c k s which were s u c c e s s f u l . The s o l i d c i r c l e s r e p r e s e n t the mean d i s t a n c e (± 1 SE) i n each of the 18 10-degree segments at which a 2 mm prey item would be a t t a c k e d (A and B) and i n (C), the mean d i s t a n c e f o r each segment at which a prey item would commence evasive a c t i o n when atta c k e d by a 2 mm p r e d a t o r . The i n n e r s e m i c i r c l e i n each f i g u r e p a s s i n g through the s o l i d c i r c l e s d e p i c t s the o v e r a l l mean d i s t a n c e ( i r r e s p e c t i v e of the angular l o c a t i o n of the stimulus) a t which the g e r r i d w i l l respond. 125 No s i g n i f i c a n t d i f f e r e n c e was observed between the two r e g i o n s ( a n t e r o l a t e r a l and p o s t e r o l a t e r a l ) f o r the mean angle r e l a t i v e , to the predator g e r r i d ( F i g . 28b) which moving prey must subtend before an a t t a c k i s e l i c i t e d . T h i s angle, 3.72° ± 0.22, was c o n s i d e r a b l y l e s s than the mean angle s t a t i o n a r y prey must subtend, 15.02° ± 2.12. The mean angle the predator must subtend before e v a s i v e a c t i o n i s taken by the prey was found to be 13.15° ± 0.95. Hunger has a s i g n i f i c a n t e f f e c t on the v i s u a l angle r e g u i r e d to e l i c i t an a t t a c k by G. r e m i g i s - F i g . 29 shows the e f f e c t of hours of food d e p r i v a t i o n on v i s u a l angle. Knowing the hours of food d e p r i v a t i o n allows c a l c u l a t i o n cf the amount of food r e g u i r e d to f i l l the gut (see S e c t i o n 3) and hence food i n the gut. F i g . 30 i l l u s t r a t e s the r e l a t i o n between amount of food i n the gut, expressed as a percent of the maximum gut c a p a c i t y , and the v i s u a l angle r e g u i r e d to e l i c i t an a t t a c k . The v i s u a l angle i s s l i g h t l y s m a l l e r with l e s s fcod i n the gut, ( i e . the h u n g r i e r the g e r r i d ) , with a 4.955 p r o b a b i l i t y of the s l o p e i n F i g . 3 0 being z e r o . The e f f e c t of g e r r i d s i z e on the angle r e g u i r e d to e l i c i t an a t t a c k i s shown i n F i g . 31. Smaller g e r r i d s r e g u i r e the prey t o subtend a l a r g e r angle, while the l a r g e r g e r r i d s r e g u i r e a s m a l l e r angle; the s m a l l e s t g e r r i d s r e g u i r e an angle of 5 . 7 1 ° ± 0.37 and the l a r g e s t , 3.23° ± 0.29. O p e r a t i o n a l l y , t h i s means t h a t f o r a standard s i z e prey item, the l a r g e r the g e r r i d , the g r e a t e r the r e l a t i v e d i s t a n c e a t which i t w i l l a t t a c k the prey. 126 Figure 29. The e f f e c t of hours of food deprivation on the mean v i s u a l angle (± 1 SE) which must be subtended by a prey item before an attack i s e l i c i t e d by adult G. remigis. 127 F i g u r e 30. The r e l a t i o n of the amount of food i n the gut, expressed as a percent of t h a t r e q u i r e d t o achieve s a t i a t i o n , to the mean v i s u a l angle (± 1 SE) r e g u i r e d to e l i c i t an a t t a c k by a d u l t G. r e m i g i s . 2 9 128 f Figure 31. The r e l a t i o n between gerrid s i z e , expressed as the sum of the lengths of the femurs and t i b i a s of the meso- and metathoracic legs, and the mean visua l angle (± 1 SE) which a prey must subtend before an attack i s e l i c i t e d . 129 F i g u r e 32. The r e l a t i o n between s t r i d e length (± 1 SE) and the mean length of the mesothoracic l e g , expressed as the sum of the l e n g t h s of the femur, t i b i a and t a r s u s , f o r the ten s i z e c l a s s e s used i n t h i s study. 31 130 Table 15 summarizes the mean d i s t a n c e s at which the d i f f e r e n t s i z e c l a s s e s of g e r r i d s w i l l a t t a c k each other when maximally s t a r v e d . 2. MOVEMENT No d i f f e r e n c e appeared to e x i s t between s p e c i e s cf g e r r i d i n s t r i d e l e n g t h . Bather, s t r i d e l e n g t h , as shown i n F i g . 32, seems to be d i r e c t l y r e l a t e d to l e g length (here measured as the sum of the mesothoracic femur, t i b i a and t a r s u s ) ; the longer the l e g s , the l a r g e r the s t r i d e . S i g n i f i c a n t s p e c i e s d i f f e r e n c e s were observed, however, with r e s p e c t to the number of l e g s t r o k e s measured over the 15 minute o b s e r v a t i o n p e r i o d ( F i g . 33). G. r e m i g i s made 4 to 6 times more s t r o k e s than any of the other f o u r s p e c i e s . This was l a r g e l y a r e s u l t of the manner i n which the d i f f e r e n t s p e c i e s responded to the water c u r r e n t s s e t up by the a i r stone i n the tank. G. r e m i g i s tended to remain i n the c u r r e n t , m a i n t a i n i n g a constant p o s i t i o n w i t h i n the tank; t h i s r e g u i r e d c o n s t a n t , r e g u l a r s t r o k e s . The other s p e c i e s , however, e i t h e r d r i f t e d with the c u r r e n t or s t a t i o n e d themselves i n areas of the tank with no c u r r e n t , making l i t t l e e f f o r t to d i r e c t t h e i r movement's a g a i n s t the c u r r e n t . In o b s e r v a t i o n s when an a i r stone was not p r e s e n t , however, G. r e m i g i s was s t i l l observed to move co n s i d e r a b l y more f r e g u e n t l y than the other s p e c i e s . As movement i n a l l f i v e l a r v a l i n s t a r s of the f i v e species^ 131 T a b l e 15. The mean d i s t a n c e (cm) a t which t h e d i f f e r e n t s i z e c l a s s e s of g e r r i d s w i l l a t t a c k each o t h e r a t maximum s t a r v a t i o n . P r e d a t o r s i z e c l a s s 1 2 3 4 5 6 7 8 9 10 1 1.24 1.26 1.29 1.32 1. 35 1.37 1. 44 1.44 1.63 1 .83 p 2 - 1.58 1.61 1.65 1.69 1.71 1.80 1.81 2.03 2.29 r e 3 - — 2.26 2.31 2.36 2. 40 2.52 2.53 2.85 3.21 y 4 — — — 3.41 3.49 3.54 3.72 3.73 4.20 4.74 s i 5 - - : - - 3. 15 3. 20 3. 36 3.37 3.79 4.28 z e 6 - - - - - • 4. 34 4.56 4.57 5. 15 5.81 c 7 — — — — — 5.64 5.66 6.37 7.19 1 a 8 — —; — — — — — 6.62 7.45 8.41 s s 9 - - - - - - - - 9.49 10.70 10 — — — — — 14.68 132 F i g u r e 33. The r e l a t i o n between G e r r i s s p e c i e s and the mean number of s t r i d e s (± 1 SE) made per i n d i v i d u a l over a 15 minute time p e r i o d . U = G. n o t a b i l i s ; R = G. r e m i g i s ; IG = G. i n c g g n i t u s ; IV = G. i n c u r v a t u s ; B = G. buenoi. 133 F i g u r e 34. The r e l a t i o n between g e r r i d s i z e , expressed as the sum of the lengths of the femurs and t i b i a s of the meso- and metathoracic l e g s , and the mean number of s t r i d e s (± 1 SE) made per i n d i v i d u a l over a 15 minute time p e r i o d , expresses i n terms of G. r e m i g i s u n i t s . (The p r o p e n s i t y to move was assumed s p e c i e s - s p e c i f i c , r e g a r d l e s s of g e r r i d s i z e , and on the b a s i s of the data i n F i g . 33, c o e f f i c i e n t s were determined f o r each s p e c i e s to s e t t h e i r frequency of movement egual to t h a t of G. remigis) . 133a 33 o o tx LU CL LU I— Z in CC LU 0 _ cn LU Q CC h-350 280 210 -140 -70 't N R IG SPECIES IV B 34 Q O CC LU 0 _ LU I— i n L U cn L U D on t-co 500 r 400 - Y = 140-6+ 5-19X 10 20 30 LEG LENGTH (MM) 134 was not measured, i n order to assess the e f f e c t of g e r r i d s i z e on s t r o k e freguency f o r each s p e c i e s , i t was assumed that the tendency to move f o r the i n s t a r s of each s p e c i e s was the same as that f o r the a d u l t , s i n c e they occur together i n the same m i c r o h a b i t a t . Thus, i f a c o r r e c t i o n c o e f f i c i e n t i s determined f o r the a d u l t s of each s p e c i e s i n terms of G. r e m i g i s , m u l t i p l i c a t i o n of the number of s t r o k e s observed f o r any i n s t a r by the corres p o n d i n g c o e f f i c i e n t should i n d i c a t e the number of s t r o k e s per u n i t time measured i n terms of G. r e m i g i s . F i g . 34 d e s c r i b e s the r e l a t i o n between g e r r i d s i z e and freguency of movement, and shows that great v a r i a t i o n was observed. There i s e v i d e n t l y no c l e a r e f f e c t of g e r r i d s i z e on the number of s t r o k e s made (the p r o b a b i l i t y of the slope being zero i s 23.3%), although the " b e s t - f i t " l i n e a r r e g r e s s i o n suggests that the s m a l l e r g e r r i d s tend to move s l i g h t l y l e s s . The amount of food i n the gut, however, d i d have a s i g n i f i c a n t e f f e c t on the freguency of movement, as shown i n F i g s . 35 and 36. again, knowing the hours of food d e p r i v a t i o n allowed c a l c u l a t i o n of the percent of gut s a t i a t i o n (see S e c t i o n 2). These data ( F i g . 36) show t h a t with i n c r e a s i n g hunger, the number of s t r o k e s made by G. r e m i g i s d e c l i n e s s i g n i f i c a n t l y , with a g e r r i d with an empty gut moving only o n e - s i x t h as much as a g e r r i d with a f u l l gut. The only b e h a v i o u r a l movement apart from locomotive s t r o k e s observed i n the g e r r i d s was grooming (excluding the v i b r a t o r y movements made by a d u l t male G. n o t a b i l i s as they courted the f e m a l e s ) . I f one second on the average i s c o n s i d e r e d the time 135 F i g u r e 35. The e f f e c t of hours of food d e p r i v a t i o n on the freguency of movement by G. r e m i g i s , measured as the mean number of s t r i d e s (± 1 SE) made per i n d i v i d u a l over a 15 minute time p e r i o d . 136 Fi g u r e 36. The r e l a t i o n of the amount of food i n the gut, expressed as a percent of t h a t r e g u i r e d to achieve s a t i a t i o n , to the mean number of s t r i d e s (± 1 SE) over a 15 minute time p e r i o d . 35 Q O CH LU 0_ LU in Dl UJ D_ to UJ D o: h-00 350 280 2101 140 70 Y = 259-4-4-27X 0 10 20 30 40 HOURS OF FOOD DEPRIVATION 36 350 280 D O cn LU o_ LU I— E 210| in CH LU Q_ CO LU Q ce i— (O 140 70 Y = 87-2 + 2-10 X 20 40 60 80 PERCENT OF GUT SATIATION 137 i n v o l v e d i n making one l e g s t r o k e (the a c t u a l l e g movement i s much f a s t e r but unless the g e r r i d i s responding to an e x t e r n a l s t i m u l u s , a s l i g h t pause u s u a l l y f o l l o w s each s t r o k e ) , the percent of the t o t a l time (15 minutes) spent by the g e r r i d s i n moving, grooming, and remaining s t a t i o n a r y i n both the l a b o r a t o r y and the f i e l d can be c a l c u l a t e d (Table 16). In terms o f movement, no s i g n i f i c a n t d i f f e r e n c e was observed between G. buenoi,< G. n o t a b i l i s , G. i n c o g n i t u s and G. i n c u r v a t u s , and so these s p e c i e s are t r e a t e d as a group r a t h e r than i n d i v i d u a l l y . Mo s i g n i f i c a n t d i f f e r e n c e i n the amount of time spent grooming (2-4%) was observed between any of the s p e c i e s i n e i t h e r the l a b o r a t o r y or the f i e l d . DISCUSSION That a predator would p r e f e r to a t t a c k prey i n f r o n t of i t r a t h e r than behind i t i s not s u r p r i s i n g . However, the s i g n i f i c a n c e of why t h i s p r e f erence i n g e r r i d s i s only f o r angles r e g u i r i n g a t u r n of l e s s than 100°, even though they are capable of d e t e c t i n g prey items i r r e s p e c t i v e of t h e i r angular l o c a t i o n , i s not apparent from t h i s study. Murphey (1971a), however, demonstrated i n G. r e m i g i s t h a t the extent of remotion of the mesothoracic l e g c o n t r a l a t e r a l to the prey i s r e l a t i v e l y c o n s t a n t o n l y as long as the t u r n produced i s l e s s than approximately 90°, Turns i n excess of 90° o f t e n i n v o l v e two powerstroke movements of the l e g c o n t r a l a t e r a l to the t a r g e t , whereas a s i n g l e powerstroke i s c h a r a c t e r i s t i c when the t a r g e t 138 Table 16. Mean percent of the t o t a l time spent by Gerris species either moving, grooming or remaining stationary. Species G. remigis G. buenoi G. n o t a b i l i s G. incurvatus G. incognitus G. remigis G. buenoi G. n o t a b i l i s G. incurvatus G. incognitus Laboratory Moving (%) S tationary (%) 32.7 65.2 7.6 35.5 6.a 90.3 F i e l d 60.6 "\ 89.8 Grooming (%) 2. 1 3.8 139 d e v i a t i o n i s l e s s than 90°. I t would thus seem that i t i s the "double powerstroke" which i s avoided, perhaps owing t c the i n c r e a s e d time i n v o l v e d i n i n i t i a t i n g the a t t a c k . Regardless, however, of the reason, f u n c t i o n a l l y , a t t a c k s r e q u i r i n g a turn i n excess of ca. 100° are avoided. The p r e f e r e n c e by g e r r i d s f o r moving prey items over s t a t i o n a r y ones i s a l s o not i n i t s e l f s u r p r i s i n g , as seme d i s c r i m i n a t o r y c a p a b i l i t y must be present or g e r r i d s would a t t a c k any f l o a t i n g o b j e c t on the water s u r f a c e . T h i s could be very time-consuming i n areas where f l o a t i n g d e b r i s c o l l e c t . G e r r i d s possess a well-developed v i s u a l system with r e l a t i v e l y l a r g e eyes p r o j e c t i n g l a t e r a l l y from the head. T h i s suggests t h a t v i s i o n as well as water s u r f a c e v i b r a t i o n , which has already been documented as s u f f i c i e n t to i n s t i g a t e an a t t a c k (Murphey, 1971a), may be i n v o l v e d . F i e l d o b s e r v a t i o n s of G. r e m i g i s on streams support the r o l e of v i s i o n i n prey d e t e c t i o n . On a f a s t moving stream, with numerous s u r f a c e r i p p l e s present, g e r r i d s were observed to p o s i t i o n themselves i n and a g a i n s t the c u r r e n t , and to respond to every prey item that f l o a t e d by r e g a r d l e s s of i t s nature. P i e c e s of bark, bubbles, as w e l l as i n s e c t s , , were a l l approached and grasped, and i f i n e d i b l e , r e l e a s e d . Since the items approached were o f t e n f l o a t i n g p a s s i v e l y with the c u r r e n t , making no r i p p l e s on t h e i r own, i t seems u n l i k e l y that water s u r f a c e v i b r a t i o n was the a t t a c k i n g s t i m u l u s , e s p e c i a l l y i n l i e u of a l l the other r i p p l e s present.^ Whether t h i s i s unigue to stream s p e c i e s i s u n c e r t a i n , however, and on the calm water s u r f a c e of a pool, r i p p l e s may indeed be 140 the main s t i m u l u s r e g u i r e d . The s u r f a c e water bug V e l i a c a r i r a i , which a l s o occurs at the edge of streams, r e a c t s t o v i s u a l prey s t i m u l i only when they are accompanied by v i b r a t i o n s t i m u l i from the same d i r e c t i o n (Meyer, 1971a, 1971b). G e r r i d s , however, being l a r g e r and p o s s e s s i n g more powerful l e g s , can p o s i t i o n themselves i n areas of s t r o n g e r c u r r e n t and hence more t u r b u l e n t water, which may mean t h a t the s t i m u l u s r e q u i r e d i s not the same. However, here again, i t i s the e f f e c t r a t h e r than the mechanism which i s of prime importance. I r r e s p e c t i v e of the exact s t i m u l i which make moving prey p r e f e r r e d , r e c o g n i t i o n that a d i f f e r e n c e i n e l i c i t i n g an a ttack e x i s t s between moving and s t a t i o n a r y prey i s a l l t h a t i s r e g u i r e d f o r t h i s study. Thus, the concept of a v i s u a l angle determining r e a c t i v e d i s t a n c e , even though v i s i o n may not always be i n v o l v e d , i s f e l t s u i t a b l e f o r use. The v a l u e s used here were obtained d i r e c t l y from the o b s e r v a t i o n s , and hence a c c u r a t e l y d e s c r i b e the data. They did not o r i g i n a t e i n theory, u t i l i z i n g such parameters as ommatidia width, the r e f r a c t i v e index of the cornea, e t c . , which would make them dependent on the v a l i d i t y of the r o l e of v i s i o n i n prey d e t e c t i o n . I t should a l s o be noted that the v i s u a l angles presented do not i n d i c a t e the d i s t a n c e at which the g e r r i d becomes aware of a nearby g e r r i d , but r a t h e r the d i s t a n c e at which a g e r r i d f i r s t responds to a nearby g e r r i d . O p e r a t i o n a l l y , t h e r e f o r e , t h i s approach i s s u i t a b l e i n the r e a l i s t i c s i m u l a t i o n of a g e r r i d * s r e a c t i v e d i s t a n c e . That the mean angle the predator g e r r i d must subtend to a prey g e r r i d before e v a s i v e a c t i o n by the prey g e r r i d i s 141 commenced i s s l i g h t l y s m a l l e r than that which the prey must subtend to the predator before the predator w i l l a t t a c k i s a l s o of i n t e r e s t . F u n c t i o n a l l y , t h i s means that f o r the predator and prey g e r r i d s used i n t h i s study (predator = G. r e m i g i s ; prey = iJD£°3i}iJ:J?s) , the mean d i s t a n c e at which the prey w i l l be attacked by the predator i s 8.6 cm i f the prey i s moving and 2. 1 cm i f i t i s s t a t i o n a r y . However, the prey w i l l commence evasive a c t i o n when the a t t a c k i n g predator's mean d i s t a n c e from i t i s 4.2 cm. I f the predator i s a l r e a d y a t t a c k i n g , s i n c e i t s l i n e of att a c k w i l l present to the prey g e r r i d the pre d a t o r ' s s m a l l e s t subtenable v i s u a l angle, namely i t s width, e v a s i v e a c t i o n w i l l commence o n l y when the predator i s r e l a t i v e l y c l o s e . T h i s mean d i s t a n c e , 1.5 cm, however, i s u s u a l l y j u s t enough to allow s u c c e s s f u l escape. The f a c t that the prey g e r r i d u s u a l l y does escape i s a r e s u l t of s p e c i f i c escape behaviour. The escape movement from a s t a t i o n a r y p o s i t i o n i s of t e n an i n i t i a l movement at r i g h t angles to the predator's l i n e of a t t a c k , f o l l o w e d by a quick c i r c l i n g around behind the pred a t o r . Since g e r r i d s are unable to stop q u i c k l y cn the water s u r f a c e , such a movement r a p i d l y p l a c e s the prey g e r r i d i n a p o s i t i o n where i t i s u n l i k e l y to be again immediately a t t a c k e d , i e . behind the g e r r i d . I f the prey g e r r i d i s r e l a t i v e l y l a r g e , i t w i l l o f t e n achieve the same e f f e c t by l e a p i n g s t r a i g h t up i n t o the a i r , a l l o w i n g the predator to skim by beneath i t . T h i s , o f t e n f o l l o w e d by a few guick s t r o k e s , i s u s u a l l y enough to again r e s u l t i n a s u c c e s s f u l escape. When a t t a c k i n g very s m a l l prey, which are unable to outrun or outjump the predator g e r r i d , escapes u s u a l l y occur only as a r e s u l t of m i s c a l c u l a t i o n by the 142 predator. I f too much momentum i s gained, the predator may be unable to stop and may s l i d e r i g h t over the prey. The predator u s u a l l y then grasps seemingly randomly around the v i c i n i t y where i t stopped, and may or may not f i n d the prey which i s beneath i t . When a t t a c k i n g moving prey, the predator u s u a l l y a t t a c k s along .a l i n e of i n t e r c e p t . Since the prey, l i k e the predator, cannot maneuver s h a r p l y on the water s u r f a c e once moving, i t s only r e c o u r s e f o r escape i s to a l t e r i t s speed and hence the time of p o t e n t i a l i n t e r c e p t . T h i s can only be accomplished by a c c e l e r a t i n g , s i n c e slowing would r e s u l t i n almost c e r t a i n capture, as the predator c o u l d then slow ^.too and a l t e r i t s l i n e of a t t a c k . Regardless, however, of the e v a s i v e response of the prey, i t i s to the prey's advantage to wait as long as p o s s i b l e b e f o r e responding to a predator g e r r i d ' s c l o s e proximity or a t t a c k . I f i t were i n i t i a l l y s t a t i o n a r y and moved too soon, i t might be a t t a c k e d when otherwise i t would be bypassed. I f the a t t a c k i s v a l r e a d y launched, premature movement would allow the predator time to a d j u s t i t s l i n e of a t t a c k , and premature jumping would cause i t to land j u s t i n time to be caught. Hence, one would expect o n l y a s l i g h t d i f f e r e n c e to e x i s t between the angle at which a prey g e r r i d responds to a predator g e r r i d and the angle r e q u i r e d to e l i c i t an a t t a c k by a predator. It was observed that the prey had the advantage. T h i s suggests, as one might guess, that c a n n i b a l i s m i n g e r r i d s i s not a f e a t u r e which has been s e l e c t e d f o r . l a r g e r g e r r i d s , at l e a s t , seem to have evolved mechanisms which tend to decrease the p r o b a b i l i t y of a s u c c e s s f u l c a p t u r e . I t should be 143 noted, that except f o r cases where only a s l i g h t d i f f e r e n c e i n s i z e e x i s t s between the predator and the prey, making the main d i f f i c u l t y that o f subduing the prey, b e h a v i o u r a l mechanisms only appear to prevent c a n n i b a l i s m . No s p e c i f i c morphological s t r u c t u r e s or defenses are known which make g e r r i d s immune to a t t a c k by other g e r r i d s . As a r e s u l t , the g r e a t e r the r e l a t i v e s i z e d i f f e r e n c e between a prey g e r r i d and a predator g e r r i d , the gr e a t e r the p r o b a b i l i t y of a s u c c e s s f u l c a p t u r e , and hence c a n n i b a l i s m . The speed and jumping a b i l i t y of s m a l l e r g e r r i d s are much l e s s than that of l a r g e r g e r r i d s , making some ca n n i b a l i s m at l e a s t seemingly unavoidable. The e f f e c t of hunger on the s i z e of the r e a c t i v e f i e l d of G. r g m i g i s i s s l i g h t , but seems to be that of an i n c r e a s i n g l y l a r g e r r e a c t i v e f i e l d with i n c r e a s i n g hunger. Whether t h i s i s a v a l i d assumption f o r the other s p e c i e s of G e r r i s as well i s unknown. However, that a hungry animal would respond to prey at a g r e a t e r d i s t a n c e from i t than would a s a t i a t e d animal would seem reasonable, although the r a t e at which the r e a c t i v e f i e l d i n c r e a s e s i n s i z e may vary with the s p e c i e s . Thus, although the s i z e of the r e a c t i v e f i e l d i s d i r e c t l y p r o p o r t i o n a l to hunger, the s i z e of the r e a c t i v e f i e l d of a hungry animal r e l a t i v e to t h a t when i t i s s a t i a t e d i s not n e c e s s a r i l y c o n s t a n t . In an e f f o r t to determine how the s i z e of the r e a c t i v e f i e l d does vary with the s p e c i e s - i n s t a r s s t u d i e d , the e f f e c t of g e r r i d s i z e on t h i s parameter was i n v e s t i g a t e d . Leg l e n g t h was the measurement used to determine g e r r i d s i z e , both to be c o n s i s t e n t with the above-mentioned s i z e c l a s s e s and because i f 144 r i p p l e s were ind e e d t h e s t i m u l u s t h a t e l i c i t e d a t t a c k , the d i s t a n c e the l e g s were a p a r t , a f u n c t i o n of t h e i r l e n g t h , might be t h e c r i t i c a l parameter a l l o w i n g o r i e n t a t i o n t o the F r e y . P r e d a t o r s i z e d i d a f f e c t the d i s t a n c e a t which a s t a n d a r d - s i z e prey i t e m would be a t t a c k e d , w i t h l a r g e r g e r r i d s a t t a c k i n g the prey i t e m at a g r e a t e r d i s t a n c e ( s m a l l e r v i s u a l angle) than s m a l l e r g e r r i d s . Whether t h i s i s a r e s u l t of t h e s m a l l e r g e r r i d s 1 b e i n g unable t o d e t e c t , o r a t l e a s t o r i e n t t o , the f a i n t e r r i p p l e s (as t h e i r l e g s a r e c l o s e r t o g e t h e r ) , or t h e r e s u l t of a complex v i s u a l mechanism i n v o l v i n g some type o f depth p e r c e p t i o n i s not c l e a r . C e r t a i n l y , t h e concept t h a t t h e r e i s a " v i s u a l a n g l e the prey must subtend b e f o r e an a t t a c k i s e l i c i t e d " i s an o v e r - s i m p l i f i c a t i o n . T h i s s t u d y i n d i c a t e s t h a t o t h e r v a r i a b l e s a r e i n v o l v e d . N e v e r t h e l e s s , r e g a r d l e s s of t h e mechanism, t h i s s t u d y shows t h a t g e r r i d s i z e does i n f l u e n c e the s i z e of the r e a c t i v e f i e l d . The much g r e a t e r a c t i v i t y o b s e r v e d by G. r e m i g i s over the o t h e r f o u r s p e c i e s presumably r e f l e c t s the m o d i f i c a t i o n s which have e v o l v e d t o a l l o w t h i s s p e c i e s t o use the stream h a b i t a t . However, by i t s e l f , i n c r e a s e d a c t i v i t y i s not s u f f i c i e n t t o remain i n p o s i t i o n on a stream. The a c t i v i t y must be d i r e c t e d a g a i n s t the c u r r e n t : such r h e c t r o p i c a c t i v i t y i s suggested i n t h i s s t u d y f o r t h i s s p e c i e s . That t h i s i n c r e a s e d a c t i v i t y i s not j u s t i n r e s p o n s e t o c u r r e n t , i s i n d i c a t e d by the f i e l d o b s e r v a t i o n s of G. r e m i g i s on p o o l s where the water s u r f a c e was not moving (as opposed t o the l a b o r a t o r y s t u d i e s where the water s u r f a c e was moving owing t o the presence of an a i r s t o n e ) . 145 Almost i d e n t i c a l p e r i o d s of time were devoted to movement under the f i e l d and l a b o r a t o r y c o n d i t i o n s . A c t i v i t y would thus seem to be the r e s u l t of an i n t e r n a l d r i v e , r a t h e r than a response to the e x t e r n a l environment. That the l a t t e r can and does i n f l u e n c e a c t i v i t y , however, i s shown by the d i r e c t e d nature of t h i s a c t i v i t y on a stream. Since G. r e m i g i s , being a stream s p e c i e s , i s somewhat unigue when compared to the f o u r pond s p e c i e s i n our study area, whether these other s p e c i e s respond to i n c r e a s i n g hunger i n the same manner as does G. r e m i g i s i s open to g u e s t i c n . G. r e m i g i s shows a s i g n i f i c a n t decrease i n o v e r a l l a c t i v i t y with i n c r e a s i n g hunger, but t h i s may simply r e p r e s e n t c o n s e r v a t i o n of energy. The maintainance of p o s i t i o n i n a f a s t - f l o w i n g stream would probably i n v o l v e a c o n s i d e r a b l e expenditure of energy. However, as the r e a c t i v e d i s t a n c e i s not s i g n i f i c a n t l y i n c r e a s e d with i n c r e a s i n g hunger, but r a t h e r remains r e l a t i v e l y constant r e g a r d l e s s of hunger, t h i s may simply r e p r e s e n t a change i n hunting s t r a t e g y with i n c r e a s i n g hunger. I t may change from an a c t i v e s a t i a t e d predator which can a f f o r d time to search f o r a mate, an o v i p o s i t i o n s i t e , or whatever, u t i l i z i n g what food items i t encounters on the way, to a hungry one which waits f o r and ambushes i t s prey. I f the above i s t r u e , then r e g a r d l e s s of the s p e c i e s of g e r r i d , i n c r e a s i n g hunger could r e s u l t i n decreased a c t i v i t y as the same c o u l d apply to pond s p e c i e s . T h i s i s assumed to be the case f o r the other s p e c i e s d i s c u s s e d i n t h i s study, even i f the above suggested e x p l a n a t i o n i s not completely accurate: no i n c r e a s e i n a c t i v i t y of the other four 146 s p e c i e s with i n c r e a s i n g hunger seemed apparent. A s i m i l a r decrease i n a c t i v i t y with i n c r e a s i n g hunger has been r e p o r t e d f o r the l a r v a of the d r a g o n f l y Aeschna cy.anea (Etienne, 1972) and f o r the l y c o s i d s p i d e r , Pardosa Vancouver! (M. Hardman, pers. comm.), which a l s o behaves as an ambush predator. In c o n t r a s t , we may note t h a t i t i s with i n s e c t s that are not predaceous, such as the b l o w f l y Phorjnia r e g i n a (Green, 1964) and the gypsy moth P o r t b e t r i a d i s p a r (Leonard, 1970), t h a t one sees an i n c r e a s e d r a t e of locomotion with i n c r e a s i n g hunger. The e f f e c t of s i z e on the p r o p e n s i t y to move i n g e r r i d s i s not c l e a r , although the r e s u l t s suggest that s m a l l e r g e r r i d s move l e s s than l a r g e r g e r r i d s . T h i s agrees with the c o n c l u s i o n s suggested on the b a s i s of r e a c t i v e d i s t a n c e . Small g e r r i d s , being unable to outrun or outjump l a r g e g e r r i d s , can most s u c c e s s f u l l y avoid being c a n n i b a l i z e d by remaining s t a t i o n a r y as much as p o s s i b l e . There would thus seem to be a s e l e c t i v e advantage f o r s m a l l g e r r i d s to move l e s s . Combining the s t r i d e length and the frequency of movement of the d i f f e r e n t s i z e c l a s s e s of g e r r i d s with the s p e c i e s -p r o p e n s i t y t o move a l l o w s c a l c u l a t i o n of the mean d i s t a n c e each of the s p e c i e s - i n s t a r s would be l i k e l y t o move i n a given time p e r i o d (Table 17). T h i s can be expressed as a r a t e i f i t i s assumed that the g e r r i d s are always moving throughout the time p e r i o d . Combining the d i s t a n c e moved with the r e a c t i v e f i e l d width, allows determination of the mean area of the swath or "encounter path" d i f f e r e n t g e r r i d s would cover as they moved acros s the water s u r f a c e (Table 18). 147 Table 17. The mean distance (m) which each of the species-i n s t a r s would be l i k e l y to move i n one hour. Insttar G. n o t a b i l i s G. remigis Larva 1 2.82 11.03 •« 2 3.35 13. 10 «' 3 4.37 16.84 » 4 6.00 22.73 " 5 9.23 31.49 Adult 6 14.18 37.84 Species G. incognitus G. buenoi G. incurvatus 1.51 2.93 2.0 1.70 3.29 2.3 1 .93 3.91 3.0 2.31 4.80 3.4 2.78 6.23 3.9 3. 37 7.63 4.6 148 T a b l e 18. The mean a r e a o f t h e swath (m 2), or "e n c o u n t e r p a t h " , t h e v a r i o u s s p e c i e s - i n s t a r s would c o v e r per hour as they moved a c r o s s the water s u r f a c e , assuming a prey s i z e of 2 mm {an average s i z e o f t e n observed i n the f i e l d ) . I n s t t a r S p e c i e s G. n o t a b i l i s G. i n c o g n i t u s G. bu e n o i G. r e m i g i s G. i n c u r v a t u s L a r v a 1 0. 1190 0.4654 0.0625 0.1213 0.0861 it 2 0.1440 0.5633 0.0717 0.1388 0.0975 n 3 0. 1967 0.7578 0.0830 0.1681 0. 1312 H 4 0.2880 1.0910 0.1016 0.2160 0. 1505 II 5 0.5003 1.7068 0. 1273 0.2990 0. 1800 A d u l t 6 0.8678 2.3158 0.1624 0.3678 0.2241 149 The basic parameters necessary for modelling the rate of prey encounter, i f the prey d i s t r i b u t i o n i s known, are thus available f o r the f i v e species of Gerris considered in t h i s pa per. 150 SECTION 5. GESRIDS AS PREDATORS AND PREY INTRODUCTION As predators, gerrids are unigue i n that their habitat, the water surface, i s a trap for most other i n s e c t s , not only for t e r r e s t r i a l forms landing from above but also for the aquatic ones emerging from below. Gerrids prey on whatever i s available on the water surface but prefer l i v e over dead prey and are adapted to detecting movement which they use as cues i n id e n t i f y i n g p o t e n t i a l prey items (see Section 4). Owing to the nature of the habitat and the fact that gerrids are more or less confined to the water surface, they are forced to be dependent for food on the items occurring i n the water surface trap. In certain s i t u a t i o n s , p a r t i c u l a r l y in mid-summer when gerrid numbers reach a peak and when small ponds dry up or become smaller, considerable crowding can occur. Under circumstances such as t h i s , and cannibalism may become s i g n i f i c a n t . This section i s primarily concerned with the predatory c a p a b i l i t i e s of gerrids with particular emphasis on siz e relationships and species i n t e r a c t i o n s . However, f i e l d observations of the a v a i l a b i l i t y of potential prey ether than gerrids, f i e l d hunger l e v e l s of the g e r r i d s , and the effectiveness of potential insect predators other than gerrids preying on gerrids are also considered. 151 MATERIALS AND METHODS 1. FIELD HUNGER LEVELS The same b a s i c procedures as were used i n S e c t i o n 3 i n determining the amounts of food i n g e s t e d by g e r r i d s were used here. The wet weight of D r o s o p h i l a was determined p r i o r to f e e d i n g , and f o l l o w i n g f e e d i n g , the f l i e s were d r i e d and reweighed. C o n t r o l s were used to determine the e f f e c t of experimental procedures, and on the b a s i s of these data, an expected t h e o r e t i c a l dry weight percentage of the i n i t i a l wet weight was determined. Any d e v i a t i o n i n dry weight f o l l o w i n g f e e d i n g from t h i s expected t h e o r e t i c a l dry weight was considered the amount eaten. Experimental procedures were as f o l l o w s : g e r r i d s c o l l e c t e d on Marion L. were immediately placed i n separate c o n t a i n e r s and fe d a known g u a n t i t y of D r o s o p h i l a . Exposure to the f l i e s was fo r 2 hr, and at the end of t h i s time, the f l i e s were removed and immediately taken back to the l a b o r a t o r y where they were d r i e d . The volume of food eaten by each g e r r i d s p e c i e s i n s t a r was then compared with the maximum amount eaten by the s p e c i e s i n s t a r when maximally s t a r v e d (see S e c t i o n 3 ) . F i e l d hunger l e v e l s are thus expressed i n terms of the percent of the gut th a t i s f i l l e d with food. 152 2. GERRIDS AS PREDATOR AND PREY A. The s i z e classes of gerrids ^ With f i v e species of G e r r i s , each with adult and f i v e l a r v a l i n s t a r s , the present study involves 30 categories of gerrids altogether. In order to reduce the number of experiments in t h i s study, ten " s i z e classes" were established as i n Section 4 based on leg length, a s i t was noted that t h i s parameter often was the c r i t i c a l one in determining whether an attack would lead to a k i l l . In struggles between gerrids of s i m i l a r s i z e , k i l l s usually require the immobilization of the prey and t h i s can only be achieved by f l i p p i n g the prey gerrid over onto i t s back. Longer legs i n the prey provide greater s t a b i l i t y on the water surface, hence reducing the probability of being k i l l e d . The ten size classes used i n t h i s study are thus the same as those used in Section 4. B. E f f e c t of species and hunger l e v e l on predation F i f t h instar larvae and adults of each of the f i v e gerrid species were used as predators to determine i f species differences i n predatory behaviour e x i s t . Only these instars were used, as preliminary experiments suggested that only the larger s i z e instars were capable of k i l l i n g e f f i c i e n t l y more than three size classes of gerrids. T r i a l s consisted of placing both a predator and a prey gerrid in a partly w a t e r - f i l l e d container, and then observing 153 the a c t i o n f o r a f i f t e e n - m i n u t e p e r i o d . In each t r i a l , the number of a t t a c k s , d e f i n e d here as a guick, d a r t i n g movement towards the prey, was r e c o r d e d , as was the time of k i l l i f t h i s occurred. Owing to the l a r g e number of g e r r i d s r e g u i r e d , the predator and prey were separated as soon as p o s s i b l e a f t e r i t was observed that the prey g e r r i d was indeed caught. When no f e e d i n g o c c u r r e d , predator g e r r i d s could be used twice cn a s i n g l e day, although they were never used the second time with the same s i z e c l a s s as was used i n the f i r s t t r i a l . Prey g e r r i d s were reused r e p e t i t i v e l y u n t i l they were e i t h e r c r i p p l e d or k i l l e d by a predator g e r r i d . Container s i z e was determined by the s i z e of the prey g e r r i d : i f i t was of s i z e c l a s s 3 or s m a l l e r , t r i a l s were i n a c l e a r p l a s t i c c o n t a i n e r 9.5 cm i n diameter and 7 cm deep, whereas i f the prey g e r r i d was of a l a r g e r s i z e c l a s s , t r i a l s were i n a c l e a r p l a s t i c c o n t a i n e r 26 cm i n diameter and 9 cm deep. Various predator hunger l e v e l s were obtained by s t a r v i n g g e r r i d s f o r known p e r i o d s of time. S t a r v a t i o n p e r i o d s of 48, 24 and 5 hr were used t o a t t a i n an empty gut, a gut 1/3 f i l l e d with food, and a gut 2/3 f i l l e d with food r e s p e c t i v e l y i n adult g e r r i d s . S t a r v a t i o n p e r i o d s of 24 and 6 hr were used to a t t a i n an empty gut and a gut 1/3 f i l l e d with food r e s p e c t i v e l y i n f i f t h i n s t a r l a r v a . These s t a r v a t i o n p e r i o d s have been shown s u f f i c i e n t to achieve the above hunger l e v e l s (see S e c t i o n 3). Prey g e r r i d s were taken from s i z e c l a s s e s 1 to 7. Hunger l e v e l s of prey g e r r i d s v a r i e d , as the same g e r r i d might have been used as both a prey and a predator on the same day. 15a Predator or prey g e r r i d s were used as they became a v a i l a b l e or were needed. However, g e r r i d s s t i l l pale i n c o l o u r from t h e i r l a s t moult or o b v i o u s l y about to moult were not used. A l l o b s e r v a t i o n s were made with a c c l i m a t e d g e r r i d s at 26°C. i n a c o n t r o l l e d environment room. C. R e l a t i v e predator-prey s i z e and k i l l success Procedures used here were i d e n t i c a l to those used above. However, i n order to determine how prey s i z e r e l a t i v e to predator s i z e i n f l u e n c e d capture s u c c e s s , i n s t e a d of j u s t f i v e s i z e c l a s s e s being used as predator, g e r r i d s from each s i z e c l a s s were presented with g e r r i d s from every s m a l l e r s i z e c l a s s and with g e r r i d s t h e i r own s i z e . G e r r i d s at only one hunger l e v e l , an empty gut, were used, as f o r most of the s p e c i e s , t h i s s t a t e maximized predatory success. In an e f f o r t to determine how g e r r i d s as prey compared with other prey items, the same o b s e r v a t i o n s as above were repeated with each of the g e r r i d s i z e c l a s s e s as predator and g r o s g p h i l a as prey. A weighted a n a l y s i s of the data was again used to c o r r e c t f o r s p e c i e s d i f f e r e n c e s . D. Environmental complexity and k i l l s uccess Predator-prey s i z e c l a s s combinations used i n t h i s part of the study were those i n which the predator g e r r i d s were ab l e to e f f i c i e n t l y c a pture and k i l l the prey g e r r i d s , with e f f i c i e n t l y being a r b i t r a r i l y d e f i n e d as more than f i v e k i l l s per 100 155 a t t a c k s . Thus, approximately equal numbers of t r i a l s (16) of 24 s i z e c l a s s combinations were observed i n each of f o u r " h a b i t a t s " : open water, "reeds" 2.54 cm a p a r t , "reeds" 5.08 cm apart and a l g a e - c l u t t e r e d water. a l l t r i a l s took p l a c e i n a p a r t l y w a t e r - f i l l e d c o n t a i n e r 26 cm i n diameter and 9 cm deep. Clean water was used i n a l l the h a b i t a t s except the l a s t , where algae clumps broke the water s u r f a c e . The water i n t h i s l a t t e r " h a b i t a t " was a l s o l e f t to stand from day to day, r e s u l t i n g i n the formation of a s l i g h t o i l y f i l m on the water s u r f a c e s i m i l a r t o t h a t observed i n n a t u r a l a l g a e - f i l l e d p o o l s . The "reeds" used were p i p e t t e s 14.5 cm long and 7 mm i n diameter painted green with a nontoxic p l a s t i c enamel. These were placed i n v e r t e d i n holes bored i n a p l e x i g l a s s d i s c which was p l a c e d i n the bottom of the c o n t a i n e r . The spacing of the holes thus determined the d i s t a n c e between the reeds, which were placed on a l l the i n t e r s e c t i o n s of e i t h e r a 2.54 cm or a 5.08 cm g r i d p a t t e r n . | Observations c o n s i s t e d of p l a c i n g the g e r r i d s i n the c o n t a i n e r and then r e c o r d i n g every f i f t e e n minutes f o r one hour whether or not a k i l l had o c c u r r e d . Owing to the very l a r g e number of t r i a l s observed (1579), i n d i v i d u a l o b s e r v a t i c n c f each c o n t a i n e r f o r the whole period was not p r a c t i c a l . As i n the p r e v i o u s experiments, a l l o b s e r v a t i o n s were made i n a c o n t r o l l e d environmental room at 26°C. under normal room l i g h t i n g c o n d i t i o n s . In an e f f o r t to determine how the presence of l i g h t a f f e c t e d g e r r i d p r e d a t i o n , 48 t r i a l s using the same procedures 156 as above were observed between size classes 10 and 2 in the "dark". , Standard red photographic darkroom lamps were used to provide i l l u m i n a t i o n . Normally, under white l i g h t , movement of the hand towards a gerrid resulted in an escape movement on the part of the g e r r i d . No such response was observed under the red l i g h t , even when the hand was only a few millimeters from the g e r r i d . This was f e l t to indicate that gerrids were unable to detect the l i g h t wavelength used. For functional purposes, then, the gerrids were in the dark. E. Effect of predator choice of prey on predation To determine i f predator gerrids p r e f e r e n t i a l l y select p a r t i c u l a r prey size classes among those which the predator gerrids were found to k i l l e f f i c i e n t l y , adult gerrids of a l l f i v e species were presented with these size classes simultaneously. adult gerrids only were chosen, as they k i l l e f f i c i e n t l y the greatest number cf prey size classes. Three prey r a t i o s were studied, the f i r s t with only one gerrid from each prey s i z e class present, i e . with numbers of each prey size class equal, and the second and t h i r d with the numbers of each prey size class not equal as shown in Table 19. adults of each species were tested separately in the f i r s t s i t u a t i o n , but owing to a shortage of prey g e r r i d s , i n the second and t h i r d t e s t s , the adult gerrids were lumped in their respective size classes (size class 10 comprised adult G. n o t a b i l i s and adult G. remigis and siz e class 8 comprised adult G. iJ2co<jnitus, adult G. incurvatus and adult G. buenoi) and these were tested against 157 Table 19. The number of prey presented from each of the prey s i z e c l a s s e s used i n determining which prey s i z e c l a s s a predator g e r r i d p r e f e r r e d when the prey types were presented i n unequal numbers. A. Prey weighted on b a s i s of l e g l e n g t h . B. Prey weighted on the b a s i s o f wet weight. , ft. Prey s i z e Number c l a s s s m a l l l a r g e a d u l t s a d u l t s 1 2 3 4 5 6 7 4 2 2 1 1 5 4 2 2 2 1 1 B. Prey s i z e Number c l a s s s m a l l l a r g e a d u l t s a d u l t s 1 2 3 4 5 6 7 8 3 1 1 7 3 2 1 1 158 the prey. Equal numbers of each predator s p e c i e s w i t h i n a s i z e c l a s s were used to p r o v i d e a mean s i z e c l a s s response i r r e s p e c t i v e of s p e c i e s . In the f i r s t and second t e s t s , , prey s i z e c l a s s e s 1-7 were used with a d u l t G. n o t a b i l i s and G. r e m i g i s , whereas prey s i z e c l a s s e s 1-5 were used with the other three s p e c i e s . In the t h i r d t e s t , i t was not p r a c t i c a l to present a l l the s i z e c l a s s e s the predator g e r r i d s were known t o k i l l e f f i c i e n t l y owing to the l a r g e biomass d i f f e r e n c e s between the prey gerrid»s weights: 54 s i z e c l a s s 1 g e r r i d s alone equal the weight of only one s i z e c l a s s 7 g e r r i d , and f o r s i z e c l a s s 10 predator g e r r i d s , a t o t a l o f 79 prey g e r r i d s would have had to have been p r e s e n t . Not enough prey g e r r i d s were a v a i l a b l e to allow t h i s and so f o r s i z e c l a s s 10 p r e d a t o r s , o n l y prey s i z e c l a s s e s 3-7 were presented as prey. For s i z e c l a s s 8 predators, prey s i z e c l a s s e s 2-5 were presented as prey. G e r r i d s were deprived of food f o r 48 hr before t e s t i n g commenced and a l l t e s t i n g was undertaken i n the l a r g e c l e a r p l a s t i c c o n t a i n e r s d e s c r i b e d above a t 26°C. i n a c o n t r o l l e d environment room. 3. OTHER INSECT PREDATORS OF GERRIDS i F i e l d o b s e r v a t i o n s suggested that of a l l the predators present i n the f i e l d s i t u a t i o n , only g y r i n i d s and no t o n e c t i d s would l i k e l y be major p r e d a t o r s of g e r r i d s . Beth i n s e c t s 159 c o e x i s t with g e r r i d s i n the f i e l d and have o c c a s i o n a l l y been observed to c a t c h g e r r i d s , & few g e r r i d s have been observed caught i n s p i d e r webs hung low over the water s u r f a c e and one g e r r i d (adult G, n o t a b i l i s ) was observed s t i l l a l i v e a f t e r being caught by a l a r g e odonate l a r v a , but such p r e d a t i o n would seem r a r e . Thus, the e f f e c t i v e n e s s of only g y r i n i d s and n o t o n e c t i d s as g e r r i d p r e d a t o r s was i n v e s t i g a t e d i n the l a b o r a t o r y . The same procedures as were used above i n determining the e f f e c t of r e l a t i v e p redator-prey s i z e on p r e d a t i o n were used here. a d u l t g y r i n i d s , f o u r t h i n s t a r n o t o n e c t i d l a r v a e and adult n o t o n e c t i d s were c o l l e c t e d i n the f i e l d , and a l l three predator types were s t a r v e d f o r 48 hr a t 26°C. before t e s t i n g commenced. G y r i n i d s , owing to t h e i r r a p i d movement, were t e s t e d i n the l a r g e p l a s t i c c o n t a i n e r s , whereas the n o t o n e c t i d s were t e s t e d i n the s m a l l p l a s t i c c o n t a i n e r s . T e s t i n g , as above, c o n s i s t e d of the p l a c i n g o f a predator and a prey g e r r i d i n the c o n t a i n e r , and then r e c o r d i n g the number of a t t a c k s made i n a 15-minute p e r i o d and whether or not a k i l l was made. Each o f the three p r e d ator types i n v e s t i g a t e d , g y r i n i d s , f o u r t h i n s t a r n o t o n e c t i d l a r v a e and a d u l t n o t o n e c t i d s , were t e s t e d with as many s i z e c l a s s e s of g e r r i d s as was p o s s i b l e . 160 RESULTS 1. FIELD FOOD AVAILABILITY AND GERRID HUNGER LEVELS Many of the a q u a t i c p l a n t s support aphid p o p u l a t i o n s and aphids were o f t e n observed on the water s u r f a c e . In the f i e l d s i t u a t i o n a very heavy aphid i n f e s t a t i o n (Bhopalosiphon n_ymj3haeae (I.).') on Nuphar and Potamogeton on Marion I. i n 1971 was observed, and i t was hard to see how prey c o u l d be l i m i t i n g f o r the small g e r r i d s at l e a s t during t h i s p e r i o d . Large numbers o f collembolans were a l s o always observed on the water s u r f a c e , and although very m o t i l e when a l i v e , the turnover r a t e of these s m a l l i n s e c t s might be q u i t e high. The main reasons f o r s u g g e s t i n g t h a t food f o r the small g e r r i d s at l e a s t i s not l i m i t i n g , however, come from the g e r r i d s themselves. Feeding g e r r i d s c o n s t i t u t e d l e s s than 1% of the t o t a l number of g e r r i d s c o l l e c t e d on s t i l l water bodies, and hundreds were c o l l e c t e d (see S e c t i o n 1 ) . Laboratory o b s e r v a t i o n s have shown that hungry g e r r i d s are tenac i o u s when f e e d i n g , and i f the prey i s small enough to be held by the f o r e l e g s , the g e r r i d can be handled p h y s i c a l l y without i t r e l e a s i n g the prey. R e l a t i v e l y s a t i a t e d g e r r i d s , i n c o n t r a s t , can be e a s i l y induced to leave the prey. Thus, the low numbers of f e e d i n g g e r r i d s c o l l e c t e d on ponds when food was known to. be present would seem t o suggest t h a t i f the g e r r i d s were not s a t i a t e d , they were a t l e a s t not p a r t i c u l a r l y hungry, as t h e i r 161 prey were not r e t a i n e d i n any number. G e r r i d s on streams (G. remigis) ' do not seem to have q u i t e t h i s abundance of food, however, as g e r r i d s i n t h i s h a b i t a t were more f r e q u e n t l y c o l l e c t e d c a r r y i n g prey items. That hunger l e v e l of the g e r r i d s may vary with temperature i s suggested by the data i n Table 20. When temperatures were low, the g e r r i d s were s a t i a t e d , but when temperatures were high (> 2 0 ° C ) , they f e d , i n d i c a t i n g t h a t the g e r r i d s were hungry. 2. GERBIDS IS PREDATOR AND PREY A. E f f e c t of s p e c i e s and hunger on p r e d a t i o n Figure 37 i l l u s t r a t e s the e f f e c t of hunger on both t o t a l percent k i l l (aggressiveness) and number of k i l l s per 100 a t t a c k s ( k i l l e f f i c i e n c y ) f o r a d u l t s of each of the f i v e s p e c i e s of G e r r i s s t u d i e d . Each value f o r each s p e c i e s at each hunger l e v e l r e p r e s e n t s the weighted mean value d e r i v e d from a l l the prey s i z e s which that p a r t i c u l a r s i z e a d u l t can k i l l e f f i c i e n t l y (see below). Thus, a d u l t G. buenoi, G. i n c o g n i t u s and G. i n c u r v a t u s , being of s i z e c l a s s 8, can k i l l e f f i c i e n t l y prey s i z e c l a s s e s 1 to 5, and so a l l these s i z e c l a s s e s were used. A d u l t G. n g t a b i l i s and G. r e m i g i s , on the other hand, being of s i z e c l a s s 10, can k i l l e f f i c i e n t l y the f i r s t seven s i z e c l a s s e s and so t h e i r mean values are c a l c u l a t e d from seven va l u e s . S i n c e the a d u l t s of the f i v e s p e c i e s are of d i f f e r e n t s i z e s , i t 162 Table 20. G e r r i d hunger l e v e l s i as the percent of the maximum gut Date L o c a t i o n n May 14/71 Marion L. 32 M ay 28/71 Marion L. 22 June 18/7 1 Marion L. 24 J u l y 16/71 Marion L. 15 J u l y 30/72 Marion L. 12 J u l y 30/72 Gate Pd. 10 the f i e l d s i t u a t i o n expresses c a p a c i t y that was empty. % gut empty Temperature <°c.) -12. 1 ± 3. 1 9.0 24.1 ± 8.6 16.5 -14.0 ± 3.2 14.5 43. 3 t 9.5 20.0 73.5 ± 9. 1 23.0 71.5 ± 11.7 24.0 1 63 F i g u r e 37. The t o t a l percent k i l l and percent k i l l per a t tack observed f o r each of the f i v e spec ie s at four hunger l e v e l s . 163a 164 was f e l t t h a t only through comparison of t h e i r o v e r a l l preformance on a l l t h e i r p o t e n t i a l prey items could a r e a l i s t i c comparison of t h e i r predatory a b i l i t i e s be a s c e r a t i n e d , s i n c e f o r any one s i z e c l a s s , r e l a t i v e predator-prey s i z e has a s i g n i f i c a n t e f f e c t on the extent of p r e d a t i o n . C o n s i d e r i n g each s p e c i e s s e p a r a t e l y , G. n o t a b i l i s , G. iS£2r.vatus and G. i n c o g n i t u s a l l showed s i g n i f i c a n t d i f f e r e n c e s i n a g g r e s s i v e n e s s over the th r e e hunger l e v e l s t e s t e d . There was no s t a t i s t i c a l d i f f e r e n c e between any of the s p e c i e s i n number of k i l l s per 100 a t t a c k s over the hunger l e v e l s t e s t e d (Table 21 )b ), S t a t i s t i c a l t e s t s on the e f f e c t of hunger d i d not i n c l u d e those v a l u e s where the g e r r i d was s a t i a t e d , as at s a t i a t i o n , none of the s p e c i e s w i l l a t t a c k prey. The r e s u l t s i n d i c a t e , then, t h a t f o r hungry g e r r i d s , two b a s i c p a t t e r n s are present. E i t h e r a g g r e s s i v e n e s s d e c l i n e s s h a r p l y a t a moderate hunger l e v e l , i e . between when the gut i s 1/3 f u l l and when i t i s 2/3 f u l l , or i t d e c l i n e s s h a r p l y at a lower hunger l e v e l , when the gut i s more than 2/3 f u l l . G. r e m i g i s and G. buenoi seem to k i l l the same percent of prey presented over most s t a t e s of hunger whereas the other three s p e c i e s do not. The s i t u a t i o n i s d i f f e r e n t , however, with r e s p e c t t o k i l l e f f i c i e n c y , or number of k i l l s per 100 a t t a c k s . G. r e m i g i s and G. i n c o g n i t u s k i l l most e f f i c i e n t l y when the gut i s only 1/3 f u l l , whereas the other s p e c i e s tend to k i l l most e f f i c i e n t l y when maximally s t a r v e d . Table 22 and Table 23 present the r e s u l t s o f s t a t i s t i c a l 165 Table 21. F-values f o r the e f f e c t of hunger on a r c s i n t o t a l percent k i l l and a r c s i n percent k i l l per a t t a c k f o r a d u l t s of each of the f i v e s p e c i e s of G e r r i s . * = s i g n i f i c a n t d i f f e r e n c e with 5% l e v e l of p r o b a b i l i t y S p e c i e s flrcsin % k i l l A r c s i n % k i l l / a t t a c k G. n o t a b i l i s G. r e m i g i s G. buenoi G, i n c u r v a t u s G. i n c o g n i t u s 12.439* (2/18) 1.708 (2/18) 0.025 (2/12) 5.971* (2/11) 4.672* (2/11) 2.448 (2/18) 2. 204 (2/18) 0.435 (2/12) 0. 434 (2/11) 3.170 (2/11) 166 Table 22. F-values between species pairs for arcsin t o t a l percent k i l l by adults at each of the three hunger leve l s tested. * = s i g n i f i c a n t difference with 5% l e v e l of probability Arcsin % k i l l 48 hr starved S B B IV IG N - 4.037 (1/12) 0.046 (1/10) 0.922 (1/10) 0.154 (1/10) B - - 2.684 (1/10) 5.785*(1/10) 1.519 (1/10) B - - - 0.967 ( 1/8) 0.015 (1/8) IV - - - 1.085 (1/8) IG -24 hr starved H - 11.0.9* (1/12) 0.030 (1/10) 0.095 ( 1/10) 0.426 (1/10) H - - 8.437*(1/10) 8.144* (1/10) 2.553 (1/10) B - 0. 174 (1/8) 0.428 (1/8) IV - - - - 0,117 (1/8) IG -5 hr starved K - 27.802* (1/12) 5.131* (1/10) 0.038 (1/9) 0.601 (1/9) B - - 0.255 (1/10) 29.783* (1/9) 29.871* (1/9) B - 3. 104 (1/7) 2.375 (1/7) IV - - - 1.000 (1/6) 167 T a b l e 23. F-values between s p e c i e s p a i r s f o r a r c s i n percent k i l l per a t t a c k b j a d u l t s at each of the th r e e hunger l e v e l s t e s t e d . * = s i g n i f i c a n t d i f f e r e n c e with a 55? l e v e l of p r o b a b i l i t y . A r c s i n % k i l l / a t t a c k 48 hr sta r v e d , N R B IV IG N - 0.311 (1/12) 1.998 (1/10) 0.008 (1/10) 0.377 (1/10) R - 1. 308 (1/10) 0.010 (1/10) 2.655 (1/10) B - 0.783 (1/8) 4.617 (1/8) IV - - - - 0.738 (1/8) IG - - - - - , 24 hr s t a r v e d N - 1.345 (1/12) 0.031 (1/10) 0.028 (1/10) 0. 139 (1/10) R - 1.470 (1/10) 3.272 (1/10) 0.407 (1/10) B - 0.094 ( 1/8) 0.234 (1/8) IV - - - - 0.774 (1/8) IG -5 hr s t a r v e d N - 10.066*(1/12) 2.224 (1/10) 0.425 (1/9) 5.762*(1/9) R - 0.868 (1/10) 4. 399 (1/9) 0.431 (1/9) B - - - 0.510 (1/7) 0.984 (1/7) IV - - - 2.364 (1/6) 168 t e s t s between p a i r s of s p e c i e s f o r aggressiveness and k i l l e f f i c i e n t l y r e s p e c t i v e l y at the three hunger l e v e l s . G. r e m i g i s was s i g n i f i c a n t l y d i f f e r e n t from G. i n c u r v a t u s a t a l l three hunger l e v e l s with r e s p e c t to aggressiveness, and with the gut only 1/3 f u l l , was a l s o s i g n i f i c a n t l y d i f f e r e n t from G. n o t a b i l i s and G. i n c o g n i t u s . The only s i g n i f i c a n t d i f f e r e n c e s with r e s p e c t to k i l l e f f i c i e n c y were between G. n o t a b i l i s and both G. r e m i g i s and G. i n c o g n i t u s when the gut was 1/3 f u l l . T a ble 24 presents the mean values of a g g r e s s i v e n e s s and k i l l e f f i c i e n c y f o r the s i z e range of prey items that f i f t h i n s t a r l a r v a e of each of the s p e c i e s can c a p t u r e . Table 25 i n d i c a t e s t h at s i g n i f i c a n t h e t e r o g e n i t y e x i s t s between the s p e c i e s , with l a r v a l G. r e m i g i s k i l l i n g about twice the number of prey items t h a t l a r v a l G. n o t a b i l i s , G. i n c u r v a t u s and G. buenoi k i l l e d . L a r v a l G. i n c o g n i t u s were a l s o more a g g r e s s i v e than the l a r v a e of the l a t t e r s p e c i e s , although not g u i t e t c the extent as were l a r v a l G. r e m i g i s . No s i g n i f i c a n t d i f f e r e n c e s i n k i l l e f f i c i e n c y were noted between l a r v a e of any of the s p e c i e s , although l a r v a l G. i n c u r v a t u s and G. i n c o g n i t u s were l e s s e f f i c i e n t than the l a r v a e of the ether s p e c i e s i n c a p t u r i n g prey items. The ranking of the s p e c i e s i n terms of t h e i r a g g r e s s i v e n e s s thus depends on whether l a r v a e or a d u l t i n s t a r s are being c o n s i d e r e d . Whether a l a r v a e or a d u l t , however, G. r e m i g i s was the most a g g r e s s i v e s p e c i e s . In the other f o u r s p e c i e s , l a r v a e g e n e r a l l y showed a lower l e v e l of a g g r e s s i v e n e s s than d i d the a d u l t s , with an average of 40% fewer k i l l s by l a r v a e than by 169 Table 24. Mean va l u e s (± 1 SE) of t o t a l percent k i l l and percent k i l l per a t t a c k observed f o r f i f t h i n s t a r l a r v a e of each of t h e f i v e s p e c i e s of G e r r i s . Species G. n o t a b i l i s G. r e m i g i s G. buenoi G. i n c u r v a t u s G. i n c o g n i t u s Percent k i l l 25.3 ± 8.34 62.4 ± 11.24 17.2 ± 3.77 26.2 ± 9.84 41.2 ± 4.92 Percent k i l l / a t t a c k 17.1 ± 8.27 22.6 ± 7.48 23.2 ± 8.74 7.9 ± 4.00 8.9 ± 2.74 170 Table 25. F-values between s p e c i e s p a i r s f o r both (A) a r c s i n t o t a l percent k i l l and (B) a r c s i n percent k i l l per at t a c k by f i f t h i n s t a r l a r v a e of each of the s p e c i e s f o l l o w i n g s t a r v a t i o n f o r 24 hr. * = s i g n i f i c a n t d i f f e r e n c e with 5% p r o b a b i l i t y l e v e l . 24 hr starved A. A r c s i n % k i l l N B B N - 6.374* (1/10) 0.015 (1/7) B - 4.683 (1/7) B IG -IG -IV 0.004 (1/8) 4. 226 ( 1/8) IG 2.043 (1/7) 1.761 (1/7) 0.037 (1/5) 12.107*(1/4) 1.731 (1/5) B. N B B IV A r c s i n % k i l l / a t t a c k • 0. 568 (1/10) 0.466 (1/7) 0.021 (1/7) 0.350 (1/8) 2.094 (1/8) 3.137 (1/5) 0.096 (1/7) 1.591 (1/7) 3.105 (1/4) 0. 286 (1/5) 171 a d u l t s . In G. r e m i g i s , a g g r e s s i v e n e s s was v i r t u a l l y i d e n t i c a l r e g a r d l e s s of the stage o f development. An i n d i c a t i o n as to how g e r r i d prey items rank i n comparison to other prey items i n terms of c a t c h a b i l i t y i s provided by the data i n F i g . 38. A l l the g e r r i d s i z e c l a s s e s t e s t e d (from 2 to 9) were able to capture l i v e D r o s o p h i l a e f f i c i e n t l y , with the l a r g e r g e r r i d s t y p i c a l l y r e q u i r i n g only one a t t a c k to make a k i l l . T h i s i s q u i t e d i f f e r e n t to when g e r r i d s are the prey item, and suggests t h a t as prey, g e r r i d s might not be favoured as a food source i f c e r t a i n other prey items were a v a i l a b l e . B. B e l a t i v e predator-prey s i z e and k i l l success The e f f e c t of r e l a t i v e p r e d a t c r - p r e y s i z e on p r e d a t i o n i s shown i n F i g . 39. A species-weighted a n a l y s i s of the data was used, owing to the d i f f e r e n t l e v e l s of aggressiveness shown by the d i f f e r e n t s p e c i e s (see above), with the r e s u l t s expressed i n terms of G. n o t a b i l i s . L a r v a l G. r e m i g i s percent k i l l s , l a r v a l i i * i n c o g n i t u s percent k i l l s , a d u l t G. remigi s percent k i l l s and a d u l t G. i n c u r v a t u s percent k i l l s were each weighted by the c o e f f i c i e n t s -0.50, -0.36, -0.20 and 1.33 r e s p e c t i v e l y . Bo weighting of k i l l s per u n i t a t t a c k was f e l t necessary as no s i g n i f i c a n t d i f f e r e n c e s were observed between the s p e c i e s . B e l a t i v e l e v e l s of aggressiveness of each of the l a r v a l i n s t a r s of the f i v e s p e c i e s was assumed the same as t h a t measured f o r the f i f t h i n s t a r l a r v a of that s p e c i e s . I t was a l s o assumed that there were no s p e c i e s d i f f e r e n c e s between how the prey 172 Figure 38. The t o t a l percent k i l l and percent k i l l per attack of each of the gerrid size classes when attacking Drosophila. • = percent k i l l ; ° = percent k i l l / a t t a c k ; - - - = 5% k i l l / a t t a c k threshold. P E R C E N T r o cn o o o I ro H O co > 9 O co H CD O 173 Figure 39. The (A) t o t a l percent k i l l and (B) percent k i l l / a t t a c k of each gerrid size class when attacking gerrids of i t s own or smaller size classes. = 5% k i l l / a t t a c k threshold. PERCENT cn o I, tn O I o I CO O PERCENT CO o i o I co o, o o _ J CO • XI m < in CO CO — CO o *QL1 174 g e r r i d s responded, s i n c e l e g length was f e l t to be the c r i t i c a l f a c t o r here and t h i s was considered i n the s i z e c l a s s d e s i g n a t i o n . For any p a r t i c u l a r s i z e p r e d a t o r , an a r b i t r a r y t h r e s h o l d of 5 k i l l s per 100 a t t a c k s was chosen as the d i v i d i n g l i n e between which s i z e c l a s s e s were e f f i c i e n t l y k i l l e d and which were not. Osing t h i s c r i t e r i o n , i s i s evident that the l a r g e r the predator g e r r i d , g e n e r a l l y the more prey s i z e c l a s s e s i t can prey upon. The only exception to t h i s i s with s i z e c l a s s 8 g e r r i d s (adults of the three s m a l l e r s p e c i e s ) which can e f f i c i e n t l y k i l l more prey s i z e c l a s s e s than can s i z e c l a s s 9 g e r r i d s ( f i f t h i n s t a r l a r v a e of the two l a r g e s p e c i e s ) . With r e s p e c t to a g g r e s s i v e n e s s , i t was found t h a t the same r e l a t i o n s h i p between predator and prey s i z e c l a s s t h a t occurs fo r k i l l e f f i c i e n c y does not always apply. O c c a s i o n a l l y , a r e l a t i v e l y l a r g e prey s i z e c l a s s w i l l s u f f e r the same percent p r e d a t i o n as w i l l a s m a l l prey s i z e c l a s s , even though the l a r g e r , prey are not as e a s i l y captured. T h i s was a r e s u l t of the d i s p r o p o r t i o n a t e l y l a r g e number of a t t a c k s l a r g e prey tend to e l i c i t from a predator, c o u n t e r a c t i n g the reduced e f f i c i e n c y on the p a r t of the predator i n handling these prey. T h i s does not, however, account f o r the reduced t o t a l percent k i l l e x h i b i t e d by the l a r g e r predators a t t a c k i n g very s m a l l prey. Here, the numbers of a t t a c k s made per predator were almost i d e n t i c a l with t h a t measured with s l i g h t l y l a r g e r prey, but the k i l l e f f i c i e n c y was lower, s u g g e s t i n g that, l i k e the l a r g e r prey items, a handling d i f f i c u l t y was present. I t would thus seem 175 t h a t these very s m a l l prey were s m a l l e r than the optimum prey s i z e t h a t these p r e d a t o r s could handle e f f i c i e n t l y . C. Environmental complexity and k i l l success The mean values of percent k i l l weighted f o r predator s p e c i e s ' a g g r e s s i v e n e s s (see above) f o r each predator s i z e c l a s s t e s t e d i n each " m i c r o h a b i t a t " are given i n Table 26. The two trends to be n o t i c e d are t h a t the l a r g e r the predator, the g r e a t e r the percent k i l l , and t h a t a d u l t g e r r i d s k i l l a g r e a t e r number of prey than do l a r v a l g e r r i d s . That l a r g e g e r r i d s would k i l l more prey was expected, as i n S e c t i o n 4 i t was noted t h a t the l a r g e r the g e r r i d , the g r e a t e r the swath through the environment the predator sweeps as i t searchs f o r f o o d . . Since a f i x e d c o n t a i n e r s i z e and time p e r i o d were used i n t h i s experiment, i t f o l l o w s t h a t the l a r g e r the g e r r i d , the g r e a t e r the chance t h a t i t w i l l thus p e r c e i v e the prey and hence capture i t . In the previous experiment, l a r v a l and a d u l t d i f f e r e n c e s i n aggressiveness were demonstrated. These are confirmed here, as i n a l l f o u r m i c r o h a b i t a t s , l a r v a l k i l l s were l e s s than the corresponding a d u l t k i l l s , even when the l a r v a e were l a r g e r than the a d u l t s . As equal numbers of t r i a l s f o r each g e r r i d s i z e c l a s s were observed i n each of the f o u r m i c r o h a b i t a t s , comparisons of the mean v a l u e s of both l a r v a e and a d u l t t o t a l percent k i l l s are f e a s i b l e . Larvae and a d u l t s were co n s i d e r e d s e p a r a t e l y owing to t h e i r d i f f e r e n t average percent k i l l s . Table 27 l i s t s these values, and to permit a comparison between m i c r o h a b i t a t s , a l s o 176 T a b l e 26. The mean v a l u e s of p e r c e n t k i l l weighted f o r p r e d a t o r s p e c i e s a g g r e s s i v e n e s s f o r each p r e d a t o r s i z e c l a s s t e s t e d i n each o f t h e f o u r m i c r o h a b i t a t s . P r e d a t o r Number o f n M i c r o h a b i t a t s s i z e prey s i z e c l a s s c l a s s e s Reeds 2.54 Reeds 5.08 Open water Algae cm a p a r t cm a p a r t 3 1 16 13.0 6.0 0. 6.0 4 1 16 13 . 0 19.0 0. 25.0 5 1 16 6.0 0. 25.0 6.0 6 2 32 10.0 ± 4.0 9.5 ± 3.5 6.5 ± 0.5 3.0 ± 3.0 7 3 48 34.3 ±11.6 11.0 ± 2.0 0. 4.7 ± 2.9 8 5 80 64.4 ± 4.9 63. 4 ± 5. 4 38.0 ±11.5 25.4 ± 7.2 9 4 64 24.5 ± 4.6 33.5 ± 8.5 14.0 ± 1.8 15.8 ±11.6 10 7 113 86. 1 ± 3.7 57.7 ± 8.8 26.9 ± 5.8 51.6 ± 3.7 177 T a b l e 27. Mean value s f o r i n s t a r and a d u l t g e r r i d percent k i l l s i n each of the f o u r m i c r o h a b i t a t s t e s t e d . Comparison c o e f f i c i e n t s are used to compare values i n each m i c r o h a b i t a t with t h a t of the open water m i c r o h a b i t a t . M i c r o h a b i t a t Larva Adult Mean C o e f f i c i e n t Mean C o e f f i c i e n t Open water 22.1 ± 4.1 1.00 77.1 ± 4.3 1.00 Reeds 5.08 cm 17.6 ± 4.5 0.80 60.1 ± 5.5 0.78 a pa r t Reeds 2.54 cm 7.8 ± 2.5 0.35 31. 5 ± 5.8 0.41 a p a r t Algae 10.0 ± 4.1 0.45 40.7 ± 5.2 0.53 178 comparison c o e f f i c i e n t s . I t i s obvious from these data that the stage of development, i e . l a r v a e or a d u l t , has v i r t u a l l y no e f f e c t on r e l a t i v e m i c r o h a b i t a t performance, with r e l a t i v e t o t a l percent k i l l s f o r both l a r v a e and a d u l t i n each m i c r o h a b i t a t being almost i d e n t i c a l . I t i s a l s o obvious that the m i c r o h a b i t a t g r e a t l y a f f e c t s the percent k i l l , with the g r e a t e s t percent k i l l being i n open water; reeds 5.08 cm a p a r t , algae and reeds 2.54 cm apart gave d e c r e a s i n g values. In an e f f o r t to determine i f v i s i o n was indeed a sense by which g e r r i d s d e t e c t e d prey, t r i a l s were run i n the dark on the open water h a b i t a t f o r one predator-prey s i z e c l a s s combination. The r e s u l t s i n Table 28 suggest t h a t the absence of l i g h t does not d r a s t i c a l l y reduce the number of k i l l s achieved, although t h e number of k i l l s was ca. 10% l e s s . Surface r i p p l e d e t e c t i o n , the only p l a u s i b l e means of prey d e t e c t i o n under these c o n d i t i o n s , would thus seem - reasonably e f f i c i e n t , a l l o w i n g g e r r i d s to p o t e n t i a l l y hunt at night almost as w e l l as during the day. In the f i e l d , the c o o l e r night temperatures might reduce a c t i v i t y to an extent which would make t h i s night p r e d a t i o n i n s i g n i f i c a n t , but the p o t e n t i a l i s n e v e r t h e l e s s t h e r e . D. E f f e c t of predator c h o i c e of prey on p r e d a t i o n The design of the p r e d a t i o n experiments thus f a r i n t h i s study have been such that only one prey item at a time was presented to a p r e d a t o r . T h i s allowed determination of the ease with which a p a r t i c u l a r prey was handled, but d i d not allow 179 Table 28. The percent k i l l in the open water microhabitat for s i z e class 10 predator gerrids preying on size class 2 prey gerrids i n both illuminated and dark conditions. Illumination Percent k i l l Mean time required for k i l l Present Absent 56 48 76.8 66.7 23.0 ± 2.0 20.6 ± 1.9 180 dete r m i n a t i o n of which prey item was a c t u a l l y p r e f e r r e d by a predator: o n l y c h o i c e experiments can do t h i s . F i g u r e 40 pr e s e n t s the r e s u l t s of c h o i c e experiments i n which a d u l t s of each s p e c i e s were presented with one of each of the prey s i z e c l a s s e s which that predator s i z e c l a s s was known to capture e f f i c i e n t l y (see above). A d u l t s of the smal l s p e c i e s p r e f e r r e d the s m a l l e r prey items, whereas a d u l t s of the two l a r g e s p e c i e s , G. n o t a b i l i s and G. r e m i g i s , s e l e c t e d l a r g e r prey items. The only s i g n i f i c a n t departure i n the data from the expected equal p r e d a t i o n on each s i z e c l a s s was f o r G. r e m i g i s . With t h i s s p e c i e s , i t appears that the m i d d l e - s i z e to l a r g e prey were p r e f e r r e d , as the l a r g e s t prey item presented was not p r e f e r e n t i a l l y chosen. A one-to-one prey choice i s not r e p r e s e n t a t i v e o f the f i e l d s i t u a t i o n , however, where c h a r a c t e r i s t i c a l l y more of the s m a l l e r i n s t a r s are present. In an e f f o r t to provide a s i t u a t i o n comparable to the f i e l d , two prey i n s t a r r a t i o s where s i z e c l a s s numbers were not equal were presented to the a d u l t g e r r i d s . The f i r s t unequal prey r a t i o t e s t e d c o n s i s t e d of prey r a t i o e d , f o r nothing b e t t e r , on the b a s i s of l e g l e n g t h . The r e s u l t s of t h i s experiment ( F i g . 41) i n d i c a t e d no s i g n i f i c a n t d e v i a t i o n away from the n u l l hypothesis where the prey are k i l l e d a c c o r d i n g to the number present, although the trend was f o r the smal l a d u l t s to s e l e c t the s m a l l e r prey and f o r the l a r g e a d u l t s to s e l e c t medium-sized prey. In the other unegual prey r a t i o t e s t e d , prey items were r a t i o e d on the b a s i s of biomass, r e s u l t i n g i n a g r e a t e r d i f f e r e n c e - between r e l a t i v e s i z e - c l a s s numbers. 181 F i g u r e 40. The numbers of each prey s i z e c l a s s k i l l e d by a d u l t s o f each of the f i v e s p e c i e s (shaded) when one of each of the prey s i z e c l a s s e s which that s i z e c l a s s a d u l t g e r r i d can k i l l e f f i c i e n t l y are presented. The s o l i d l i n e r e p r e s e n t s the expected r a t i o . G. notabi l is 201 2 3 4 5 PREY SIZE C L A S S G. remigis 20i S 15-1 ct: u m Z) 10 1 2 3 4 5 6 7 PREY SIZE C L A S S 20 G. incognitus 2 3 4 PREY SIZE CLASS G. buenoi 1 2 3 4 PREY SIZE CLASS -5 181c G. incurvatus 2 3 4 PREY SIZE C L A S S 182 Figure 41. The numbers of each prey size c l a s s k i l l e d by adult gerrids when the numbers of the prey s i z e classes were ratioed as to t o t a l leg length present. 182a E £ x 30 CD O O UJ 15 0 G. notabi t is G. remigis Q • 20 -i ° 15-1 10 cr UJ DD => 5 2 3 4 5 PREY SIZE CLASS 7 NUMBER KILLED TOTAL LEG LENGTH (mm) _ A ro O o O O -* . 1 i ~~U m < r o c o cn m P cr c • fD O 183 U n f o r t u n a t e l y , the s m a l l e s t prey s i z e c l a s s e s had to be omitted, but among those s i z e c l a s s e s t h a t were presented, s i m i l a r r e s u l t s as were obtained i n the previous experiment were observed ( F i g . 42). The smal l a d u l t s p r e f e r r e d the sma l l e r s i z e c l a s s e s and the l a r g e a d u l t s p r e f e r r e d medium-sized prey. I t should be noted t h a t f o r both prey r a t i o s , t h i s s e l e c t i o n was f o r both the s i z e c l a s s k i l l e d and the s i z e c l a s s attacked f i r s t . That the l a r g e r prey r e g a r d l e s s of predator s i z e were igno r e d i s thus not a r e s u l t of t h e i r being a b l e to escape the predator g e r r i d , but i s r a t h e r a r e s u l t o f a d e f i n i t e preference f o r s m a l l e r prey. 3. OTHER INSECT PREDATORS OF GERRIDS The r e s u l t s of g y r i n i d p r e d a t i o n are shown i n F i g . 43a. G y r i n i d s are somewhat unusual p r e d a t o r s i n that on encountering a prey item, they tend to c o r r a l i t by swimming g u i c k l y i n t i g h t c i r c l e s around the prey, yet a l l the while launching a t t a c k s . The e f f e c t i s f o r a d i s p r o p o r t i o n a t e l y l a r g e number of a t t a c k s to be launched, and although they are only r e l a t i v e l y e f f i c i e n t at c a p t u r i n g f i r s t i n s t a r g e r r i d l a r v a e , f o r a l l the prey s i z e c l a s s e s , they were able t o k i l l a r e l a t i v e l y l a r g e number of the prey presented. G y r i n i d s were unable to k i l l g e r r i d s ^ l a r g e r than s i z e c l a s s 6 owing to the great l e g span of the l a r g e r g e r r i d s . When a t t a c k i n g a l a r g e g e r r i d , the only p o r t i o n of the o g e r r i d t h at can be grasped by a g y r i n i d are the l e g s , as the body i s suspended too high above the water s u r f a c e . I f only one 184 Figure 42. The numbers of each prey size c l a s s k i l l e d by adult gerrids when the numbers of the prey s i z e classes present were ratioed as to t o t a l biomass present. 184a <; G. buenoi G. incognitus G incurvatus \~ S?4 LU E i — ~ O LU a 2 3 4 5 PREY SIZE CLASS 184b U J E8 - , j l , i— B» G. notabilis G. remigis 2 3 4 5 6 PREY SIZE C L A S S ' 185 F i g u r e 43. The t o t a l percent k i l l and percent k i l l / a t t a c k of (A) g y r i n i d s , (B) f o u r t h i n s t a r l a r v a e n o t o n e c t i d s and (C) a d u l t n o t o n e c t i d s when a t t a c k i n g each of the ten s i z e c l a s s e s of g e r r i d s . 0 = t o t a l percent k i l l ; • = percent k i l l / a t t a c k ; - -= 5% k i l l / a t t a c k t h r e s h o l d . 185a 100-1 75 50 25 H 0 100 75 LU 50 o cr LU ^ 25-1 0 100 75-50-25-o GYRINIUS 9 0 -o-~T ? ? f FOURTH INSTAR LARVAE NOTONECTID © o e o 0 n i I i o o o ADULT NOTONECTID © r ^ r 5 6 , PREY T — ° ? °t 8 . 9 10 a 186 leg i s grasped, the g e r r i d can u s u a l l y f r e e i t s e l f by jumping g u i c k l y , although part of the l e g may be eaten, G y r i n i d s , possessing chewing mandibles, b i t e t h e i r prey and hence i n g e s t t h e i r food r e l a t i v e l y g u i c k l y . I f both l e g s on one s i d e of the body can be grasped s i m u l t a n e o u s l y by the g y r i n i d , the g e r r i d i s hobbled and cannot f r e e i t s e l f . The g y r i n i d then eats i t s way up the g e r r i d ' s l e g s u n t i l i t reachs the body, which i s then a l s o eaten. The e f f e c t i v e n e s s of n o t o n e c t i d s as g e r r i d predators are shown i n F i g . 43b and 43c. L i k e the g y r i n i d s , both s i z e s of n o t o n e c t i d t e s t e d were able to capture g e r r i d prey up to about s i z e c l a s s 7. However, u n l i k e the g y r i n i d s , n o t o n e c t i d s are r e l a t i v e l y more e f f i c i e n t at c a t c h i n g g e r r i d s , but they a t t a c k l e s s . The net e f f e c t i s f o r the t o t a l percent k i l l by n o t o n e c t i d s to be s l i g h t l y l e s s than t h a t f o r g y r i n i d s . Regardless of r e l a t i v e e f f e c t i v e n e s s , however, these data demonstrate t h a t both n o t o n e c t i d s and g y r i n i d s are e f f e c t i v e p r e d a t o r s of g e r r i d s and have the p o t e n t i a l to s i g n i f i c a n t l y i n f l u e n c e t h e i r numbers. DISCUSSION The g r e a t e r a g g r e s s i v e n e s s demonstrated by some of the s p e c i e s over the o t h e r s may be r e l a t e d to the the s p e c i f i c h a b i t a t s p r e f e r r e d by the s p e c i e s . The c h a r a c t e r i s t i c a l l y a g g r e s s i v e nature of G. r e m i g i s may thus be r e l a t e d to t h i s s p e c i e s * p r e f e r r e d h a b i t a t being on streams, where food items 187 are presented o n l y once, as they are swept by on the c u r r e n t . It would seem of s e l e c t i v e advantage f o r t h i s s p e c i e s to .check every item f l o a t i n g p a s t . Whether or not the observed h e t e r o g e n i t y among the other s p e c i e s i s a r e a l phenomenon or not i s open to q u e s t i o n . The r e l a t i v e l y l a r g e sample s i z e s i n v o l v e d support i t s v a l i d i t y , but why on the b a s i s of whether l a r v a or adult the d i f f e r e n t s p e c i e s should be ranked d i f f e r e n t l y i n terms of a g g r e s s i v e n e s s i s not immediately obvious. A l l the i n s t a r s of a s p e c i e s c o h a b i t the same m i c r o h a b i t a t and so would a l l presumably be exposed to the same problems r e l a t i n g to food c a p t u r e . That G. buenoi and G. i n c o g n i t u s which do shew l a r v a l -a d u l t d i f f e r e n c e s and which are the most a g g r e s s i v e of the pond s p e c i e s are those found c l o s e s t i n s h o re may be s i g n i f i c a n t . Prey items i n t h i s m i c r o h a b i t a t often r e s u l t from i n s e c t s i f a l l i n g o f f emergent v e g e t a t i o n or jumping o f f the shore, and with many p o t e n t i a l s u r f a c e s to crawl out of the water on, they might be expected to leave the water s u r f a c e r e l a t i v e l y g u i c k l y . I f t h i s i s indeed the case, then one might expect a g e n e r a l l y higher l e v e l of a g g r e s s i v e n e s s f o r these two s p e c i e s as a whole, but not j u s t f o r one p a r t i c u l a r s t a g e . ^ Prey s e l e c t i o n by a predator i s i n f l u e n c e d by many v a r i a b l e s . Those c h a r a c t e r s of the prey i n p a r t i c u l a r which determine the e x t e n t of p r e d a t i o n are r e l a t i v e predator-prey s i z e , m o b i l i t y , d e f e n s i v e c a p a b i l i t i e s , p a l a t a b i l i t y and the prey's r e l a t i v e abundance, both to the predator and to other p o t e n t i a l prey items. The manner i n which these parameters a f f e c t p r e d a t i o n , however, vary. Some determine the e f f i c i e n c y 188 of a t t a c k , i e . the number of k i l l s made per u n i t number of a t t a c k s , whereas o t h e r s determine the p r e f e r e n c e a predator has for a p a r t i c u l a r prey item, i e . the t o t a l number of a t t a c k s which w i l l be launched by the p r e d a t o r . Thus, a prey may be e a s i l y and e f f i c i e n t l y captured, but because i t i s u n p a l a t a b l e , may be avoided and hence not preyed upon. K i l l e f f i c i e n c y and prey preference have been i n v e s t i g a t e d i n t h i s study as both parameters i n f l u e n c e the extent of c a n n i b a l i s m among g e r r i d s . G e r r i d s are r a p t o r i a l i n s e c t s and l i k e the mantid ( H o l l i n g , 1966), one might expect that there s h o u l d be a range of prey s i z e s which are grasped e f f i c i e n t l y . T e r r e s t r i a l prey items are r e l a t i v e l y h e l p l e s s on the water s u r f a c e and are o f t e n h e l d immobile by the s u r f a c e t e n s i o n . T h i s means t h a t they do not always have to be a c t i v e l y subdued and hence, are o f t e n not grasped i n the c o v e n t i o n a l manner. With such prey, a g e r r i d simply uses i t s f o r e l e g s to maintain p o s i t i o n on the prey, thus p e r m i t t i n g r e l a t i v e l y s m a l l g e r r i d s to e a t very l a r g e prey. I t i s only with very s m a l l prey items and with prey which are a l s o mobile on the water s u r f a c e t h a t g r a s p i n g problems a r i s e . G e r r i d s have never been observed to feed upon prey too s m a l l to be grasped and i t would seem that they must hold such prey f o r i n s e r t i o n of the p r o b o s c i s . S i m i l a r l y , p o t e n t i a l prey items such as other g e r r i d s and v e l i i d s which are very a c t i v e on the water s u r f a c e have to be p h y s i c a l l y subdued before f e e d i n g can commence and so here too, g r a s p i n g of the prey i s o f t e n r e q u i r e d . Thus, r e g a r d l e s s of the nature of the prey, one would expect a minimum s i z e t h r e s h o l d to 189 e x i s t below which prey are not e f f i c i e n t l y handled. One would expect a maximum si2e t h r s e h o l d t o e x i s t only f o r mobile prey, as only these prey items need to be subdued. Maximum t h r e s h o l d s i n prey s i z e were determined f o r each g e r r i d s i z e c l a s s s t u d i e d , i n d i c a t i n g t h a t f o r any p a r t i c u l a r s i z e g e r r i d , g e r r i d s e x i s t which although s m a l l e r than the predator, cannot be preyed upon. However, i t was only with the l a r g e s t and the s m a l l e s t g e r r i d s i z e s s t u d i e d together t h a t an i n d i c a t i o n o f a minimum s i z e t h r e s h o l d became e v i d e n t , and even here, the prey were e f f i c i e n t l y caught. Thus, f o r the purpose of i n v e s t i g a t i n g c a n n i b a l i s m i n t h i s genus, the concept c f a minimum s i z e t h r e s h o l d need not be c o n s i d e r e d , as f u n c t i o n a l l y , i t does not e x i s t . S i m i l a r r e s u l t s have been r e p o r t e d f o r the c o c c i n e l l i d b e e t l e A d a l i a decempunctata f e e d i n g on the aphid Microlophium e v a n s i (Dixon, 1959). Each s u c c e s s i v e c o c c i n e l l i d i n s t a r l a r v a e captured l i v e f i r s t i n s t a r l a r v a e aphids more e f f i c i e n t l y than the previous c o c c i n e l l i d i n s t a r l a r v a e , and was capable of c a p t u r i n g e f f i c i e n t l y a g r e a t e r number of aphid i n s t a r s . The r e l a t i v e s i z e d i f f e r e n c e between c o c c i n e l l i d and aphid was not gr e a t enough to i n d i c a t e a minimum s i z e t h r e s h o l d , but a maximum s i z e t h r e s h o l d was evident f o r each c o c c i n e l l i d i n s t a r l a r v a e . With r e s p e c t to prey p r e f e r e n c e , i t i s assumed t h a t no d i f f e r e n c e i n p a l a t a b i l i t y e x i s t s between g e r r i d s of d i f f e r e n t s i z e s and s p e c i e s . However, d i f f e r e n c e s do e x i s t between g e r r i d s from the d i f f e r e n t s i z e c l a s s e s i n amount of e f f o r t 190 required for subduing, potential gain i n energy for the predator, and perhaps in s t i m u l i presented to the predator. Obviously, for example, as a prey item approachs a predator i n s i z e , more and more e f f o r t w i l l be reguired on the part of the predator to subdue the prey. Optimal foraging theory predicts that there should be a threshold equal to the average capture rate (energy gain) in the environment and that any prey encountered which gives a lower capture rate during i t s handling should be bypassed (Charnov, 1973). Thus, more energy might be gained over a unit time period when handling prey from a smaller size c l a s s , since small prey are c h a r a c t e r i s t i c a l l y more abundant than large prey, and sc long as the prey are not too small, more e f f i c i e n t l y captured. The average capture rate with these prey would thus often be r e l a t i v e l y ' higher. It i s suggested further that the i n t e r n a l state of the predator might be involved, as the potential energy gain can only be measured in terms of the average capture rate. Thus, i f a predator i s very hungry, i e . has a low average capture rate, i t might be l i k e l y to attack a larger prey which requires more subduing than would a predator having i t s gut 2/3 f i l l e d with food. It might chance the r i s k of losing the prey since any prey would represent an energy gain. In t h i s respect, then, i t i s int e r e s t i n g to note that the f i v e species of Gerris studied do not seem to prefer d i f f e r e n t s i z e classes of prey at d i f f e r e n t l e v e l s of hunger. G. remigis and G. buenoi k i l l roughly the same percentage of prey from each size class apparently regardless of state of hunger, whereas the other three species k i l l fewer prey at the lowest hunger l e v e l tested, but s t i l l 191 show no measureable preference f o r s m a l l e r prey. I t would thus seem that i n t h i s s i t u a t i o n , the energy gain on the part of the predator must always exceed the l o s s i n v o l v e d i n c a p t u r i n g and subduing the prey. However, s i n c e the average capture r a t e under f i e l d c o n d i t i o n s i s unknown, p r e d i c t i o n s as to what s i z e c l a s s e s should be p r e f e r r e d i n the f i e l d are impossible to assess. I t was o r i g i n a l l y thought t h a t i n the m i c r o h a b i t a t with reeds 2.54 cm /apart, the reeds might impede movement of the l a r g e s t g e r r i d s to an e x t e n t which would even f u r t h e r reduce t h e i r percent k i l l from that expected. However, the data do not support t h i s h y p o t h e s i s , as r e g a r d l e s s of g e r r i d s i z e , r e l a t i v e k i l l was not s i g n i f i c a n t l y d i f f e r e n t . I t would thus seem t h a t the main f a c t o r d e c r e a s i n g percent k i l l amongst the reeds was the cover they p r o v i d e d , not the impedement of pursuing p r e d a t o r s . However, t h i s does not e x p l a i n the reduced p r e d a t i o n i n t h e algae h a b i t a t . There, v i s u a l cover was not decreased to too g r e a t an e x t e n t , as the algae d i d not protrude above the water s u r f a c e very f a r . I f prey d e t e c t i o n was p a r t l y by s u r f a c e r i p p l e d e t e c t i o n , , however, cover f u n c t i o n a l l y might be i n c r e a s e d , as the r i p p l e s would not be able to move very f a r before being absorbed. Regardless of the reason, though, o p e r a t i o n a l l y , a l g a e works to the advantage of the prey by r e d u c i n g predator e f f e c t i v e n e s s . The above experiments always i n v o l v e d the p r e s e n t a t i o n of only one s i z e c l a s s to the predator at a time. What do the r e s u l t s of c h o i c e experiments suggest concerning prey i 192 preference? In t r i a l s where equal numbers (one) of each prey s i z e c l a s s ,were presented, the two l a r g e g e r r i d s p e c i e s (G. r e m i g i s and G. n o t a b i l i s ) p r e f e r r e d the m i d d l e - t o - l a r g e prey s i z e c l a s s e s and the three s m a l l s p e c i e s (G. fcuenoi, G. iJSS°ajlli2§ a n ^ G . incurvatus) p r e f e r r e d the s m a l l e s t prey s i z e c l a s s e s . Since the a d u l t s of the l a r g e s p e c i e s have been shown to e x h i b i t h a n d l i n g problems with the s m a l l e s t prey s i z e c l a s s e s , t h i s p r e f e r e n c e f o r l a r g e r g e r r i d s may r e s u l t from avoidance of these s m a l l s i z e c l a s s e s . The s m a l l a d u l t s , having no such handling d i f f i c u l t i e s , would not be expected to avoid the s m a l l e s t s i z e c l a s s e s , and t h i s was observed. That the s m a l l a d u l t s and l a r g e a d u l t s both avoided the l a r g e s t prey s i z e c l a s s e s i n c h o i c e experiments where unegual prey s i z e c l a s s numbers were presented supports t h i s i n t e r p r e t a t i o n . I t suggests that those prey s i z e c l a s s e s p r e f e r r e d are those which are most e f f i c i e n t l y captured by t h a t s i z e c l a s s a d u l t , as with both egual and unegual prey r a t i o s , the l a r g e s t prey s i z e c l a s s e s presented were avoided. J u s t as the s m a l l e s t prey are s l i g h t l y but nonetheless l e s s e f f i c i e n t l y caught, so are the l a r g e s t prey, although f o r the purposes of t h i s study, they are s t i l l r e l a t i v e l y e f f i c i e n t l y captured when compared to ether g e r r i d s i z e c l a s s e s . That t h i s i s simply not j u s t a r e s u l t of the g r e a t e r numerical abundance of the s m a l l e r prey i n the l a t t e r experiments i s suggested by the s i m i l a r i t y of the r e s u l t s obtained from the egual prey s i z e c l a s s numbers c h o i c e experiments. The extent of c a n n i b a l i s m among g e r r i d s , then, would seem 193 t o depend on the a v a i l a b i l i t y of e f f i c i e n t l y captured prey items. I f the p r e f e r r e d prey i s t h a t which i s most e f f i c i e n t l y captured, nonmotile prey, p a r t i c u l a r l y l i v e prey trapped by the s u r f a c e f i l m , would l i k e l y be p r e f e r r e d over r e l a t i v e l y l a r g e g e r r i d s , as they can r e s i s t only weakly at b e s t . Even SJE2soj2hila which do not break the s u r f a c e f i l m completely but r a t h e r act as i f they s t i c k to i t , moving j e r k i l y and with d i f f i c u l t y , can be r e a d i l y captured by g e r r i d s t h e i r cwn s i z e . Observations of p o t e n t i a l prey items on water s u r f a c e s i n the f i e l d suggest t h a t on l a k e s and ponds, small p o t e n t i a l prey items are l i k e l y always p r e s e n t . I t may w e l l be, however, that t h e r e i s a shortage of food items f o r the l a r g e r g e r r i d s . In the pond h a b i t a t under normal c o n d i t i o n s , then, i f c a n n i b a l i s m of g e r r i d s where handling d i f f i c u l t i e s are present i s o c c u r r i n g , i t i s l i k e l y at a r e l a t i v e l y low l e v e l . Only the most e f f i c i e n t l y caught g e r r i d s would be preyed upon. That some ca n n i b a l i s m always occurs seems l i k e l y , as the ease with which these g e r r i d s are caught approaches t h a t with which nonmctile prey are captured. The s i t u a t i o n may be d i f f e r e n t on streams, however, owing to the p o s s i b l e shortage of food t h a t may e x i s t i n t h i s h a b i t a t . M o r t a l i t y of the s m a l l to m i d d l e - s i z e g e r r i d s would a l s o seem to r e s u l t from p r e d a t i o n by i n s e c t s other than g e r r i d s . Both g y r i n i d s and n o t o n e c t i d s a r e found i n the same h a b i t a t as g e r r i d s and both have been shown capable of preying on g e r r i d s . Both i n s e c t s , however, being i n the water r a t h e r than on i t , are r e s t r i c t e d to water r e l a t i v e l y ' f r e e of f l o a t i n g d e b r i s and 194 a l g a e . G e r r i d s may thus have a refuge away from these preda tors i n t ha t they can move over and l i v e i n debr i s -choked waters , a h a b i t a t l a r g e l y avoided by these two p reda to r s . 195 SECTION 6. THE MODEL INTRODUCTION That the s p e c i e s of G e r r i s s t u d i e d are not e c o l o g i c a l homologues has been demonstrated. However, owing t o the great number of d i f f e r e n c e s between the s p e c i e s and the c h a r a c t e r i s t i c s of the genus as a whole, the conseguences a r i s i n g when d i f f e r e n t s p e c i e s c o e x i s t are not immediately obvious. Which s p e c i f i c d i f f e r e n c e s between the s p e c i e s are l i k e l y to weigh most h e a v i l y i n c o e x i s t i n g s i t u a t i o n s , and how important c a n n i b a l i s m might be among g e r r i d s are unknown. In an attempt to answer these guestions, i t was decided to c o n s t r u c t a computer model capable of s i m u l a t i n g c o e x i s t i n g s p e c i e s . T h i s s e c t i o n i s concerned with the o r g a n i z a t i o n of the data d e s c r i b e d i n the preceeding s e c t i o n s to c r e a t e such a model and i n the model's p r e d i c t i o n s . The main i n t e r e s t s here, t h e r e f o r e , l i e not i n the s p e c i e s themselves, but i n the e c o l o g i c a l r e l a t i o n s h i p s between the s p e c i e s . MATERIALS AND METHODS The model d e s c r i b e d here was designed f o r use on the I.E.M. model 360 computer. F o r t r a n IV was the language used i n programming the model. A l l the data used i n the model was de r i v e d from experiments and o b s e r v a t i o n s i n the f i v e preceeding 196 s e c t i o n s . An experimental components a n a l y s i s type of approach as developed by H o l l i n g (1965, 1966) was used i n the c o n s t r u c t i o n of a mechanistic model ( F i g . 44 and Appendix 1) . The model was s i m p l i f i e d whenever p o s s i b l e , but as i t simulated the i n t e r a c t i o n s of f i v e p o t e n t i a l l y c o e x i s t i n g s p e c i e s , a c e r t a i n amount of complexity was unavoidable. Many of the same components used by H o l l i n g were contained i n the model and l i k e H o l l i n g ' s models, the present model d e s c r i b e s the predator's a c t i v i t i e s once i t has reached the s i t e where i t d i d i t s hunting. I t does not i n c l u d e the l o c a t i o n of these hunting areas by the predator. The same four b a s i c primary components used by H o l l i n g (1966) were used i n t h i s model, namely: (1) r a t e of s u c c e s s f u l s e a r c h , (2) time predators are exposed to prey, (3) time spent h a n d l i n g prey and (4) hunger. A l l the eguations and submodels used i n the model were i n v o l v e d i n the d e r i v a t i o n and m o d i f i c a t i o n of these primary components. The v a l u e s f o r the parameters used i n the b a s i c v e r s i o n of the model are shown i n Appendix 2. D e s c r i p t i o n s of the major processes i n the model are as f o l l o w s : 197 J Figure 44. A simple flow diagram describing the model constructed in the present study. / Input a r r a y s ; I n i t i a l i z e L „, 1 V J I L r Input newly hatched young Organize s p e c i e s - i n s t a r s i n t o s i z e c l a s s e s | Determine hunger, e t c . of I the s i z e c l a s s e s Determine p o t e n t i a l preda t i c n . j i _ j 1 1 1 L Determine preda a c t u a l t i c n r .J |Determine new hunger, e t c . | | of the s i z e c l a s s e s | Su b t r a c t m o r t a l i t y i D a i l y output 1 4, Seasonal output .j 198 A* Growth and m o u l t i n g S p e c i e s i n s t a r growth was based on t h e number of degree-days accumulated each day measured i n the water a t Area 3 on Marion Lake i n 1971. These v a l u e s were determined f o r each day by p l o t t i n g a s i n e c u r v e between the maximum and minimum tem p e r a t u r e s measured i n t h e f i e l d . S i n c e g e r r i d s are on the water s u r f a c e and are not i n the water, on sunny days, t h e i r body t e m p e r a t u r e s became g r e a t e r than water t e m p e r a t u r e s . A m u l t i p l i c a t i o n c o e f f i c i e n t o f 3.0 was found most r e a l i s t i c i n s i m u l a t i n g what t h e accumulated degree-days by g e r r i d s might have been. T h i s c o e f f i c i e n t was maximal on the summer s o l s t i c e (June 21) and decreased by 0.01 e v e r y day away from t h i s d a t e . The number of degree-days accumulated i n t h i s manner by t h e g e r r i d s of each d a i l y c o h o r t o f each s p e c i e s was m o n i t o r e d , and at t h e b e g i n n i n g o f each i t e r a t i v e day, the i n s t a r o f each c o h o r t was d e t e r m i n e d from t h e s e d a t a . Ecdyses and i n p u t of f i r s t i n s t a r g e r r i d s f o l l o w i n g e c l o s i o n was thus a t a s p e c i f i c t i m e once each i t e r a t i v e day. B. P r e d a t o r - p r e y u n i t s The 30 s p e c i e s - i n s t a r s o f t h e f i v e g e r r i d s p e c i e s were each a s s i g n e d i n t o one of ten s i z e c l a s s e s on the b a s i s c f l e g l e n g t h as d e s c r i b e d i n S e c t i o n 4. P r e d a t i o n was between the g e r r i d s c o m p r i s i n g t h e s e s i z e c l a s s e s , w i t h the g e r r i d s i n each s i z e c l a s s c a p a b l e of p r e y i n g o n l y on g e r r i d s i n i t s own or a s m a l l e r s i z e c l a s s . S i n c e each s p e c i e s responded t o temperature d i f f e r e n t l y , i t was n e c e s s a r y t o weight t h e response t o 199 temperature by the s i z e c l a s s a c c o r d i n g to the numbers of the d i f f e r e n t s p e c i e s comprising t h a t s i z e c l a s s . T h i s weighting of s i z e c l a s s parameters was r e g u i r e d whenever s p e c i e s d i f f e r e n c e s e x i s t e d , with maximum gut c a p a c i t y , food d e s i r e d , r a t e of movement and a g g r e s s i v e n e s s examples of other weighted s i z e c l a s s v a l u e s . Those parameters such as maximum r e a c t i v e d i s t a n c e , s t r i d e length and k i l l e f f i c i e n c y which were f e l t c h a r a c t e r i s t i c of the s i z e of a g e r r i d i r r e s p e c t i v e of s p e c i e s were not weighted. C. Number of predators and prey Since the model was designed to simulate the i n t e r a c t i o n s over an e n t i r e season, the i n i t i a l g e r r i d s f e d i n t o the system were overwintered a d u l t s . These a d u l t s commenced breeding immediately i n a l l the s p e c i e s except G. n o t a b i l i s , which r e g u i r e d a temperature t h r e s h o l d to be exceeded before breeding c o u l d commence (although the f i e l d data suggested that t h i s should be 15°C., 20°G. was used i n an attempt to c o r r e c t f o r a c t u a l body temperatures e x p e r i e n c e d ) . O v i p o s i t i o n r a t e was based on accumulated degree-days. A d a i l y m o r t a l i t y f a c t o r of 1 % of the number present i n each d a i l y c o h o r t , i n a d d i t i o n t o t h a t r e s u l t i n g from p o t e n t i a l p r e d a t i o n by g e r r i d s , was a p p l i e d to every i n s t a r of each s p e c i e s on the b a s i s of data obtained by Vepsalainen (1971c). F o l l o w i n g breeding, m o r t a l i t y of the overwintered a d u l t s i n c r e a s e d g r e a t l y , and i n the model, a l l the a d u l t s were presumed dead 10 days a f t e r 90% of t h e i r eggs had been l a i d . 200 The e f f e c t of t h i s l a t t e r m o r t a l i t y was to temporally separate the overwintered a d u l t s from the summer a d u l t s , as the number of degree-days r e g u i r e d f o r g e r r i d s to l a y 90% of t h e i r eggs was l e s s than-that r e q u i r e d by the l a r v a e to reach maturity. In a l l the s p e c i e s except G. r e m i g i s , f o l l o w i n g completion of development as an a d u l t , those a d u l t s d e s t i n e d f o r o v e r w i n t e r i n g (macropters i n the b i v o l t i n e s p e c i e s ) were assumed to l e a v e the l a k e , as g e r r i d s overwinter i n a t e r r e s t r i a l h a b i t a t . In the b i v o l t i n e s p e c i e s , however, those g e r r i d s which had developed during t h e i r f o u r t h i n s t a r under long days with an i n c r e m e n t a l change of daylength were used to suggest which a d u l t s became summer breeding imagos. I t was these l a t t e r g e r r i d s t h a t gave r i s e to a p a r t i a l second g e n e r a t i o n . T h i s mechanism f o r morph det e r m i n a t i o n was r e p o r t e d f o r G. odontogaster by Vepsalainen (1971b), and s i n c e the a c t u a l mechanism i n the g e r r i d s s t u d i e d i s unknown, t h i s was adopted. A h a l f hour decrease i n day l e n g t h f o l l o w i n g the summer s o l s t i c e was used to s e t a date a f t e r which any i n s t a r not a l r e a d y advanced to the a d u l t stadium would become an o v e r w i n t e r i n g a d u l t . Thus, any g e r r i d which had reached t h i s stage by t h i s date w i l l breed as soon as i t matures, and any g e r r i d which has not reached t h i s stage by t h i s date w i l l o v e r winter. 201 D. Temporal p a t t e r n of hunting bouts The b a s i c i t e r a t i v e u n i t of the model was one hour. P r e d a t i o n , the number of k i l l s made by each predator s i z e c l a s s on each prey s i z e c l a s s , was determined each hour f o r 24 hr, as i t was assumed that g e r r i d s hunt both at n i g h t and during the day. Both f e e d i n g experiments and p r e d a t i o n experiments i n d i c a t e d t h at the presence of l i g h t was not r e g u i r e d f o r these a c t i o n s to occur. O p e r a t i o n a l l y , however, l e s s p r e d a t i o n would u s u a l l y occur at night owing to the t y p i c a l l y c o o l e r temperatures and hence lower metabolism of the g e r r i d s . E. C a l c u l a t i o n of food i n the gut The amount of food i n the gut was determined i n each i t e r a t i o n hour. I f the food volume present i n the gut at the beginning of the i t e r a t i o n was l e s s than a s p e c i f i e d percent of the maximum gut c a p a c i t y , the amount of f o o d p r e s e n t i n the gut was s e t equal to t h i s amount. The s p e c i f i e d amount of food always present represented the food obtained from food items other than g e r r i d s . At the end of each i t e r a t i o n hour, the amount of food i n i t i a l l y present p l u s that obtained during the hour was d i g e s t e d , i e . reduced, by an amount determined from the nega t i v e e x p o n e n t i a l e g u a t i c n i n S e c t i o n 3. D i g e s t i v e r a t e v a r i e d , depending on whether the g e r r i d s were l a r v a e or a d u l t s , and was modified by temperature a c c o r d i n g to i t s e f f e c t on food consumption. 202 F. Estimated handling time and area searched Handling time comprised the amount of time spent feeding plus capture time (the amount of time spent s e a r c h i n g f o r a prey i t e m ) . Feeding time was a maximum of two hours f e r the l a r g e s t g e r r i d s t e s t e d i f they were maximally s t a r v e d . Since t h i s time p e r i o d exceeded the length cf the b a s i c i t e r a t i v e u n i t (one hour) , the g e r r i d s i n each s i z e c l a s s were assigned to one of two s t a t e s : those feeding and these not f e e d i n g . Those fee d i n g were not capable of hunting, and hence the amount of time spent hunting by the g e r r i d s i n each s i z e c l a s s v a r i e d , as time spent hunting per g e r r i d was the average among a l l the g e r r i d s present i n the s i z e c l a s s . Since both predator and prey g e r r i d moved, the o p e r a t i o n a l rate of movement between any two s i z e c l a s s e s was the sguare root of the sum of the sguares of t h e i r r e s p e c t i v e r a t e s of movement. S i z e c l a s s r a t e s of movement were i n f l u e n c e d by both s p e c i e s and the amount of fcod present i n the gut. Reactive d i s t a n c e of the predator was a l s o i n f l u e n c e d by the amount of food present i n the gut. Combining t h i s value with the o p e r a t i o n a l r a t e of movement allowed c a l c u l a t i o n of the swath through the environment swept by the g e r r i d . S i n c e a f i n i t e area (50 m2) of water s u r f a c e was being simulated, the area unswept was e<-£>, where S r e p r e s e n t s the area swept d i v i d e d by 50. 20 3 G. Calculation of potential number cf prey k i l l e d A random d i s t r i b u t i o n of the gerrids ever the 50 m2 simulation area was assumed. The number of encounters between each predator-prey size class combination was determined from the area swept and the prey density. The number of these encounters which resulted in k i l l s depended on the e f f i c i e n c y with which the prey was k i l l e d , predator aggressiveness, prey preference by the predator, the degree of environmental complexity, and the amount cf feed desired by the predator. The mean size of the prey r e l a t i v e to the mean amount of food desired by the predator determined whether a predator could k i l l more than one prey item per i t e r a t i o n i n t e r v a l . H. Calculation of actual number cf prey k i l l e d The potential number of each prey size class k i l l e d by each predator size class was determined assuming that these two size classes interacted independently of the other eight size classes. Hence, depending on the number of gerrids in a l l ten siz e classes, the t o t a l number of gerrids in a l l the size classes k i l l e d by a size class could easily exceed the number which that size class was actually capable of k i l l i n g . S i m i l a r l y , the tot a l number of prey in a size class k i l l e d by a l l i t s potential predator s i z e classes could exceed the number of prey actually present in that size c l a s s . The potential number of each prey size class k i l l e d by each predator size class was thus modified by the number of predators and prey actually present, to provide the actual numbers of each size 204 c l a s s k i l l e d . The d i s t r i b u t i o n of t h i s actual mortality over the species comprising the prey size c l a s s was weighted as to the number of each species present in that s i z e class. The numbers k i l l e d were subtracted from each si z e class at the end of each hourly i t e r a t i o n . Once the actual numbers k i l l e d by each si z e class had been determined, the actual volume of food gained and the actual handling time was determined i n preparation for the next i t e r a t i o n . RESULTS Figure 45 demonstrates the temporal pattern of breeding by each species when alone in 1971 as predicted by the model. It can be seen that although the predicted dates of the f i r s t appearance of f i r s t i nstar larvae of each species agree reasonably well with the observed f i e l d data, differences e x i s t , indicating that the measure of degree-days accumulated by the gerrids was only approximated. This was expected, for whenever a value has to be guessed, as with/the c o e f f i c i e n t r e l a t i n g water temperatures to gerrid body temperatures, errors a r i s e . More attention i s thus obviously needed i n determining the actual body temperatures experienced by gerrids in the f i e l d s i t u a t i o n i f a more exact simulation of the observed f i e l d data i s desired. The prediction of greatest interest here i s that G. 205 Fi g u r e 15. The p r e d i c t e d occurrence and numbers of f i r s t i n s t a r l a r v a e of each of the f i v e s p e c i e s produced when each s p e c i e s was considered s e p a r a t e l y and the stomachs were always 15% f u l l . 205a MY ' JN ' JU ' AU 1 i££3S.Si£HS i s a t r i v o l t i n e s p e c i e s . T h i s was not i n i t i a l l y obvious from the f i e l d data, as no attempt was made to i d i s t i n g u i s h between summer g e n e r a t i o n s . Restudying the f i e l d data, however, supports the model's p r e d i c t i o n s that G. i°£23IJitus might be t r i v o l t i n e i n a t l e a s t some h a b i t a t s , as summer a d u l t s were c o l l e c t e d on Marion L. as e a r l y as June 8 i n 1972. Since the data i n F i g . 45 are the p r e d i c t e d r e s u l t s when each of the f i v e s p e c i e s was run s e p a r a t e l y i n the model, the numbers of f i r s t i n s t a r l a r v a e present r e f l e c t o n l y the consequences of i n t r a s p e c i f i c c a n n i b a l i s m . U n f o r t u n a t e l y , the a c t u a l numbers of f i r s t i n s t a r l a r v a e p r e d i c t e d cannot a c c u r a t e l y be r e l a t e d to the f i e l d data, as the data obtained i n the model r e p r e s e n t the t o t a l number of g e r r i d s present i n the 50 m2 simulated a r e a . Observed f i e l d numbers were based on f i v e , 1 m2 samples, and s i n c e the d i s t r i b u t i o n of g e r r i d s i n the f i e l d i s somewhat contagious, depending on the v e g e t a t i o n p a t t e r n , a c c u r a t e v a l u e s r e p r e s e n t i n g the t o t a l number of g e r r i d s present may not have been ob t a i n e d . N e v e r t h e l e s s , the p r e d i c t i o n s are of i n t e r e s t , as they suggest when peaking of numbers may have occurred f o r each of the s p e c i e s i n the f i e l d . Only numbers of G. buenoi and G. i n c u r v a t u s are p r e d i c t e d to peak i n l a t e summer, i n agreement with the observed f i e l d data. I t should be noted, however, t h a t s i n c e development, l i k e egg l a y i n g , i s temperature dependent, the model's p r e d i c t i o n s here, l i k e those above, are onl y as v a l i d as the assumptions used i n the model. One would t h e r e f o r e not expect the model's 207 p r e d i c t i o n s to e x a c t l y d u p l i c a t e the f i e l d o b s e r v a t i o n s . For comparison of d i f f e r e n t s i m u l a t i o n s , perhaps the best measure of how the s p e c i e s are a f f e c t e d i s i n terms of the number o f a d u l t females produced during the course of the season. I f two e c o l o g i c a l homologues i n egual numbers happen to invade or are simulated to invade a new h a b i t a t s i m u l t a n e o u s l y , then the winning s p e c i e s e v e n t u a l l y w i l l be that which produces the g r e a t e s t number of female progeny which s u r v i v e to reproduce per u n i t time. In t h i s model, u n i t time would be one summer, as the model has not been c o n s t r u c t e d t o i n c l u d e o v e r w i n t e r i n g . In the b i v o l t i n e s p e c i e s , a d u l t females are of two types: those which breed the same season they mature, and those that w i l l o verwinter to breed the f o l l o w i n g s p r i n g . To d i s t i n g u i s h the r e l a t i v e numbers of each female type, the p a t t e r n of p r e s e n t a t i o n shown i n F i g . 46 w i l l be used i n the f o l l o w i n g t a b l e s to present the r e s u l t s of each run of the model. Two measures of s p e c i e s success are thus a v a i l a b l e : the t o t a l number of females produced, i e . i n c l u d i n g both female types, and the t o t a l number of females which w i l l overwinter and e s t a b l i s h the s p e c i e s 1 p o p u l a t i o n s i n the f o l l o w i n g year. Table 29 shows the p r e d i c t e d e f f e c t of hunger l e v e l on the number of a d u l t females produced when each s p e c i e s was run s e p a r a t e l y i n the model. Mean hunger l e v e l of each g e r r i d was c o n t r o l l e d i n the model by modifying the percent of the maximum gut c a p a c i t y . I t should be noted that i f the guts of the g e r r i d s were always f i l l e d with food, no ca n n i b a l i s m would o c c u r , and the g r e a t e s t number of females would be produced. 208 Figure 46. The manner of presentation of predicted numbers of adult female progeny produced during the season by any one species. 2S£ 208a | Overwintered | a d u l t s | I I | A d u l t female | s p r i n g | o f f s p r i n g Breeding o f f s p r i n g | I i— | C v e r w i n t e r i n g | o f f s p r i n g | I | Adult female summer | o f f s p r i n g r ~% I I | Breeding I | o f f s p r i n g | Over w i n t e r i n g | o f f s p r i n g A d u l t female f a l l o f f s p r i n g | | Overwintering! | o f f s p r i n g | | T o t a l | breeding | o f f s p r i n g j T o t a l o v e r w i n t e r i n g o f f s p r i n g 209 Table 29. The p r e d i c t e d number of a d u l t female progeny produced at the end of the summer at each of three hunger l e v e l s when each s p e c i e s was considered s e p a r a t e l y . Species G. i n c u r v a t u s G. i n c o g n i t u s G. n o t a b i l i s G. r e m i g i s G. buenoi Gut 25% f u l l 10 Gut 50% f u l l 10 Gut 75% f u l l 10 1 0 117 117 164 104 13 104 13 141 23 861 1005 1458 0 861 0 1005 0 1481 "J04 874 104 lol8 lui 14 81 10 10 48 48 35 48 0 35 0 35 0 296 193 203 1 295 2 191 2 20 1 7 15 11 0 7 0 15 0 11 49 312 37 206 ~37 2*12 10 10 10 10 10 463 0 463 "0 463 186 0 186 '0 "186 162 127 35 809 0 809 127 844 10 10 446 0 446 "O 44 6 162 0 162 "0 162 167 129 38 842 0 842 129 88 0 10 10 409 0 409 '0 409 162 0 162 0 162 202 170 52 1229 0 1229 776" 1281 210 S i n c e no animal i n the f i e l d i s always s a t i a t e d , however, t h i s c o n d i t i o n was never simulated. Among the hunger l e v e l s s i m u l a t e d (guts 1/4, 1/2 and 3/4 f i l l e d with f o o d ) , the p r e d i c t i o n s here are of i n t e r e s t . G. i n c o g n i t u s , G. r e m i g i s and G. n o t a b i l i s should each produce the g r e a t e s t number of female progeny when food i s i n s h o r t supply, whereas the g r e a t e s t number of G. i n c u r v a t u s and G. buenoi females to s u r v i v e would be when food was only s l i g h t l y l i m i t i n g . These p r e d i c t i o n s presumably r e f l e c t the i n t r a s p e c i f i c i n t e r a c t i o n s r e s u l t i n g from the observed d i f f e r e n t e f f e c t s of hunger on s p e c i e s aggressiveness and s p e c i e s k i l l e f f i c i e n c y . Table 30 presents the p r e d i c t e d r e s u l t s when G. n o t a b i l i s was c o n s i d e r e d alone at three i n i t i a l d e n s i t i e s of overwintered a d u l t s . In a l l three s i m u l a t i o n s , the gut was assumed 3/4 f i l l e d with food. I t can be seen t h a t although the number of progeny produced i n c r e a s e s with i n c r e a s i n g i n i t i a l numbers of g e r r i d s , the r a t e of progeny i n c r e a s e d e c l i n e s . T h i s suggests, as was expected, that t h e r e i s a t h r e s h o l d of number of overwintered a d u l t s present above which a f u r t h e r i n c r e a s e i n number w i l l have no e f f e c t on the number of progeny produced. The main f e a t u r e to be noted here, however, i s that the t o t a l d e n s i t i e s of overwintered a d u l t females used i n most of the model's s i m u l a t i o n s (20-100) are well below t h i s t h r e s h o l d . These d e n s i t i e s were a l s o those commonly observed i n the f i e l d s i t u a t i o n . The conseguences of a l l the s p e c i e s c o e x i s t i n g i n the same 211 Table 30. The predicted number of adult female progeny produced at the end of the season from varying numbers of overwintered G. n o t a b i l i s . 10 25 459 0 459 50 402 0 702 830 0 830 459 702 830 \ 212 m i c r o h a b i t a t are shown i n Table 31. At a low hunger l e v e l (gut 3/U f i l l e d ) , o n l y G. i n c o g n i t u s i s s u c c e s s f u l i n producing a l a r g e number of young, and G. i n c u r v a t u s i s e l i m i n a t e d e n t i r e l y from the system. I n c r e a s i n g hunger has no e f f e c t i n a l t e r i n g the outcome, but r e s u l t s i n an even b e t t e r s u r v i v a l of G. iJQ£03ili*Jas females, and the a d d i t i o n a l e l i m i n a t i o n of both G. r e m i g i s and G. buenoi from the system. The e x p l a n a t i o n f o r the success of G. i n c o g n i t u s , c o n s i d e r i n g i t s r e l a t i v e l y low f e c u n d i t y , i s the low minimum temperature f o r growth demonstrated by t h i s s p e c i e s and hence the e a r l y s t a r t to breeding. In the e a r l y s p r i n g , the l a r v a e of other s p e c i e s grow only very s l o w l y i f a t a l l owing to the c o o l temperatures whereas G. i n c o g n i t u s l a r v a e grow r e l a t i v e l y g u i c k l y . G. i n c o g n i t u s l a r v a e are thus a b l e to reach a s i z e which allows them to prey on the i n s t a r s of the other s p e c i e s , and t h i s combined with i t s a b i l i t y to have three g e n e r a t i o n s , would seem s u f f i c i e n t to e f f e c t i v e l y e l i m i n a t e the other c o e x i s t i n g s p e c i e s . In the f o l l o w i n g simulated s i t u a t i o n s , hunger i s always low, with the gut always 3/1 f i l l e d with food. The p r e d i c t i o n s made by the model should thus be i n t e r p r e t e d with t h i s i n mind. < I t i s not r e a l i s t i c to simulate G. r e m i g i s c o e x i s t i n g with other s p e c i e s on the pond h a b i t a t , s i n c e i t occurs p r i m a r i l y on streams. Table 32 p r e s e n t s the p r e d i c t i o n s obtained when only the four pond s p e c i e s are thus c o n s i d e r e d . I t can be seen that G- inco£nitus i s s t i l l dominant, although the other three s p e c i e s do produce s l i g h t l y more progeny and no s p e c i e s i s 213 Table 31. The p r e d i c t e d number of a d u l t female progeny produced at the end of the season at each of three hunger l e v e l s when a l l f i v e s p e c i e s are c o n s i d e r e d t o g e t h e r . Species Gut 25% f u l l Gut 50% f u l l Gut 75% f u l l G. i n c u r v a t u s 10 10 10 0 0 0 0 _0 0 0 _0 0 ~, 0 0 0 0 0 0 G. i n c o g n i t u s 10 10 10 37 38 37 37 0- 38 0 37 0 285 213 214 0 285 0 213 0 214 37 285 38 2 1 1 37 214 G. n o t a b i l i s 10 10 10 3 10 12 0 3 _ 0 _J0 0 J2 0 3 0 10 0 ~12 G. buenoi 10 10 10 0 4 5 0 0 0 _4 0 5 0 0 0 4 0 5 G. buenoi 10 10 10 0 0 1 g g o o o _1 o o ~o o o ~ T 214 Table 32. The predicted number of adult female progeny produced at the end of the season when only the four pond species are considered together. G. n o t a b i l i s G. incurvatus G. buenoi G. incognitus (i) 10 10 . 1 0 10 18 2 8 0 18 0 2 0 8 0 ~"l8 0 2 0 8 (i i ) 10 10 10 ! 16 1 6 0 16 0 1 0 6 43 43 0 261 0 261 43 261 35 35 0 229 1 228 7 0 7 36 235 0 16 0 1 0 6 ( i i i ) 10 10 10 3 24 5 10 24 0 24 0 5 0 10 24 0 148 _ 0 148 0 24 0 5 0 10 24 148 (iv) 20 20 20 10 17 0 3 0 17 0 0 0 3 0 17 0 0 0 3 (continued) 43 43 258 0 1 11 257 0 11 44 268 214-a G. n o t a b i l i s G. i n c u r v a t u s G. buenoi G. i n c o g n i t u s (v) 20 20 20 3 34 5 11 22 0 34 0 5 0 11 22 0 140 _0 140 0 34 0 5 0 1 1 ~22 "140 (vi) 25 25 . 25 1 188 16 29 8 0 188 8 8 10 19 8 0 122 208 6a 0 .122 0 208 _0 64 0 188 8 130 ~10 227 ~ 8 ~64 215 e n t i r e l y e l i m i n a t e d from the system. In an e f f o r t to determine what the r a t i o of G. i n c o j j n i t u s must be r e l a t i v e to the other s p e c i e s present to allow c o e x i s t e n c e of a l l f o u r s p e c i e s , the model was run with v a r y i n g p r o p o r t i o n s of the f o u r pond s p e c i e s present. Only at the lowest G. i n c o g n i t u s d e n s i t y t e s t e d , when only one p a i r of G. i n c o g n i t u s was i n i t i a l l y present compared to 25 p a i r s cf each of the other three s p e c i e s , was the e f f e c t of i t s i n i t i a l h e a d start reduced enough to permit even' a moderate est a b l i s h m e n t of the other three s p e c i e s i n the system. The advantage e s t a b l i s h e d by the low minimum growth t h r e s h o l d of G. i n c o g n i t u s i s thus of major s i g n i f i c a n c e . I t should be noted that c o n s i d e r i n g only the t h r e e s m a l l s p e c i e s together (G. incognijbus, G. i n c u r v a t u s and G. buenoi) the r e s u l t s show again dominance by G. i n c o g n i t u s , i n d i c a t i n g t h a t G. buenoi and G. i n c u r v a t u s numbers were not decimated by G. n o t a b i l i s . Table 33 presents the p r e d i c t e d r e s u l t s when only the s p e c i e s i n h a b i t a t i n g the open water pond h a b i t a t (G. buenoi, G. ii}£iJ£JSS.tus a n d G. n o t a b i l i s ) are c o n s i d e r e d together. a l l three s p e c i e s are capable of c o e x i s t i n g , even when only one p a i r of each of the three s p e c i e s are c o n s i d e r e d . The a b i l i t y of G. n o t a b i l i s to grow at lower temperatures than the the ether two s p e c i e s seems to have l i t t l e e f f e c t here, presumably because the u n i v o l t i n e nature of G. n o t a b i l i s , leaves i t unable to f o l l o w up the advantage i n i t i a l l y o b t a i n e d . I f the suggested 15°C. t h r e s h o l d which must be exceeded before breeding of G. n g t a b i l i s can commence i s removed, the r e s u l t s are s t i l l the same. The 216 T a b l e 33. The p r e d i c t e d number o f female progeny produced when o n l y t h e open water pond s p e c i e s a r e c o n s i d e r e d t o g e t h e r . * = the temperature t h r e s h o l d which must be exceeded f o r G. U 2 i a b i l i s to commence b r e e d i n g has been removed. G. n o t a b i l i s G. i n c u r v a t u s G. buenoi ( i ) 10 10 10 435 0 435 0 435 ( i i ) 10* 10 436 0 436 0 436 ( i i i ) 10 460 0 460 0 460 (i v ) 10 10 431 0 431 0 431 (v) 10 14 18 9 5 5 13 277 163 0 277 0 163 9 282 5 176 i 10 9 18 7 2 6 12 274 233 0 274 0 • 233 7 276 6 245 10 19 9 10 390 0 390 "9 400 21 14 7 410 0 410 417 10 104 176 91 13 107 69 513 670 0 513 0 670 91 526 ?07 739 217 s l i g h t temporal breeding s e p a r a t i o n produced by t h i s t h r e s h o l d thus has l i t t l e e f f e c t . S i m u l a t i o n of c o e x i s t e n c e of the two u n i v o l t i n e s p e c i e s (Table 34), G. n o t a b i l i s and G. r e m i g i s , suggests that G. not a b i l i s would dominate, presumably owing to the g r e a t e r f e c u n d i t y of t h i s s p e c i e s . The g r e a t e r aggressiveness of G. r e m i g i s would not help, as i t would be d i r e c t e d a g a i n s t i t s own young as w e l l . The delayed i n i t i a t i o n of breeding by G. £2tabilis again has l i t t l e e f f e c t on the f i n a l outcome. DISCUSSION The main concern of t h i s s e c t i o n has been the s y n t h e s i s of much of the data obtained i n the preceeding f i v e s e c t i o n s i n t o a working model designed to simulate the i n t e r a c t i o n s between g e r r i d s . Models can be very v a l u a b l e and elegant t o o l s , i n that they allow t e s t i n g of hypotheses which, owing perhaps to t h e i r complexity and l e n g t h , would be i m p r a c t i c a l to study using more c o n v e n t i o n a l t e c h n i q u e s . However, the value of a model and i t s p r e d i c t i o n s can only be r e a l i z e d i f the r i g h t c r i t e r i a are used t o assess i t . The number of reproducing a d u l t females i n one g e n e r a t i o n per u n i t female of the preceeding generation has been used as a measure of the r e l a t i v e success of a s p e c i e s i n a number of s t u d i e s (DeBach, 1969; G i l b e r t and Hughes, 1971). One main advantage of t h i s c r i t e r i o n i s t h a t i t takes i n t o account 218 T a b l e 34. The p r e d i c t e d number of female progeny produced at the end of the season when the th r e e s m a l l s p e c i e s and the two l a r g e s p e c i e s are con s i d e r e d s e p a r a t e l y , * = the temperature t h r e s h o l d which must be exceeded before G. n o t a b i l i s N can commence breeding has been removed. (i) G. i n c u r v a t u s 10 ( i i ) ( i i i ) 1 0 33 0 32 0 G. buenoi 10 32 G. n o t a b i l i s 10 387 0 0 3 87 387 G. n o t a b i l i s 10* 413 0 4J3 0 413 G. i n c o g n i t u s 10 36 36 212 0 1 13 211 0 13 37 224 G. .remigis 10 62 0 0 62 62 G. r e m i g i s 10 18 J) 0 18 "l8 219 m o r t a l i t y of the immature stages because only the female progeny which mature are counted. In t h i s study, t h i s c r i t e r i o n joer se cou l d not be s t r i c t l y used, as no data on o v e r w i n t e r i n g s u r v i v a l of each of the s p e c i e s were a v a i l a b l e . I f the number cf females d e s t i n e d f o r o v e r w i n t e r i n g are used, and winter s u r v i v a l d i s c r i m i n a t e s a g a i n s t the s p e c i e s d i f f e r e n t l y , then any c o n c l u s i o n s d e r i v e d from f a l l numbers would be wrong. For comparative purposes, however, the only measure a v a i l a b l e i n t h i s study were the number of a d u l t females which mature, and so the assumption here i s that winter s u r v i v a l does not d i s c r i m i n a t e a g a i n s t the s p e c i e s d i f f e r e n t l y . Obviously t h i s i s a major assumption, and one which c e r t a i n l y needs f u t u r e i n v e s t i g a t i o n . Another c o n s i d e r a t i o n i s that the breeding success c f a female depends on when i n the season the female breeds. Thus, an o v e r w i n t e r i n g female of a b i v o l t i n e s p e c i e s might produce, owing to heavy c a n n i b a l i s m i n the s p r i n g , only a few females, of which some w i l l be d e s t i n e d to overwinter and some w i l l be de s t i n e d to breed again the same year. The l a t t e r females, perhaps owing to an absence of other g e r r i d s i n l a t e summer, might be very s u c c e s s f u l , producing many a d u l t female progeny. What c r i t e r i o n should thus be used to assess r e p r o d u c t i v e success of the p o p u l a t i o n of a s p e c i e s d u r i n g the season? G i l b e r t and G u t i e r r e z (1973) r e c o g n i z e d t h i s problem when they s t a t e d t h a t "maximizing i n d i v i d u a l " f i t n e s s " i s c e r t a i n l y not the same t h i n g as minimizing the p r o b a b i l i t y of e x t i n c t i o n " . The problem here i s weighting the success of an i n d i v i d u a l 220 a g a i n s t the p r o b a b i l i t y of the s p e c i e s going e x t i n c t . To avoid t h i s , G i l b e r t and G u t i e r r e z (1973) have determined the " f i t n e s s " of the p o p u l a t i o n f o r a number of t i m e - i n t e r v a l s , and i t i s the sum of a l l these " f i t n e s s e s " which the p o p u l a t i o n should, i n t h e o r y , attempt to maximize. In a sense, t h i s has been done here, with the time i n t e r v a l s i n the b i v o l t i n e and t r i v o l t i n e s p e c i e s being one g e n e r a t i o n . The number of adult^ daughters produced by the a d u l t female o f f s p r i n g of the overwintered females have been considered as progeny of the overwintered females, thereby e s t a b l i s h i n g a p o p u l a t i o n concept. S i n c e i t has already been assumed that winter s u r v i v a l has no r e l a t i v e e f f e c t s , the success of a s p e c i e s can thus be measured i n terms of the number of o v e r w i n t e r i n g a d u l t s . One important p o i n t to s t r e s s with r e s p e c t to the present model i s the f a c t t h a t the data used i n the c o n s t r u c t i o n of the model were obtained from l a b o r a t o r y experiments. Only the environmental temperature data were taken from the f i e l d r e s u l t s . I t i s t h e r e f o r e of i n t e r e s t to note that the p r e d i c t i o n s of the present model agree f a v o u r a b l y with those r e s u l t s observed i n the f i e l d . S i n c e no e f f o r t was made to formulate a model, and then through varying the parameters, o b t a i n good approximations of census h i s t o r i e s , the present model i s f e l t to i n d i c a t e some of the mechanisms c o n t r o l l i n g the system being s t u d i e d . Cannibalism i n G e r r i s would thus seem t c be an important process whereby po p u l a t i o n numbers may be r e g u l a t e d . I t can l e a d to s p e c i e s e x t i n c t i o n s i n c o e x i s t i n g s i t u a t i o n s , and where on l y one s p e c i e s i s p r e s e n t , r e s u l t s i n a lower number of surviving progeny. 222 GENERAL DISCUSSION A c o n s i d e r a b l e body of i n f o r m a t i o n concerning the b i o l o g y of g e r r i d s has been accumulated i n t h i s study. I t has been shown that the f i v e s p e c i e s of G e r r i s are not e c o l o g i c a l homologues and t h a t both temporal and s p a t i a l d i f f e r e n c e s e x i s t between the s p e c i e s . Two of the s p e c i e s , G. r e m i g i s and G. iS£23£iiS§» p r e f e r s p e c i f i c h a b i t a t s unigue to that s p e c i e s , and where the s p e c i e s p r e f e r the same h a b i t a t , as do G. n o t a b i l i s , G. i n c u r v a t u s and G. buenoi, m i c r o h a b i t a t d i f f e r e n c e s e x i s t . T h i s i n f o r m a t i o n , however, does not demonstrate why these d i f f e r e n c e s may e x i s t and what the conseguences might be i f the s p e c i e s d i d not have such h a b i t a t d i f f e r e n c e s . Such g u e s t i c n s can only be framed in a model, as only i n a mcdel i s i t p r a c t i c a l to s i m u l a t e the numerous i n t e r a c t i o n s of a l l the combinations of c o e x i s t i n g s p e c i e s which are of i n t e r e s t . I t i s of v a l u e , t h e r e f o r e , to c o n s i d e r the i m p l i c a t i o n s of the p r e d i c t i o n s of the model i n terms c f how do the d i f f e r e n c e s between the s p e c i e s i n f l u e n c e c o e x i s t e n c e . I t should be pointed out once again t h a t , the v a r i o u s b i t s and p i e c e s of data that went i n t o the model were de r i v e d from independent study of thousands of g e r r i d s i n the l a b o r a t o r y . The model i s thus nothing but a t o o l designed to answer b i o l o g i c a l guestions based on the above data, and by i t s e l f , has no i n t r i n s i c value. The f i r s t major p r e d i c t i o n by the model i s that i n a h a b i t a t i n which G. i n c o g n i t u s breeds throughout the season, no other s p e c i e s produces a s i g n i f i c a n t number of a d u l t female progeny. Even a few breeding G. i n c o g n i t u s i s s u f f i c i e n t to 223 e s t a b l i s h t h i s one as the dominant s p e c i e s . T h i s p r e d i c t i o n i s supported by the observed f i e l d data. On 0 I n l e t , the o n l y area s t u d i e d where G. i n c o g n i t u s bred throughout the season, G. i n c o g n i t u s was dominant. On Marion Lake and Gate Pond, where other s p e c i e s were pr e s e n t , G. i n c o g n i t u s was only present e a r l y i n the season and l a r g e l y l e f t these areas i n l a t e June f o r some reason. Thus the the success o f G. i n c g g n i t u s i n a c o e x i s t i n g s i t u a t i o n i s because of (1) i t s a b i l i t y to u t i l i z e temperatures f o r growth lower than those capable of being u t i l i z e d by the other s p e c i e s , and (2) i t s p o t e n t i a l f o r being b i v o l t i n e and even perhaps t r i v o l t i n e , which allows i t t o c a p i t a l i z e on the h e a d s t a r t c r e a t e d i n (1). Hence, only i n areas where G. iH£23Uiius i s present a l l summer does i t e l i m i n a t e the other s p e c i e s present. G. n o t a b i l i s , which has a minimum temperature f o r growth on l y s l i g h t l y above that of G. i n c o g n i t u s , i s u n i v o l t i n e and hence i s unable to e x p l o i t any advantages obtained from an e a r l y s t a r t . The second major p r e d i c t i o n of the model i s that G. n o t a b i l i s , G. i n c u r v a t u s and G. buenoi are capable of c o e x i s t i n g together i n the same h a b i t a t f o r at l e a s t one season. Long term c o e x i s t e n c e was not s t u d i e d i n the present study. I t i s thus of i n t e r e s t to note that i n the f i e l d on the open water h a b i t a t , where a l l three s p e c i e s c o e x i s t , s p a t i a l s e p a r a t i o n (expressed i n terms of p r e f e r r e d d i s t a n c e form shore) was found to e x i s t . T h i s supports the c o m p e t i t i v e displacement p r i n c i p l e , and may be the mechanism which allows c o e x i s t e n c e between these s p e c i e s over a number of y e a r s . I f they were to c o e x i s t i n the same 224 m i c r o h a b i t a t , i n theory at l e a s t , given enough time, the s p e c i e s producing the g r e a t e s t number of reproducing female progeny would be expected to g r a d u a l l y e l i m i n a t e the other two s p e c i e s . U n f o r t u n a t e l y , however, as i n the case of M i t c h e l l ' s (1969) study, the e v o l u t i o n a r y h i s t o r i e s of the s p e c i e s being s t u d i e d are unknown. Hence, although i t has been shown t h a t i n theory the t h r e e open water pond s p e c i e s can c o e x i s t i n the same h a b i t a t f o r a t l e a s t one season, i t must be concluded that there i s no b a s i s f o r arguing that c o m p e t i t i o n i n the past caused s p e c i e s of G e r r i s to s p e c i a l i z e so as to d i v i d e resources among them. F i n a l l y , G. r e m i g i s would seem to demonstrate adaptive r a d i a t i o n w i t h i n the genus to e x p l o i t a new h a b i t a t . I t i s capable of c o e x i s t i n g with the pond s p e c i e s i n theory, but i t appears that i t has gone i t s own way and has s p e c i a l i z e d to e x p l o i t the stream h a b i t a t . In c o n c l u s i o n , c a n n i b a l i s m would seem to be a major f a c t o r capable of i n f l u e n c i n g c o e x i s t e n c e i n G e r r i s . I t can determine which s p e c i e s are capable of c o e x i s t i n g together. F i g u r e 47 summarizes the c h a r a c t e r i s t i c s of the f i v e s p e c i e s which appear important i n t h e i r c o e x i s t e n c e . Many problems s t i l l remain to be i n v e s t i g a t e d and the model should be put to independent experimental v e r i f i c a t i o n . Much data i s s t i l l needed before the model can be expected to run f o r s e v e r a l seasons. Only then can a complete understanding of c o e x i s t e n c e i n G e r r i s be a t t a i n e d . The data presented here, however, are f e l t to l a y the groundwork f o r f u t u r e study, and l i k e most b i o l o g i c a l s t u d i e s , i n doing so, 225 Figure 47. which seem A of summary of the c h a r a c t e r i s t i c s of the f i v e major importance i n th e i r coexistence. species 225a i 1 I , I 1 G. remigis | I I | Isolated by stream | | and cold water | | preferences. j 1^  I I G. incognitus | | Isolated by preference | | f o r l i t t e r e d water ) | surface habitat. j | Dominates i n sympatric | | situations owing to | | i n i t i a l headstart | | obtained in breeding. j G. buenoi G. incurvatus G. n o t a b i l i s Prefer open water habitat. Able to coexist i n short-term sit u a t i o n s . 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Ent. exp. appl., 13: 107-121. 232 APPENDIX 1 SUBROUTINE EGG(DTEMP,N,NAF,NINAA,M,NEG,KN,XMORT) C AEG = MAXIMUM ACCUMULATIVE NUMBER OF EGGS LAID BY EACH SPECIES C. BEG = Y - INTERCEPT OF EGGLAYING EQUATION C NEDA = NUMBER OF DEGREE - DAYS AT DAY M C NENA = NUMBER OF MATURE FEMALES AT DAY M C NINAA = NUMBER OF EGGS LAID BY EACH SPECIES ON DAY M C S EG = SLOPE IN EGGLAYING EQUATION DIMENSION NEG (5) DIMENSION DTEMP (5,1) DIMENSION NEDA (5,100) , NENA (5, 100) ,M (5) ,SEG (8) ,BEG(8) DIMENSION AEG (8) DIMENSION NAF (5) ,NINAA (5) IF(KN.NE.O) GO TO 1 C INITIALIZES PARAMETERS 10=6 IN=5 999 FORMAT (8F6. 2) 998 FORMAT (F6.0) 997 FORMAT (8F6. 4) DO 3 J = 1,5 M (J) =0 DO 3 J J = 1,100 NEEA(J,JJ) = 0 3 NENA (J, JJ) = 0 READ(IN,997) (SEG(J) ,J=1,8) READ (IN,999) (AEG (J) , J= 1, 8) READ(IN,999) (BEG (J) , J= 1,8) C TERM = MAXIMUM NUMBER OF DAYS OVER-WINTERING ADULTS LAY EGGS BEFORE C OLD AGE MORTALITY COMMENCES AND DATE OF SWITCH TO 2ND GENERATION C EGG-LAYING READ (IN, 998) TERM TTERM = TERM + 10. GO TO 9 C IF THE FEMALES ARE TURNED ON, THE NUMBER OF DEGREE-DAIS SINCE THEY C WERE TURNED ON IS DETERMINED AND THE NUMBER OF EGGS TO EE ADDED TO C THE SYSTEM IS CALCULATED 1 DO 25 JL = 1,5 IF (NEG (JL) ) 25 ,25,26 26 M (JL) = M (JL) +1 IF (M (JL) .GT.99) M (JL) = 99 MM = M (JL) C ADD DEGREE-DAYS TO DAILY TOTALS AND MOVE TOTALS AHEAD ONE DAY DEGEG = DTEMP (JL,N) IF ( DEGEG.LT.O.) DEGEG = 0. NMN = DEGEG + 0.5 IF (MM-1)30,30 ,31 31 DO 24 J = 2,MM JE = J - 1 IF (JE. EQ. 1) NXEDA = NEDA (JL , JE) NNEDA = NXEDA NX EDA = NEDA (JL, J) IF(JE.EQ.1)NEDA (JL,JE) = DEGEG + 0.5 233 + 0.5 EACH DAi AHEAD ONE DAY AND INPUTS 24 NEBA(JL,J)=NNEDA + DEGEG + 0.5 30 IF (MM.EQ.1)NEDA (JL,MM) = DEGEG C MOVES TOTALS OF MATURE FEMALES FOR C NEW ADULTS IF (MM - 1) 32,32,33 33 DO 34 J - 2,MM JEN = J - 1 IF (JEN.EQ. 1) NXENA = NENA (JL, JEN) NNENA = NXENA NXENA = NEN A (JL, J) IF (JEN. EQ. 1) NENA (JL, JEN) = NAF (JL) 34 NENA (JL,J) = NNENA C DETERMINES DAILY AND OLD AGE MORTALITY OLD=0. DO 2 J=1,MM NDAI= (NENA (JL,J)*XMOBT) + 0.5 NENA (JL, J) = NENA (JL , J) - NDAI NNED=NEDA (JL,J) IF(JL.EQ. 1. AND.NNED.GT.295) OLD IF (JL.EQ.2.AND.NNED.GT.225)OLD IF (N. GT.TTERM) GO TO 4 IF (JL.EQ.3.AND.NNED. GT. 211) OLD IF (JL.EQ. 4. AND.NNED.GT. 155) OLD IF (JL.EQ.5.AND.NNED.GT.198)OLD GO TO 5 4 IF (JL.EQ.3.AND.NNED.GT.259)OLD IF (JL. EQ. 4. AND.NNED.GT. 180) OLD IF (JL.EQ. 5. AND.NNED.GT. 286) OLD 5 NOLD = (OLD * NENA (JL, J) ) + 0.5 NENA (JL,J) = NENA (JL,J) - NOLD IF (NENA (JL, J) .LT.O) NENA (JL, J) =0 2 CONTINUE 32 IF (MM. EQ. 1) NENA(JL,MM) = NAF (JL) C DETEBNINES NUMBER OF EGGS LAID ON BASIS OF SUMMED DEGREE-DAYS AND C NUMBER OF MATURE FEMALES NOTEG = 0 IF(N.LT.TTERM.OR.N.GT.TTERM) GO TO 40 DO 41 JN=3,5 ( (NNED - 295)/5.) * 0. 05 ( (NNED - 225)/5.) * 0. 05 ( (NNED - 21D/5.) * 0. 05 ( (NNED - 155)/5.) * 0. 05 ( (NNED - 198) /5.) * 0. 05 ( (NNED - 259)/5.) * 0. 05 ( (NNED - 180)/5.) * 0. 05 ( (NNED - 286)/5.) * 0. 05 41 40 JJ=JN+3 SEG (JN) = AEG(JN) = SEG (JJ) AEG(JJ) BEG (JN) = BEG (JJ) DO 36 J=1,MM IF (NENA (JL, J) .EQ.O) X EG = NEBA (JL , J) YEG = AEG (JL) / (1. + IF IF IF IF IF GO TO 36 EXP (BEG (JL) - (SEG(JL) * XEG) ) ) (JL. EQ. LAND.YEG.GT. 203.) YEG = 203. (JL.EQ.2. AND.YEG.GT.98. ) YEG = 98. (JL.EQ.3.AND.YEG.GT.72.) YEG = 72. (JL.EQ.4.AND.YEG.GT.123.) YEG = 123. (JL.EQ.5.AND.YEG.GT.153.) YEG = 153. IF (YEG.LT.O.)YEG = 0. YEG = YEG * NENA(JL,J) XXEG = NEDA (JL, J) - NMN Z EG = AEG(JL)/(1. + EXP(BEG(JL) - (SEG (JL) * XXEG))) 234 IF (JL.EQ.1.AND. ZEG. GT. 203. ) ZEG= 203. - 0.5 IF (JL.EQ. 2.AND.ZEG.GT.98.) ZEG = 98. - 0.5 IF (JL.EQ.3.AND.ZEG.GT.72. ) ZEG = 72. - 0.5 IF (JL. EQ.4.AND.ZEG.GT. 123.) ZEG = 123. - 0.5 IF (JL.EQ.5.AND.ZEG.GT. 153. ) ZEG = 153. - 0 . 5 IF (ZEG.LT.O.) ZEG = 0. ZEG = ZEG * NENA (JL,J) IF(N.GT.40. AN E. MOD (N , 1 60) .EQ.O) WRITE (10,995) YEG , ZEG 995 FORMAT (4X,2 (F6.2,2X) ) NOTEG = NOTEG 4 YEG -ZEG + 0.5 36 CONTINUE N IN AA (JL) = NOTEG 25 CONTINUE 9 RETURN END 4 C*AD = SLOPE IN HUNGER EQUATION (INSTAR OR ADULT) C*ADI = SPECIES INSTAR MAXIMUM DAILY INTAKE AT 26 C. C*AHU = SPECIES INSTAR MAXIMUM GUT CAPACITY C ASYM •= MEAN SIZE CLASS MAXIMUM GUT CAPACITY C*AT = Y-INTERCEPT OF TEMPERATURE REGRESSION C*BT = UNIT INCREMENT OF TEMPERATURE REGRESSION C*DV = SIZE CLASS PREFERENCE VALUE C*EFF = SIZE CLASS KILL EFFICIENCY C F = FOOD IN GUT C FDES = MEAN SIZE CLASS FOOD DESIRED (SIZHG - F) C FFDES = INITIAL MEAN SIZE CLASS FOOD DESIRED (SIZHG -' F) C*HA = SPECIES CONSTANT IN EFFECT OF HUNGER ON % KILL C*HB = SPECIES CONSTANT IN EFFECT OF HUNGER ON % KILL C HP = SPECIES % KILL COEFFICIENT RELATIVE TO NO FOOD IN GUT C HUGIN = MEAN INSTAR FOOD NEEDED C L = NUMBER OF CONSECUTIVE DAYS WITH TEMPERATURE ABOVE THE THRESHOLD C REQUIRED TO COMMENCE EGGLAYING C M = NUMBER OF DAYS SINCE SPECIES COMMENCED EGGLAYING C*MICRO = DAY THRESHOLD BEFORE WHICH IMAGOS OF BIVOLTINE SPECIES C ARE SHORTWING C MISC = NUMBER OF KILL ITEMS OF BIOMASS WT1 DISCARDED C MWK = MEAN WEIGHT OF KILL ITEM BY SIZE CLASS C NAF = NUMBER OF FEMALES TO MATURE ON EACH DAY C NAFB = NUMBER OF EACH SPECIES MATURING TO BREED ON DAY N C*NCDM = NUMBER OF DEGREE DAYS BETWEEN SPECIES - INSTABS MOULTS C NDIAP = NUMBER OF SPECIES-ADULTS THAT ENTER DIAPAUSE ON DAY N C NDP = NUMBER OF EACH SPECIES ENTERING DIAPAUSE ON DAY N C NEG = OFF - ON SWITCH FOR SPECIES TURNED ON TO REPRODUCTION C NIDA = NUMBER OF SPECIES- DEGREE DAYS ON DAY N C NGER = NUMBER OF SPECIES-INSTARS ON DAY N C NIN = NUMBER IN SPECIES-INSTAR COHORT ON DAY (N-1) C*NINA = NUMBER OF ANIMALS AT DAY N C NINS = NUMER IN SPECIES - INSTAR COHORTS C HK = NUMBER OF PREY POTENTIALLY KILLED C NKDT = NUMBER OF SIZE CLASS KILLED PER DAY C NKST = NUMBER OF SIZE CLASS KILLED PER ITERATION C NKT = TOTAL NUMBER ACTUALLY KILLED BY SIZE CLASS C*NOSIZ = INSTAR SIZE CLASS NUMBER C NRR = NUMBER OF MATURE ADULTS THAT ARE KILLED THROUGH PREDATION EACH C DAY 235 C NSIOO = NUMBER IN SIZE CLASS AT BEGINNING OF DAY C NSIZ = NUMBEB IN SIZE CLASS COHORTS C NSIZO = NUMBER IN SIZE CLASS AT BEGINNING OF ITERATION C NSIZ1 = SIZE CLASS NUMBEB STILL FEEDING AT TIME (T + 1) C NWT = TOTAL BIOMASS OF KILL BY SIZE CLASS C PROX = PROPORTION NO. PREDATORS PRESENT IS TO NUMBER C POTENTIALLY OBTAINED C PROY = PROPORTION NUMBER PRESENT IS TO NUMBER POTENTIALLY KILLED C*RATES = SPECIES-INSTAR RATE OF MOVEMENT (M/HR) C RATE = SIZE CLASS RATE OF MOVEMENT (M/HR) - GUT FULL C RATT = PBOPOBTION OF EACH SIZE CLASS SURVIVING AT THE END OF THE DAY C*RFW = SIZE CLASS MAXIMUM REACTIVE DISTANCE (CM) C RRATE = SIZE CLASS RATE OF MOVEMENT ADJUSTED FOR HUNGER C SIZHG = MEAN SIZE CLASS FOOD NEEDED C SIZHM = MAXIMUM SIZE CLASS FOOD NEEDED C*SPF = SIZE CLASS % KILL COEFFICIENTS BELATIVE TO POND SPECIES C T = NUMBER OF MINUTES OUT OF THE HOUR THE PREDATORS ARE HUNTING C*TEMP = TEMPERATURE FOR DAY N C TH = TEMPERATURE COEFFICIENT (% OF MAXIMUM DESIRED) C TINS = DAILY BREEDING ADULT CHANCE MORTALITY C TOLD = DAILY BREEDING ADULT OLD AGE MORTALITY C TTH = MEAN SIZE CLASS TEMPERATURE COEFFICIENT C W = USEABLE DRYWEIGHT BIOMASS C*WX = USEABLE BIOMASS OF EACH SIZE CLASS (DBYWEIGHT PERCENT) C WT1 = BIOMASS OF KILL NOT ASSIMILATED C*WW = WET WEIGHT OF EACH SPECIES-INSTAB C WZ = WET WEIGHT OF MEAN SIZE CLASS INDIVIDUAL C*XDDGH = SPECIES TEMPERATURE THBESHOLD FOB EGGLAYING C XFF = SIZE CLASS % KILL COEFFICIENTS BELATIVE TO G. BUENOI C XNK = NUMBER OF PREY,SIZE CLASS ACTUALLY KILLED BY PREDATOR SIZE CLASS C YFF = SIZE CLASS % KILL COEFFICIENTS RELATIVE TO MAXIMUM % KILL REAL MWK ,NWT DIMENSION NKT (10) ,MWK (10) , SIZHM (10) DIMENSION AT (5) ,BT (5) ,FDES (10) DIMENSION T(10) ,T1 (10) ,NSIZ1 (10) ,F1 (10) ,NK (10,10) ,WX (10) ,WZ(10) DIMENSION PBOY(10) ,HUGIN(5,8) DIMENSION TH (5) , SIZHG (10) ,FFDES (10) ,DV (10 , 10) ,XNK( 10, 10) , PROX (10) DIMENSION HUT (10), AD(2) DIMENSION NOSIZ (5,7) ,AHU (5,8) ,,ASYM (10) , F (10) , BATES (5, 7) , EFF ( 1 10, 10) ,RFW (10, 10) DIMENSION NKDT (10) ,WW (5,7) , RATE (10) DIMENSION NSIZO (10) ,NSIOO (10) DIMENSION RATT (10), WT1(10), MISC(10) DIMENSION TINS (5) , TOLD (5) ,TTH (10) DIMENSION NKST (10) ,W (10) , RRATE (10) DIMENSION NIN(5), ADI(5,8), MICRO (5) , NDIAP(5) DIMENSION NAF (5) ,XDDGH (5) , NEG (5) ,L (5) , M (5) DIMENSION NIDA (5, 150) , NINA (5,150) ,NDDM (5,9) ,NINS (5,8) ,NSIZ(12) , 1 TEMP (154) ,NAFB (5,155) DIMENSION NRR (5) ,NDP (5, 155) ,NGER (5,8,155) DIMENSION SPF (10.) ,XEF (10 ) ,HA (5, 3) , HB (5, 3) , HP (5) , YEP (10) DIMENSION NINAA(5) ,DTEMP (5,154) EQUIVALENCE (NI NS (1 , 1) , N11) , (NINS (2, 1) , N21 ) , (N IN S (3, 1 ) , N3 1 ) , (NIN 1S (4,1) ,N41), (NINS (5,1) ,N51) EQUIVALENCE (HUGIN (3, 2) ,H32) , (HUGIN (4 ,2) ,H42) , (HUGIN (5 ,2 ) , H52) , 236 1 (HUGIN (1, 2) , H12) , (HUGIN (2,2) ,H22) , (HUGIN (3 ,3) ,H33) , (HUGIN(4,3) 1,H43), (HUGIN (5,3) ,H53) , (HUGIN (1, 3) , H13), (HUGIN (2, 3 ) , H23 ), (HUGI 1N (3, 4) , H34)', (HUGIN (4 ,4) ,H44) , (HUGIN (5, 4) , H54) , (HUGIN (3, 5) , H35) 1, (HUGIN (5,5) ,H55) , (HUGIN (1 , 4) , H14) , (HUGIN (2 ,4 ) ,H24 ) , (HUGIN(4,5 1),H45), (HUGIN (3,6) ,H36) , (HUGIN (5, 6) , H56) , (HUGIN ( 1, 5) , H 15) EQUIVALENCE (HUGIN (4, 6) , H46) , (HUGIN (3 ,7) ,H37) , (HUGIN (4 ,7 ) , H47 ) 1, (HUGIN (5,7) ,H57) , (HUGIN (3,8) , H38) , (HUGIN (4, 8) , H48) , (HUGIN (5,8 1),H58), (HUGIN (1, 6) , H16) , (HUGIN (2 , 6) ,H26) , (HUGIN (1 ,7) , H17) , (HUG 1IN (2,7) ,H27) , (HUGIN (1 ,8) ,H18) , (HUGIN (2, 8) , H28) EQUIVALENCE (NINS (3 ,2) ,N32) , (NINS (4 ,2) ,N42) , (NINS (5, 2) ,N52) , (NI 1NS (1,2) ,N12) , (NINS(2,2) ,N22) , (NINS (3 ,3 ) , N3 3 ) , (NI NS (4 , 3 ) , N43 ) , ( 1 NINS (5,3) ,N53) , (NINS (1 ,3) , N13) , (NINS (2, 3) ,N23) , (NINS (3, 4) , N 34) 1, (NINS (4,4) ,N44) , (NINS (5,4) ,N54) , (NINS (3 ,5) ,N35 ) , (NINS (5,5) , N5 1 5 ) , (NINS (1 ,4) ,N14) , (NINS (2,4) ,N24) , (NINS ( 4, 5) , N 45) , (NINS(3,6), 1N36) , (NINS (5,6) ,N 56) , (NINS (1,5) ,N15) , (NINS (2 ,5) , N25) EQUIVALENCE (NINS (4 ,7) , N47) , (NINS (5, 7) , N57) , (NINS (3 1,8),N38), (NINS (4,8) ,N48) , (NINS (5 ,8) ,N58) , (NINS (1, 6) , N 16) , (NINS 1(2,6),N26), (NINS(1, 7) ,N17) , (NINS (2 ,7) ,N27) , (NINS (1 ,8) ,N18 ) , (NI 1NS (2,8) ,N28) EQUIVALENCE (HUGIN (2, 5) , H25) , (NINS (4 ,6) ,N46 ) , (NINS ( 3 , 7 ) , 1N37) EQUIVALENCE (TH ( 1) , TH 1) , (TH(2),T2), (TH(3),T3), (TH(4),T4), (TH 1(5),T5) EQUIVALENCE ( S P F ( 1 ) , E 1 ) , (SPF (2) ,E2) , ( S P F ( 3 ) , E 3 ) , ( S P F ( 4 ) , E 4 ) , 1 (SPF (5) ,E5) , ( S P F ( 6 ) , E 6 ) , ( S P F ( 7 ) , E 7 ) , ( S P F ( 8 ) , E 8 ) , ( S P F ( 9 ) , E 9 ) , 1 (SPF(10) , E10) EQUIVALENCE (XE F ( 1 ) , X 1 ) , (XEF(2),X2), (XEF(3),X3), ( X E F ( 4 ) , X 4 ) , 1 (XEF(5) ,X5) , (XEF(6) ,X6) , (XEF (7) ,X7) , (XEF (8) ,18), (XEF(9),X9), 1 (XEF (10) ,X10) EQUIVALENCE (YEF ( 1) , Y1) , {YEF (2) , Y2) , ( Y E F ( 3 ) , Y 3 ) , ( Y E F ( 4 ) , Y 4 ) , 1 (YEF (5) ,Y5) , ( Y E F ( 6 ) , Y 6 ) , (YEF (7) ,Y7) , (YEF(8),Y8), ( Y E F ( 9 ) , Y 9 ) , 1 (YEF(10) ,Y10) , (HP(1),G1), (HP (2) ,G2) , (HP(3),G3), (HP(4),G4), 1 (HP (5) ,G5) EQUIVALENCE (NSIZ (1) ,NS1) , (NSIZ (2) , NS 2) , (NSIZ ( 3) , NS3) , , (NSIZ (4) , 1NS4) , (NSIZ (5) ,NS5) , (NSIZ (6) ,NS6) , (NSIZ (7) ,NS7) , (NSIZ (8 ) , NS8) , 1 (NSIZ (9) ,NS9) , (NSIZ (10) , NS 10) KN = 0 IN = 5 10=6 TI = 24. SITS NUMBER OF DEGREE-DAYS AT DAY J AND NUMBER OF SPECIES AT CAY J = 0 J=1 NOTABILIS;J=2 REMIGIS ;J=3 INCOGNITUS ;J=4 INCURVATUS;J = 5 BUENOI DO 1 JJ=1,5 DO 1 J = 1,150 NAFB ( J J , J ) =0 NDP (J J , J) -0 NIDA(JJ,J) = 0 DO 2 J = 1,5 DO 2 J J = 1,150 NINA ( J , J J ) = 0 SETS NUMBERS OF SPECIES-INSTARS AT DAY J = 0 DO 3 J = 1,5 DO 3 J J = 1,8 NINS ( J , J J ) = 0 SETS NUMBER OF MATUBE FEMALES AT DAY - 1 = 0 237 DO 11 J = 1,5 L(J) = 0 NRR (J) = 0 MICRO (J) = 0 NEG (J) = 0 NIN(J) = 0 11 NAF (J) = 0 C READS IN MAXIMUM GUT CAPACITIIES DO 89 J=1,5 89 READ (IN, 999) (AHU (J,K) ,K=1,8) C READS IN TEMPERATURE REGRESSION PARAMETERS, C HUNGER CURVE SLOPES, RATE, RFW, AND W READ (IN,3000)AT READ (IN, 3000) BT , READ <IN,3001)AD READ (IN,3004) (SPF(J) ,J=1,10) DO 4113 J=1,5 READ (IN, 3006) (HA(J,K) ,K=1,3) 4113 READ (IN,3006) (HB (J, K) , K= 1 ,3) DO 96 J=1,5 READ (IN,3002) (WW (J,K) , K= 1 ,7 ) 96 READ (IN, 3003) (RATES (J,K) ,K=1,7) DO 240 J=1,10 240 READ (IN, 3004) (RFW(J,K) ,K=1,10) READ (IN,3004) WX C READS IN SIZE CLASS PREFERENCE VALUES, SIZE CLASS KILL EFFICIENCIES, C AND INSTAR SIZE CLASS VALUES DO 88 J=1, 10 88 READ (IN,3004) (DV (K, J) ,K=1,10) DO 87 J = 1,10 87 READ (IN,3004) (EFF (K, J) , K= 1, 10) BO 86 J=1,5 86 READ (IN,1002) (NOSIZ(J,K),K=1,7) C SITS NUMBERS IN SIZE CLASSES = 0 DO 5 J = 1,12 5 NSIZ(J) = 0 READ (IN,3000) (XDDGH (J) , J= 1,5) C READS IN NUMBER OF DEGREE-DAYS AT WHICH SPECIES-INSTARS MOULT C AND OVERWINTERED ADULTS ' READ(IN, 1001) (NINA ( J , 1) , J=1 ,5) DO 4 3 — 1,5 4 READ (IN, 1000) (NDDM (J,K) ,K=1 ,9) 1000 FORMAT (9 (14 ,2X) ) 1001 FORMAT(5(13,3X)) 1002 FORMAT (7 (12,4X) ) 3000 FORMAT (5F6. 2) 3001 FORMAT (5F6.3) 3002 FORMAT (7F6. 0) 3003 FORMAT (7F6.2) 3004 FORMAT (10F6. 3) 3005 FORMAT (10F8.0) 3006 FORMAT (3F6. 2) 2224 F0RMAT(1X,I3,3X,I9,5X,I9,8X,I9,6X,I9,4X,I9) 2223 FORMAT(//4X, 'G. NOTABILIS*,4X,*G. REMIGIS' ,4X , *G. INCOGNITUS',2X, 1*G. INCURVATUS*,4X,«G. BUENOI*) / 238 READ (IN, 3001) XY, XMORT, EU VIR ,TC ,TCC C XMORT = 55 DAILY MORTALITY; NTERM = MAXIMUM NUMBER OF DAYS GERRIDS LIVE READ (IN, 1003) NTERM , (MICRO (J) ,J=3,5) 1003 FORMAT (4 (13 ,3X) ) C READS IN DAILY TEMPERATURES N=0 DO 529 JJ=1, 154 529 READ (IN,3000) (DTEMP (J, JJ) , J= 1, 5) READ(IN,3014) TEMP 3014 FORMAT (10F6.2) 999 FORMAT (8F6.0) DO 95 J=1,5 95 READ (IN,999) (ADI(J,K) ,K=1,8) READ (IN,3005) THET NTWET=TWET 10 N=N+1 IF (N. EQ. 160. AND.MOD (N, 1) .EQ.O) WRITE (10,106) N 106 FORMAT (4X,'DAY',4X,13) LJ = 0 DO 733 J=1,5 DO 732 JJ=1,N IF (NINA (J, JJ) . EQ.O) LJ = LJ + 1 732 CONTINUE 733 CONTINUE LLJ = 5 * N IF (LJ.EQ.LLJ) WRITE (10,731) 731 FORMAT(2X,'ALL SPECIES NUMBERS EQUAL ZERO') IF (LJ.EQ.LLJ) GO TO 201 C ADDS DEGREE-BAYS TO DAILY TOTALS AND MOVES TOTALS AHEAD ONE DAY TEMP (N)=TEMP (N) +TCC IF(N.GT.52) NTN=N-52 IF (N.EQ.52.0R.N.LT.52) NTN=52-N DO 991 JT=1,5 DTEMP (JT,N)=DTEMP (JT,N)* (TC - (0.01 * NTN) ) DEGDY = DTEMP(JT,N) IF (DEGDY. LT.O. ) DEGDY = 0. IF (N-1) 7,7,6 6 DO 12 J = 2,N JN= J -1 IF (JN.EQ.1) NXIDA =NIDA(JT,JN) NNIDA = NXIDA NXIDA = NIDA (JT, J) IF (JN. EQ. 1) NIDA (JT, JN) = DEGDY + 0.5 12 NIDA(JT,J) = NNIDA + DEGDY + 0.5 7 IF(N.EQ.1) NIDA (JT,N) = NDDM (JT,8) + 1 991 CONTINUE C MOVES TOTAL OF ANIMALS FOR EACH DAY AHEAD ONE DAY IF (N-1)9,9,8 8 DO 17 J = 1,5 DO 20 J J = 2,N JN = J J - 1 IF (JN.EQ.1)NXINA = NINA(J,JN) NNINA = NXINA NXINA = NINA (J,JJ) IF (JN. EQ. 1) NINA ( J , JN) = NINAA (J) 239 20 NINA (J,JJ) = NNINA 17 CONTINUE C DETERMINES NUMBER OF ADULTS GOING INTO DIAPAUSE AND SUMMER BREEDING C ADULTS DO 245 J= 1,5 NDIAP(J)=0. NAF(J)=0 IF (NEG (J) . EQ. 0) GO TO 217 DO 242 JJ=1,N IF(NIDA(J,JJ) .LT.NDDM(J,8)) GO TO 242 NXY=NIDA (J,JJ) - DEGDY IF (NXY. EQ.NDDM ( J , 8) .OR.NXY.GT.NDDM (J*8) )GO TO 242 IF (N.LT.MICRO (J)) GO TO 29 NDIAP (J) =NDIAP (J) +NINA(J,JJ) IF (J.EQ.2)GO TO 242 NINA (J, JJ) =0 GO TO 242 29 IF(NEG(J) .EQ.O.OR.NEG (J) .LT.O) GO TO 242 NAF (J) = NAF (J) + (NINA (J, JJ)/2) 242 CONTINUE 247 NDP (J,N) = NDIAP (J) NAFB (J,N) =NAF (J) 245 CONTINUE IF (N. EQ. 160.AND.HOD(N, 1) .EQ.O) WRITE (10 , 1 001) NDIAP C DETERMINES OLD AGE MORTALITY OF BBEEDING ADULTS C GEBBIDS LIVE A MAXIMUM OF (TERM) DAYS TERM = NTERM + 10 DO 932 I = 1,5 TOLD (I) = 0. DO 933 I I = 1,N OLD = 0. N ID = NIBA(I,II) IF (I.EQ. 1 . AND. NID.GT. 1045)OLD = ( (NID - 1045)/5.) * 0.05 IF(I.EQ.2.AND.NID.GT.900.AND.N.LT.75)OLD = ((NID -1900)/5.) * 0.05 IF (N.GT.TERM) GO TO 131 IF (I.EQ.3. AND. NID.GT.896)OLD = ( (NID - 896)/5.) * 0. 05 IF (I.EQ. 4. AND. NID.GT.777) OLD = ( (NID - 777)/5.) * 0. 05 IF (I.EQ.5. AND. NID.GT.808)OLD = ( (NID - 808)/5.) * 0. 05 GO TO 130 IF (I.EQ.3. AND. NID.GT.944)OLD = ( (NID - 944)/5.) * 0. 05 I F ( I . EQ.4. AND. NID.GT.802) OLD = ( (NID - 802)/5.) * 0. 05 IF (I.EQ.5. AND. NID.GT.896)OLD = ( (NID - 896)/5.) * 0. 05 130 IF (OLD. GT. 1.) OLD = 1. NOLD = (NINA (I,II) * OLD) +0.5 NINA (I, II) = NINA ( I , II) - NOLD 933 TOLD (I) = TOLD (I) + NOLD 932 CONTINUE IF (N.EQ.160.AND.MOD (N,1).EQ.0)WRITE (10, 3000)TOLD C ASSIGNS NUMBERS TO INSTARS ON THE BASIS OF THE SUMMED DEGBEE-DAYS C AND DETEBMINES TOTAL INSTAB FOOD NEEDED AT 26 DEGBEES CENTIGRADE C CORRECTS FOR POSITION WITHIN STADIUM C INSTAR 7=ADULT STADIUM; INSTAR 8=MATURE ADULT; INSTAR 1=EGG 9 DO 14 J = 1,5 DO 21 J J = 1 ,8 240 NINS(J,JJ) =0 CFF = 0. J J J = J J + 1 DO 16 K = 1,N IF (NIDA (J,K) - NDDM(J,JJ)) 16,16,15 15 IF (NIDA (J,K) - NDDM (J,JJJ))32,32, 16 32 NINS(J,JJ) = NINS(J,JJ) • NINA(J,K) IF (JJ.EQ.8) CFF=0. IF ( J J . EQ. 8) GO TO 16 NDIFF = NDDM (J,JJJ) - NDDM(J,JJ) DIF = NIBA(J,K) - NDDM(J,JJ) CF = (DIF/NDIFF) * 100. IF (CF.LT.0.40.OR.CF.EQ.0.40) CFH = (32.3 + (1.5 * CF))/100. IF (CF.GT.0.40) CFH = (102.4 - (0.25 * CF))/100. CFH = ADI(J,JJ) * CFH * NINA(J,K) CFF = CFF + CFH 16 CONTINUE HUGIN(J,JJ) = CFF NGER (J , J J , N) =NINS ( J , JJ) 21 i CONTINUE 14 CONTINUE C FOR THE ADULTS, IT IS ASSUMED THAT THE NUMBER OF MALES EQUALS THE C NUMBER OF FEMALES, HENCE ADI IS THE AVERAGE OF BOTH SEXES H18 = ADI (1 ,8) * N18 H28 = ADI (2, 8) * N28 H38 = ADI (3,8) * N38 H48 = ADI (4, 8) * N48 H58 = ADI (5,8) * N58 IF (N. EQ. 160.AND.MOD(N, 1) .EQ.O) WRITE (10,1000) NINS IF (N.EQ.160.AND.MOD (N,1).EQ.0)WRITE(IO,400) HUGIN 400 FORMAT(10(F7.1,2X)) C DETERMINES DAILY MORTALITY DO 931 1=1,5 TINS (I) = 0. DO 930 11=1,8 MMORT = 0.5 + (XMORT * NINS (I, II) ) NINS (I, II) = NINS (I, II) - MMORT 930 IF(II.EQ.8) TINS (I) = TINS (I) + MMORT 931 CONTINUE IF (N.EQ.160.AND.MOD (N,1).EQ.0)WRITE(IO, 3000)TINS C CHECKS TO SEE IF FEMALES HAVE BEEN TURNED ON TO REPRODUCTION - THIS C REQUIRES FIVE CONSECUTIVE DAYS WITH TEMPERATURES ABOVE THE C THRESHOLD DESIGNATED FOR THAT SPECIES (XDDGH (SP) ) DO 18 K = 1,5 IF (NEG (K) ) 19,19,18 C IF THE FEMALES ARE NOT TURNED ON, THE TEMPERATURE IS CHECKED TO SEE C IF IT IS ABOVE THE THRESHOLD 19 NCF = 0 IF (TEMP (N) .GT. XDDGH (K) ) NCF = 1 IF(NCF) 13, 13, 22 13 L(K) = 0 GO TO 18 22 L (K) = L (K) + 1 IF(L(K) . EQ. 5) NEG (K) = 1 IF (L (K) .EQ.5) M (K) = 0 241 18 CONTINUE C DETERMINES NUMBER OF ADULT FEMALES THAT HAVE MATURED IF (N.EQ. 160. AND.MOD(N, 1) .EQ.O) WRITE (10,1019) NEB DO 35 J =1,5 IF(NEG(J) ) 35, 35,248 248 IF (M (J) . NE. 0) GO TO 35 NAF(J) =NINS ( J , 8) /2 NAFB (J,N) = NAF (J) 35 CONTINUE' IF (N.EQ. 160. AND. MOD (N, 1). EQ. 0) WRITE ( 1 0 , 1019) NIN IF (N.EQ. 1 60. AND.MOD (N, 1) .EQ.O) WRITE (10,1019) NAF DO 31 J =1,5 31 NINAA (J) = 0 CALL EGG(DTEMP,N,NAF,NINAA,M,NEG,KN,XMORT) IF (N. EQ. 160.AND.BOD(N, 1) .EQ.O) WRITE (10,1019) NINAA 1019 FORMAT (5 ( 1 3 ,2X)) KN = KN +1 C SUMS SPECIES-INSTARS TO FORM SIZE CLASSES NS 1 = N32 N42 • N52 NS2 = N12 N22 + N33 + N43 + N53 NS 3 = N13 + N23 • N34 + N44 • N54 NS4 = N35 + N55 NS5 N14 + N24 • N45 NS6 = N36 + N56 NS7 N15 • N25+ N46 NS8 = N37 + N47 • N57 + N38 + N48 + N58 NS9 — N16 + N26 NS10 = N17 + N27 + N18 + N28 NSIZ(11) = N11 + N21 + N31 + N41 + N51 C DETERMINES SIZE CLASS AGGRESIVENESS AT MAXIMUM STARVATION C CORRECTED FOR RELATIVE RATES OF MOVEMENT AND REACTIVE DISTANCE (B=1) DO 80 J=1 ,10 80 IF(NSIZ (J) .EQ.O) NSIZ (J) =10000000 X1= ( (N32*E3)+ (N42*E4) + (N52*E5))/NS1 X2= ( (N 12*E 1) + (N22*E2) + (N33*E3) • (N43*E4) + (N53*E5) ) /NS2 X3= ( (N13*E1) + (N23*E2) + (N34*E3) + (N44*E4) + (N54*E5) )/NS3 X4= ( (N3 5*E3) • (N55*E5) )/NS4 X5= ( (N14*E1) + (N24*E2) + (N45*E4) ) /NS5 X6= ( (N36*E3) + (N56*E5) )/NS6 X7= ( (N 15*E 1) + (N25*E2) • (N46*E4)) /NS7 X8= ( ( (N37 + N38)*E8)+ ( (N47 + N48) *E9) + ( (N57+N58) *E10) ) /NS8 X9= ( (N 16*E1) + (N26*E2) )/NS9 X10=(((N17+N18)*E6)+((N27+N28)*E7))/NSlO C DETERMINES TEMPERATURE COEFFICIENT FOR EACH SPECIES DO 33 J = 1,5 TH(J) =(AT(J) + (BT(J) * TEMP (N) ) ) / (AT (J) + (BT (J) * 26.)) IF (J.HE. 2) GO TO 33 IF (TEMP (N) . LT. 19. ) GO TO 33 TH(2) = (1916. -(27.38 * TEMP (N) ))/(1916. - (27.38 * 26)) 33 CONTINUE IF (N.EQ. 160. AND.MOD (N, 1) .EQ.O) WRITE (10,401 )TH 401 FORMAT (5 (F7.2 ,2X) ) C DETERMINES MEAN SIZE CLASS FOOD NEEDED SIZHG (1) =((H32 * T3) + (H42 * T4) + (H52 * T5)) / NS 1 SIZHG(2) =((H12 * TH 1) + (H22 * 12) + (H33 * T3) + (H43 * 242 1T4) + (H53 * T5) ) / NS2 SIZHG(3) = ((H13 * TH 1) + (H23 * T2) • (H34 * T3) + (H44 * 1T4) + (H54 * T5)) / NS3 SIZHG (4) = ( (H35 * T3) + (H55 * T5) ) / NS4 SIZHG (5) = ((H14 * TH 1) + (H24 * T2) + (H45 * T4)) / NS5 SIZHG (6) = ( (H36 * T3) + (H56 * T5) ) / NS6 SIZHG(7) =((H15 * TH 1) + (H25 * T2) + (H46 * T4) ) / NS7 SIZHG (8) = ( (H37 * T3) + (H47 * T4) + (H57 * T5) + (H38 * 1T3) • (H48 * T4) + (H58 * T5) ) / NS8 SIZHG (9) = ((H16 * TH 1) + (H26 * T2) ) / NS9 SIZHG(10) = ((H17 * TH1) + (H27 * T2) + (H18 * TH 1) + (H28 1 * T2)) / NS10 IF (N.EQ. 160. AND. MOD (H, 1) . EQ.O) WRITE (10, 400) SIZHG C DETERMINES MAXIMUM SIZE CLASS FOOD NEEDED SIZHM (1) = (H32 + H42 + H52) / NS 1 SIZHM(2) = (H1.2+ H22 + H33 + H43 + H53) / NS2 SIZHM (3) = (H13 + H23 + H34 + H44 + H54) / NS3 SIZHM(4) = (H35 + H55) / NS4 SIZHM (5) = (H14 + H24 + H45 ) / NS5 SIZHM(6) = (H36 • H56) / NS6 SIZHM (7) = (H15 + H25 • H46 ) / NS7 SIZHM (8) = (H37 + H47 • H57 + H38 + H48 + H58) / NS8 SIZHM (9) = (H16 + H26) / NS9 SIZHM(10) = (H17 + H27 + H18 + H28) / NS10 IF (N.EQ.160.AND.MOD (N,1).EQ.O)WRITE(10, 400) SIZHM DO 81 J=1,10 IF (NSIZ (J) . NE. 10000000) GO TO 81 SIZHM (J) =0. SIZHG (J)=0. NSIZ (J) =0 XEF (J)=0. 81 CONTINUE TAREA = 50. C DETERMINES MEAN SIZE CLASS MAXIMUM GUT CAPACITY DO 42 J = 1,10 IF (NSIZ (J) . EQ. 0) ASYM(J)=0. IF (NSIZ (J) .EQ.O) GO TO 42 XNO=0. DO 409 K = 1,5 DO 410 KK = 2,8 JK=KK-1 IF (NOSIZ (K, JK) - J) 410,43,410 43 XNO = (NINS (K,KK) * AHU (K,KK) ) • XNO 410 CONTINUE 409 CONTINUE ASYM (J) = XNO / NSIZ (J) 42 CONTINUE IF (N.EQ. 160.AND.MOD (N, 1) .EQ.O) WRITE (10,400) ASYM DO 74 J = 1,10 T1 (J)=0. F1 (J)=0. NSIZ1(J)=0. NSIOO (J) = NSIZ (J) F(J)=XY * ASYM(J) WZ (J)=0 243 TTH(J)=0. RATE (J) =0. 74 W (J) = 0. C DETERMINES SIZE CLASS RATE OF MOVEMENT AND SIZECLASS USEABLE C DRYWEIGHT BIOMASS DO 263 1=1,5 DO 264 IJ=2,8 JJJ=IJ - 1 IF (NINS (I,IJ) .EQ.O) GO TO 264 JJJJ=NOSIZ(I,JJJ) RATE (JJJJ) = (NINS(I,IJ) * RATES (I, JJJ) * TH (I) ) + RATE (JJJJ) WZ (JJJJ) = (NINS(I,IJ) * WW(I,JJJ)) + WZ(JJJJ) TTH (JJJJ) = (NINS(I,IJ) * T H ( I ) ) + T T H ( J J J J ) , 264 CONTINUE 263 CONTINUE DO 265 1=1, 10 IF (NSIZ (I) .EQ.O) GO TO 265 TTB(I) = TTH (I) /NSIZ (I) WZ(I) = SZ (I)/NSIZ (I) W (I) = WX (I) * WZ (I) * 0.01 RATE(I)= RATE (I)/NSIZ (I) 265 CONTINUE IF (N.EQ. 160. AND. MOD (N, 1) . EQ. 0) WRITE (IO, 300 5) W IF (N. EQ. 160.AND.MOD(N, 1) .EQ.O) WRITE (IO ,3004 ) RATE C MAXIMUM VALUE OF ,MJ EQUALS NUMBER OF ITERATIONS PER DAY DO 52 MJ=1,24 DO 3402 J=1,10 3402 NSIZO(J) = NS1Z(J) IF (N.EQ. 160. AND. MOD (N, 1) . EQ. 0. AND. MJ. EQ. 1) WRITE (IO, 405)NSIZO C J EQUALS PREDATOR SIZE CLASS DO 411 J = 1,10 IF (NSIZ (J) .NE. 0) GO TO 350 DO 351 JJ=1,10 351 NK(J,JJ)=0 350 IF (NSIZ (J) .EQ.O) GO TO 411 C J J EQUALS PREY SIZE CLASS ^ DO 40 J J = 1,10 IF (NSIZ (JJ) .EQ.O) NK(J,JJ)=0 IF (NSIZ (JJ) .EQ.O) GC TO 40 IF( JJ-J) 121, 121,40 121 R=0. C R = CODE INDICATING POTENTIAL NUMBER KILLED GREATER THAN NUMBER C OF PREDATORS C HN = NUMBER OF POTENTIAL PREY PREDATOB HAS EATEN IN HOUR MN=0 C DETERMINES INITIAL PARAMETERS C NOTE: ANIMALS ALWAYS HAVE XY % OF THEIR GUT FULL FROM FEEDING ON C OTHER FOOD ITEMS BESIDES GERRIDS IF (N.EQ. 160. AND. MOD (N,1). EQ.O. AND. MJ.EQ. 1) WHITE (IO , 40 5) N SIZ IF(N.EQ.160.AND.MOD(N,1).EQ.0.AND.MJ.EQ.1)WRITE(IO,405)NSIZ1 405 FORMAT(10I6) NHUNT = NSIZ(J) - NSIZ1 (J) T(J) = 60. * {((60. * NHUNT) + (Tl (J) * NSIZ1 (J) ) ) / (60. * (NHUNT 1+ NSIZ1 (J)) ) ) F (J) = ((NHUNT * F(J)) + (NSIZ1 (J) * F 1 ( J ) ) ) / NSIZ (J) FR = F(J) / ASYM(J) IF (FR.LT.XY) F (J) = XY * ASYM (J) C DETERMINES SIZE CLASS AGGRESSIVENESS IN RELATION TO FOOD IN GUT C ASSUMED MAXIMUM ENCOUNTERS ATTACKED WHEN % KILL IS AT MAXIMUM FR=F (J) /ASYM (J) IF (FR.LT.0.33) NF = 1 IF(FR.GT.0.32.AND.FR.LT.0.66) NF = 2 IF (FR.GT.0.65) NF = 3 DO 3407 JK=1,5 3407 HP (JK) = HA (JK ,NF) + (HB (JK, NF) * FR) DO 4316 JZ=1,10 IF (NSIOO (JZ).EQ.O)NSIOO(JZ) = 10000000 4316 CONTINUE Y1 = ( (G3*N32) + (G4*N42) + (G5*N52) )/NSIOO (1) Y2 = ( (G1*N12) + (G2*N22) + (G3*N33) + (G4*N43) + (G5*N53) ) /NSICC{2) Y3 = ( (G1 *N13) + (G2*N23) + (G3*N34) + (G4*N44) + (G5*N54) )/NSIOO (3 ) Y4 = ( <G3*N35) • (G5*N55) )/NSIOO (4) Y5 = ( (G 1*N14) + (G2*N24) + (G4*N45) )/NSIOO (5) Y6 = ( (G3*N36) + (G5*N56) )/NSIOO (6) Y7 = ( (G1*N15) +(G2*N25) + (G4*N46) ) /NSIOO (7) Y8 = ( (G3* (N37+N38) ) + (G4* (N47 + N48)) + (G5* (N57+N58) ) ) /NSIOO (8) Y9 = ( (G1*N16) + (G2*N26))/NSIOO (9) Y10 = ( (E1* (N17 + N18) ) • (E2* (N27+N28) ) )/NSIOO (10) DO 4317 JZ=1,10 IF (NSIOO (JZ).EQ.10000000)YEF(JZ)=0. 4317 IF (NSIOO (JZ) .EQ. 10000000) NSIOO (JZ)=0 IF (N.EQ.160.AND.MOD (N,1).EQ.0.AND.MJ.EQ.1)WRITE(IO, 2003)J,JJ 2003 FORMAT (2X, 12, 2X,12) IF (N.EQ. 160. AND. MOD (N, 1) . EQ.O. AND. MJ. EQ. 1) WRITE (IO, 1006) T (J ) IF (N. EQ. 160.AND.MOD (N, 1) . EQ . 0 . AND . M J .EQ . 1) WRITE (IO , 1006) F(J) 1006 FORMAT (2X,10F10.2) C DETERMINES AREA SEARCHED DO 992 JR=1,10 992 RRATE(JR) = (0.29 +(0.71 * FR) ) * RATE (JR) FF=F (J) NIZ=NSIZ (JJ) 41 RVEL1 = RRATE (J) RV EL2 = RRATE (JJ) RVEL = ( (R VEL1*RVEL1) • (RVEL2* RVEL2) ) **0. 5 RZW = RFW(J,JJ) * (0.69 +(0.31 * FR) ) * 0.02 ASER = RVEL * RZW ASER= ASER / TAREA UNSWP = EXP (-ASER) ASER = TAREA - (TAREA * UNSWP) C DETERMINES NUMBER OF ENCOUNTERS DEN = NSIZ(JJ) / TAREA ENC = ASER * DEN * (T (J) / 60.) *NSIZ(J) IF (N.EQ. 1 60. AND. MOD (N, 1) . EQ . 0 . AND . M J .EQ . 1) WRITE (IO ,34 03) ENC . 3403 FORMAT (F8.3) C DETERMINES POTENTIAL NUMBER OF J J KILLED BY J C AGGRESIVENESS DETERMINES TOTAL NUMBER OF ENCOUNTERS WHICH C ACTUALLY RESULT IN AN ATTACK FDES(J) = (SIZHG (J) - F ( J ) ) IF (MN. EQ. 0) FFDES(J) = FDES (J) DV(J,JJ) = 1. . 245 ZEFF = ENC * XEF(J) * YEF (J) NK(J,JJ) = (EFF(J,JJ) * ZEFF * DV(J,JJ) * ENV18 ) + 0.5 IF (N.EQ. 160. AND. HOD (N,1) .EQ . 0 . AND. H J. EQ. 1) WRITE (IO, 1010) 1EFF ( J , JJ) ,DV ( J , JJ) 1010 FORMAT (2X,2F10.2) IF (NK (J,JJ) .GT.NSIZ (J) ) R = 1. C DETERMINES IF NUMBER KILLED IS GREATER THAN NUMBER OF PREDATORS IF (NK (J,JJ) .GT.NSIZ (J) ) NK(J,JJ) = NSIZ (J) IF (R.EQ.O.) NK(J,JJ) = (MN * NSIZ(J)) + NK(J,JJ) IF(N.EQ.160.AND.MOD(N,1).EQ.O.AND.MJ.EQ.1)WRITE(IO,1004) 1ASER ,ENC , NK ( J , JJ) 1004 FORMAT (2X,2 (F10.2,2X),16) C DETERMINES HANDLING TIME, ETC. FOR THOSE HUNTING AGAIN IN THE HOUR IF (R.NE. 1 ) F (J)=FF IF (R. NE. 1) NS IZ (JJ) =N IZ IF (R.NE.1) GO TO 40 C CITERMINES IF FOOD DESIRED GREATER THAN FOOD OBTAINED IF (FDES (J).EQ.W (JJ).OR.FDES (J).LT.W (JJ) ) GO TO 40 39 NFEED = NK(J,JJ) C H = POTENTIAL HANDLING TIME C ENCR = POTENTIAL ENCOUNTER RATE HT=120. IF ( J . EQ. 1) HT = 60. IF (J.EQ.2) HT = 90. H = (W (JJ) / ASYM (J) ) * HT IF (H.GT.HT) H = HT IF (W (JJ) .GT.ASYM (J) ) W (J J) = ASYM(J) ENCR = 60./ NK (J,JJ) C = (0.5 * ENCR) + (0.5 * ENCR * NFEED) T (J) = C + H IF (T (J) . GT.60.) GO TO 40 F (J) = F (J) + W (JJ) NSIZ(JJ) = NSIZ(JJ)' - NK(J,JJ) T(J) = (60. - T ( J ) ) MN = MN • 1 R=0. GO TO 41 40 CONTINUE IF (N.EQ. 160. AND.MOD (N, 1) . EQ . 0 .AND . M J . EQ . 1 ) WRITE (10,153 ) 1 (NK (J,K) ,K=1 ,10) 411 CONTINUE C DETERMINES ACTUAL PREDATION DO 53 J = 1,10 IF (NSIZ (J) .EQ.O) W (J) = 0. XNKT = 0. IF (NSIZ (J) .EQ.O) GO TO 370 DO 55 J J =1,10 JX=JJ-J IF(JX.GT.O) GO TO 370 C XNKT = TOTAL NUMBER OBTAINED BY PREDATOR J NTX=NK (J,JJ) IF (NTX.GT.NSIZ (J) ) NTX=NSIZ (J) XNKT = XNKT + NTX 55 CONTINUE 370 IF(XNKT.EQ.O.) PROX(J) = 1. IF (XNKT.EQ.O.) GO TO 53 PROX (J) = NSIZ(J) / XNKT IF (PROX (J) .GT. 1. ) PROX (J) = 1. 53 CONTINUE DO 56 J J = 1,10 YNKT = 0. IF (NSIZ (JJ) .EQ.O) GO TO 371 DO 57 J = 1,10 JX=JJ-J IF(JX.GT.O) GO TO 371 C YNKT = TOTAL NUMBER OF PREY J J KILLED YNKT = YNKT • NK(J,JJ) 57 CONTINUE 371 IF (YNKT. EQ. 0.) PROY(JJ) * 1. IF (YNKT.EQ.O.) GO TO 56 PBOY(JJ) = NSIZ(JJ) / YNKT IF (PROY (J J) . GT. 1. ) PROY (JJ) = 1. 56 CONTINUE DO 122 J = 1,10 NKT(J) = 0 NWT(J) = 0 DO 58 J J = 1,10 JX=JJ-J IF (JX. GT. 0) XNK ( J , JJ) = 0. IF (JX.GT.O) GO TO 372 XNK(J,JJ) = NK(J,JJ) * PROX(J) * PROY (JJ) IF (XNK (J,JJ) .GT. 0.. AND. XNK (J, J J) . LT. 1. ) XNK(J,JJ) = 1. 372 IF (XNK(J,JJ) .EQ.O.) NK(J,JJ) = 0 NXNK= XNK (J , JJ) + 0. 5 NKT (J) = NKT (J) • NXNK 58 NWT(J) = NWT(J) + (NXNK * W (JJ) ) IF (MOD (N,200).EQ.O)WRITE (10,400) (XNK (J,I) ,1= 1, 10) IF (MOB (N, 200) .EQ.O) WRITE (10,1005) (NK (J ,1) ,1=1 ,10 ) 1005 FORMAT (2X,1016) IF (NWT (J) .EQ.O) MWK(J) = 0 IF (NWT (J) .EQ.O) GO TO 122 RST=NWT(J) MWK (J) = (EST / NKT (J) ) 122 CONTINUE IF (N.EQ. 160. AND. MOD (N, 1) . EQ.O. AND. MJ. EQ. 1) WRITE (10, 3005) MWK IF (N. EQ. 160.AND.MOD(N, 1) .EQ . 0 .AND . MJ . EQ. 1) WRITE (10,3005) NWT IF (N.EQ.160.AND.MOD (N,1).EQ.0.AND.MJ.EQ.1)WRITE (10,405)NKT DO 71 J J = 1,10 NKST (JJ) = 0 DO 71 J = 1,10 71 NKST(JJ) = NKST(JJ) • XNK(J,JJ) IF (N.EQ. 160. AND. MOD (N, 1) . EQ . 0. AND . M J.EQ . 1) WRITE (10,2002) 2002 FORMAT (2X,» SIZE CLASS NUMBEB KILLED PER ITERATION'/) IF (N.EQ. 160. AND. MOD (N, 1) . EQ . 0 . AND . M J .EQ . 1) WRITE (10,405 ) NKST DO 239 J=1,10 IF (MJ. EQ. 1) NKDT(J) = 0 NKDT(J) = NKDT (J) + NKST (J) 239 CONTINUE C DETERMINES MEAN KILL RATE, HANDLING TIME, FEEDING TIME DO 46 J = 1, 10 IF (NSIZ (J).EQ.O) GO TO 83 IF (N.EQ. 160.AND.MOD(N, 1) .EQ.O.AND.MJ.EQ.1) WRITE (10,405) NKT IF (NKT (J) .EQ.O) GO TO 83 C NM = NUMBEB OF PREY PREDATOR HAS EATEN IN ONE HOUR C H = HANDLING TIME C ENCR = ENCOUNTER RATE NM = 0 ENCR = 60. / NKT(J) NFEED •= NKT (J) IF (NKT (J) .GT.NSIZ (J) ) NFEED = NSIZ (J) HT = 120. IF ( J . EQ. 1) HT = 60. IF (J.EQ.2) HT = 90. IF (FFDES(J) - MWK (J) ) 47,48,48 48 H = (MWK(J) / ASYM(J)) * HT IF (H. GT. HT) H=HT IF (MWK (J) .GT. ASYM (J) ) W (J) = ASYM (J) MISC(J) .= 0 WT 1 (J) = 0 . GO TO 45 47 H - ((ASYM(J) - F (J) ) / ASYM (J) ) * HT IF (H.GT.HT) H=HT MISC (J) = NKT (J) WT1(J) = MWK(J) - FFDES(J) 45 C = (0.5 * ENCR) + (0.5 * ENCR * NFEED) IF(N.EQ.160.AND.MOD(N,1) .EQ.O.AND.MJ.EQ.1)WRITE (10,1006)C IF (N.EQ. 160. AND. MOD (N, 1) . EQ.O. AND. MJ. EQ. 1) WRITE ( 1 0 , 1006) H T(J) = C 4 H X = T(J) NSIZZ = NSIZ(J) 70 NSIZ(J) = (NM * NSIZ(J)) + NSIZ(J) IF (NKT (J) .LT.NSIZ (J) .OR.NKT (J) .EQ.NSIZ (J) ) GO TO 44 NFEED = NKT (J) - NSIZ (J) NM = NM +1 GO TO 70 44 T (J) = (NM * T(J)) + T(J) IF (N.EQ. 160. AND. MOD (N, 1). EQ.O. AND. MJ. EQ. 1) WRITE ( 10 , 1006) T (J) NSIZ (J) = NSIZZ C DETERMINES PARAMETERS FOR THOSE HUNTING AND THOSE FEEDING C AND SUBTBACTS MORTALITY NSIZ (J) = NSIZ(J) - NKST (J) IF (N.EQ. 160. AND. MOD (N, 1) .EQ.0.AND.MJ.EQ.1)WRITE (10,405) NSIZ RSI=NSIZ(J) RAT = RSI / NSIZO (J) NFEED = (RAT * NFEED) + 0.5 IF (T (J) - 60.) 49,49,50 50 NJ = 60 / X IF (X.GT.60.) GO TO 976 F (J) = F (J) + (NJ * MWK (J) ) GO TO 977 976 F (J) = F (J) 977 NSIZ1(J) = N FEED IF (NSIZ1 (J).EQ.O) T1 (J) = 0 . IF (NSIZ1 (J) .EQ.O) F1(J) = 0. IF (NSIZ1 (J).EQ.O) GO TO 51 248 F1(J) = ((F(J) * NFEED) + NWT(J) - (NJ * MWK(J) * NSIZC(J) 1) - (WT1 (J) * MISC(J)))/ NSIZ1 (J) T1 (J) = 120. - T ( J ) GO TO 51 83 MISC (J) = 0 NSIZ(J) = NSIZ(J) - NKST(J) WT1 (J) = 0. 49 I F (NSIZ (J) . EQ. 0) F (J) =0. IF (NSIZ (J).EQ.0)GO TO 373 IF(NKT(J) . EQ. 0) MWK (J) = 0 IF (NKT (J) .EQ.O) WT1 (J) = 0 F(J) = F(J) • MWK(J) - WT1(J) 373 NSIZ1 ( J ) = 0 F1(J) = 0. ' T1 (J)=0. T ( J ) = 60. C DETERMINES FOOD LOST OWING TO DIGESTION AND SUBTRACTS THIS IF (NSIZ (J) .EQ.O) GO TO 46 51 MMJ=1 IF(J.EQ.8.0R.J.EQ. 10)MMJ=2 A = 0.9 * ASYM ( J ) IF( F(J) . GT. A) F(J) = A HS = (ALOG (ASYM (J) / (ASYM (J) - F (J) ) ) ) / AD (MMJ) IF (N.EQ. 1 60. AND.MOD (N, 1) . EQ.O.AND.MJ.EQ.1) WRITE (10 ,1006 ) F (J) HS = HS - (1. * TTH (J) ) IF(HS.LT.O.)HS=0. F (J) = ASYM(J) * ( 1. - EXP(-AD(MMJ) * HS) ) IF (N.EQ. 160. AND.MOD (N, 1) . EQ.O.AND.MJ.EQ.1) WRITE (10 , 1006) F(J) IF (F1 (J) .EQ.O.) GO TO 46 IF (F1 (J) . GT. A) FI (J) = A HS 1 = (ALOG (ASYM (J) / (ASYM (J) - F 1 ( J ) ) ) ) / AD(MMJ) IF(N.EQ.160.AND.MOD(N,1).EQ.0.AND.MJ.EQ. 1) WRITE (10, 1006) F1 (J) HS1 = HS1 - ( 1. * TTH (J) ) IF (HS1.LT.0.)HS1=0. F1(J) = ASYM(J) * (1. - EXP(-AD(MMJ) * HS1) ) IF (N. EQ. 160. AND. MOD (N,1). EQ.O. AND. MJ. EQ. 1)WRITE (10, 1006)F1 (J ) 46 CONTINUE 52 CONTINUE C DETERMINES NEW DAILY NUMBEBS AFTEB DAY'S PREDATION IF (N.EQ.160.AND.MOD (N,1).EQ.0)WRITE(10,2000)NSIOO 2000 FORMAT(2X,'NSIOO',2X,10(15,2X)) IF (N.EQ. 160. AND. MOD (N, 1) . EQ.O) WRITE (10, 200 1) NKDT 2001 FORMAT (2X, • NKDT * , 2X, 1 0 (15, 2X) ) DO 75 J = 1,10 RSIO=NSIOO (J) IF (RSIO.EQ.O) RATT (J) = 0. IF (RSIO.EQ. 0) GO TO 75 RATT (J) = NSIZ(J) / RSIO 75 CONTINUE BO 888 J=1,5 NR=NINS (J,8) K=NOSIZ (J,7) NOR=BATT(K)*NR NRB (J) =NR-NOR 888 CONTINUE 249 DO 76 J = 1,5 DO 77 J J - 2,8 J J J = J J + 1 J J J J = J J - 1 DO 78 K = 1,N IF(NIDA(J,K) - NDDM (J, JJ) )78,78,79 79 IF(NIBA(J,K) - NDDM ( J , J J J) ) 123 ,1 23 ,78 123 KK = NOSIZ (J,JJJJ) IF (NINA (J , K) . EQ. 0) GO TO 78 NINA(J,K) = (NINA(J,K) * RATT (KK) ) +0.5 78 CONTINUE 77 CONTINUE 76 CONTINUE IF (N.NE.160.OH.MOD(N,1).NE.O)GO TO 413 WRITE(IO, 111) WRITE (10,107) 107 FORMAT(2X,'DEGREE DAYS'/) WRITE (IO, 103) (NIDA (1 , J) , J=1,N) 103 FORMAT (10 (18, 2X) ) WRITE (10,111) 111 FORMAT(//) WRITE (10,108) 108 FORMAT(2X,'SPECIES NUMBER AT END OF DAY'/) DO 99 J=1,5 WRITE(IO, 153) (NINA (J,K) ,K=1 ,N) 153 FORMAT (10 (18,2X) ) WRITE(IO, 111) 99 CONTINUE WRITE (IO, 109) 109 FORMAT (2X,'SPECIES INSTAR NUMBERS AT BEGINNING OF DAY'/) WRITE(IO, 104) ( (NINS(J,KK) ,KK=1 ,8) ,J=1 ,5) 104 FORMAT (8 (18,4X)/) WRITE (IO, 111) WRITE (10,110) 110 FORMAT(2X,'SIZE CLASS NUMBERS AT END OF DAY'/) WRITE (10,1 16) NSIZ 116 FORMAT(8(18,4X)) WRITE (IO, 111) 413 IF (N - NTWET)10,201,201 201 WRITE (10,3540) 3540 FORMAT(//4X,'NUMBER OF GERRIDS ENTERING DIAPAUSE') WRITE (10,2223) DO 2225 J=1,N 2225 WRITE (10,2224)J, (NDP (K,J),K=1,5) WRITE(IO, 2228) 2228 FORMAT (//4X,'NUMBER OF ADULTS MATURING TO BREED') WRITE(IO, 2223) DO 2229 J=1,N 2229 WRITE (IO, 2224) J , (NAFB (K, J) ,K=1 ,5) 2226 FORMAT (//4X,» NUMBER OF FIRST INSTARS') WRITE(IO, 2226) WRITE (10,2223) DO 2227 J=1,N 2227 WRITE (10,2224) J , (NGER (K,2, J) ,K= 1,5) CALL EXIT END Appendix 2 0. 57. 164. 382. 743. 1266. 1790. 1790. 0. 77. 217. 472. 885. 1569. 1739. 1739. 0. 32. 78. 199. 397. 647. 725. 725. 0. 36. 97. 180. 343. 671. 619. 619. 0. 28. 81. 153. 339. 596. 446. 446. -85.2 44.6 33.0 302.7 207.4 95.2 70.7 27,3 26.6 28.0 0. 182 0.047 0.13 0.13 0.75 0.30 0. 12 0. 12 0.07 0.75 0. 12 0. 42 1.00 1.72 0.84 0.00 -2.18 -0.85 1.00 1.00 3.00 0.00 0.00 -3.03 1.00 1.00 3.00 0.00 0.00 -3.03 1.00 2.32 1.18 1.09 -2.93 -1.18 1.00 1.61 1.17 0.00 -1.84 -1.18 254. 901. 2654. 6576. 14890 .47730 .47730 • 2.82 3.35 4.37 6.00 9.23 14. 18 14. 18 363. 1274. 3497. 8507. 22100 .43380 .43380 • 11.03 13.10 16.84 22.73 31.49 37.84 37.84 130. 369. 1151. 2794. 5439. 13640 . 13640 • 1.51 1.70 1.93 2.31 2.78 3. 37 3.37 147. 480. 1013. 2305. 5689. 11435 .11435 • 2.93 3.29 3.91 4.80 6.23 7.63 7.63 109. 385. 828. 2257. 4848. 7966. 7966. 2.08 2.31 3.05 3.42 3.93 4.64 4.65 1.24 0. 0. 0. '0. 0. 0. 0. 0. 0. 1.26 1.58 0. 0. 0. 0. 0. 0. 0. 0. 1. 29 1.61 2.26 0. 0. 0. 0. 0: 0. 0. 1.32 1.65 2.31 3.41 0. 0. 0. 0. 0. 0. 1. 35 1. 69 2. 36 3.49 3. 15 0. 0. 0. 0. 0. 1.37 1.71 2.40 3.54 3. 20 4. 34 0. 0. 0. 0. 1.44 1. 80 2.52 3.72 3.36 4.56 5.64 0. 0. 0. 1.44 1.81 2.53 3.73 3.37 4.57 5.66 6. 62 0. 0. 1.63 2.03 2.85 4.20 3.79 5.15 6.37 7.45 9. 49 0. 1.83 2.29 3.21 4.74 4.28 5.81 7. 19 8. 41 10 .70 14 .68 26. 7 26.7 26. 7 26.7 26.7 26.7 26.7 31.7 31 .7 31 .9 0. 0. 1 . 1. 1. 1. 0.75 0. 5 0. 5 0. 4 0. 0. 0. 0. 0. 1. 1. 0.75 0. 75 0. 5 0. 0. 0. 0. 0. 0. 0.75 1. 1. 0. 75 0. 0. 0. 0. 0. 0. 0. 0.75 1. 1. 0. 0. 0. 0. 0. 0. 0. 0. 5 0. 75 0. 75 0. 0. 0. 0. 0. 0. 0. 0. 0. 5 0. 5 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 4 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.086 0.400 0. 122 0. 114 0.059 0. 254 0. 263 0. 239 0. 0. 0. 0.022 0.027 0.113 0.124 0.239 0. 511 0. 415 0. 0. 0. 0.021 0.01 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. . 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. .0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 2 3 5 7 9 2 3 5 7 9 1 2 3 4 6 1 2 3 5 7 1 2 3 4 6 20.0 9.0 9.0 9.0 9.0 50 0 2 50 50 0 110 179 251 333 0 144 215 279 338 0 129 196 246 3 16 0 95 159 221 279 0 88 163 226 290 .75 .01 0.8 3.0 5.0 90 85 85 85 0. 00 0.00 0. 00 0. 00 0.00 2. 50 1.10 3. 40 1. 10 1. 10 3. 00 1. 29 4. 00 1. 29 1.29 0. 50 0.00 1. 50 0. 00 0.00 0. 20 0.00 1. 00 0. 00 0.00 2. 59 1.10 3. 50 1. 10 1. 10 4. 30 2. 20 5. 30 2. 20 2.20 4. 20 2.00 5. 20 2. 00 2.00 4. 20 2.00 5. 20 2. 00 2.00 6. 00 3.70 7. 00 3. 70 3.70 6. 00 3.70 7. 00 3. 70 3.70 4. 20 2.00 5. 20 2. 00 2.00 1. 79 0.30 2. 80 0. 30 ,0.30 1. 80 0.40 2. 80 0. 40 •' 0.40 0. 20 0.00 1. 20 0. 00 0.00 0. 20 0.00 1. 20 0. 00 0.00 0. 70 0.00 1. 50 0. 00 0.00 2. 40 0.70 3. 30 0. 70 0.70 1. 50 0.00 2. 50 0. 00 0.00 1. 20 0.00 2. 20 0. 00 0.00 4. 30 2. 20 5. 30 2. 20 2.20 6. 70 4.50 7. 69 4. 50 4.50 8.00 5.70 9. 00 5. 70 5.70 6. 20 4.00 7. 20 4. 00 4.00 6. 00 3.70 7. 00 3. 70 3.70 8. 00 5.70 9. 00 5. 70 5.70 9. 20 7.00 10. 20 7. 00 7.00 9. 70 7.50 10. 70 7. 50 7.50 4. 50 2.20 5. 50 2. 20 2.20 4. 20 2.00 5. 20 2. 00 2.00 4. 80 2.40 5. 80 2. 40 2.40 3. 50 1.20 4. 50 1. 20 1.20 3. 20 1.00 4. 20 1. 00 1.00 0.041 0.073 0.200 0.138 0.095 0.015 0. 0.082 0. 135 0.217 0. 0.015 0.125 0.014 0.101 0. 0. 0.030 0.037 0.066 0. 0. 0.026 0. 0.056 0. 0. 0. 0. 018 0.030 0. 0. 0. 0. 0.012 0. 0. 0. 0. 0. 10 10 10 10 8 8 8 8 8^  8 446 576 750 3000 406 509 675 3000 396 511 685 3000 343 456 622 3000 342 438 610 3000 2.80 3.00 2.80 5.70 4.00 2.30 3.70 3.20 3.00 3.50 3. 20 2.30 2.00 2.50 6. 20 6.20 5.00 4.20 5.00 5.20 5.00 2.00 0. 80 1.80 1. 20 1.50 2.80 4.70 4.00 5.70 8.50 5.70 5.80 3.50 7. 50 6.20 7.00 5.20 4.00 5.30 8.30 10.50 11.50 13.00 13.50 15.50 16. 20 16.20 16. 20 16.00 14.00 15.20 15.50 15.80 15.80 0. 40 0.80 0.60 3.50 1.70 0.20 1.70 1.00 0.80 1.20 1.00 0.20 0.00 0.50 4.,10 4.00 2.70 2.00 2.70 3.00 2.70 0.30 0.00 0.00 0.00 0.00 0. 60 2.50 1.89 3.60 6. 20 3.50 3. 40 1.20 5. 20 4.00 4.70 3.00 1.70 2.90 5.90 8.20 9. 20 10.70 1 1. 20 13.20 14.00 14.00 14.00 13.70 11.70 13.00 13.20 13.40 13.40 3.80 4.00 3. 80 6.70 5.00 3.30 4.70 4.20 4.00 4.50 4.20 3.30 3.00 3.50 7. 19 7.20 6.00 5.20 6.00 6.20 6.00 3.00 1.80 2.80 2.20 2.50 3.80 5.70 5.00 6.70 9.50 6.70 6.80 4.50 8.50 7.20 8.00 6.20 5.00 6.30 9.30 11.50 12.50 14.00 14.50 16.50 17.20 17.20 17. 20 17.00 15.00 16.20 16.50 16.80 16.80 0.40 0.80 0.60 3.50 1.70 0.20 1.70 1.00 0.80 1.20 1.00 0.20 0.00 0.50 4. 10 4.00 2.70 2.00 2.70 3.00 2.70 0.30 0.00 0.00 0.00 0.00 0.60 2.50 1.89 3.60 6.20 3.50 3.40 1.20 5.20 4.00 4.70 3.00 1.70 2.90 5.90 8.20 9.20 10.70 11.20 13.20 14.00 14.00 14.00 13.70 11.70 13.00 13.20 13.40 13.40 0.40 0. 80 0.60 3.50 1.70 0.20 1.70 1.00 0.80 1.20 1.00 0.20 0.00 0.50 4.10 4.00 2.70 2.00 2.70 3.00 2.70 0.30 0.00 0.00 0.00 0.00 0.60 2.50 1.89 3.60 6.20 3.50 3.40 1.20 5.20 4.00 4.70 3.00 1.70 2.90 5.90 8.20 9.20 10.70 11.20 13.20 14.00 14.00 14.00 13.70 11.70 13.00 13.20 13.40 13.40 4.40 16.50 16. 50 16.50 11.50 15.50 13. 20 14.80 15.80 14.30 15.00 15.20 15. 20 15.20 15.00 14.50 9.70 10.80 12. 20 11.00 11.00 12.20 13. 30 10.30 8.80 9.50 10.00 10.80 10.20 10.80 11. 20 11.50 11.50 9.00 7.50 6.00 5.80 6.80 6.80 5.50 5.50 5.20 4. 20 5.50 4.50 4.20 4.80 5.50 5.50 5.20 4.20 5.20 5. 50 5.80 6.00 2.90 5. 10 14.20 17.50 14.20 17.50 14.20 17.50 9.20 12.50 13.20 16.50 11.00 14.20 12.40 15.80 13.40 16.80 11.90 15.30 12.70 16.00 13.00 16.20 13.00 16.20 13.00 16.20 12.70 16.00 12.20 15.50 7.50 10.70 8.40 11.80 10.00 13.20 8.70 12.00 8.70 12.00 10.00 13.20 10.90 14.30 7.90 11.30 6.40 9. 80 7.20 10.50 7.70 11.00 8.40 11.80 8.00 11.20 8.40 11.80 9.00 12.20 9.20 12.50 9.20 12.50 6.70 10.00 5. 20 8.50 3.70 7.00 3. 40 6.80 4.40 7.80 4. 40 7.80 3.20 6.50 3.20 6.50 3.00 6.20 2.00 5.20 3.20 6.50 2. 20 5.50 2.00 5.20 2.40 5.80 3.20 6.50 3. 20 6.50 3.00 6.20 2.00 5.20 3.00 6.20 3. 20 6.50 3.40 6.80 3.70 7.00 2.90 2.90 14.20 14.20 14.20 14.20 14.20 14.20 9.20 9.20 13.20 13.20 11.00 11.00 12.40 12.40 13.40 13.40 11.90 11.90 12.70 12.70 13.00 13.00 13.00 13.00 13.00 13.00 12.70 12.70 12.20 12.20 7.50 7.50 8.40 8.40 10.00 10.00 8.70 8.70 8.70 8.70 10.00 10.00 10.90 10.90 7.90 7.90 6.40 6.40 7.20 7.20 7.70 7.70 8.40 8.40 8.00 8.00 8.40 8.40 9.00 9.00 9.20 9.20 9.20 9.20 6.70 6.70 5.20 5.20 3.70 3.70 3.40 3.40 4.40 4.40 4.40 4.40 3.20 3.20 3.20 3.20 3.00 3.00 2.00 2.00 3.20 3.20 2.20 2.20 2.00 2.00 2.40 2.40 3.20 3.20 3.20 3.20 3.00 3.00-2.00 2.00 3.00 3.00 3.20 3.20 3.40 3.40 3.70 3.70 5 .80 3 . 4 0 6 . 80 3 . 4 0 3 . 4 0 6 . 0 0 3 . 7 0 7 . 00 3 .70 3 . 7 0 5 . 30 2 .90 6 . 30 2 . 9 0 2 . 9 0 4 . 5 0 2 . 2 0 5 . 50 2 . 2 0 2 . 2 0 3 . 50 1.20 4 . 50 1.20 1 .20 2 . 8 0 0 .40 3 . 80 0 . 4 0 0 . 4 0 2 . 5 0 0 . 30 3 . 50 0 . 3 0 0 . 3 0 1 .50 0 . 0 0 2 . 50 0 . 0 0 0 . 0 0 0 . 8 0 0 . 0 0 1. 80 0 .00 0 . 0 0 1 .20 0 . 0 0 2 . 20 0 . 0 0 0 . 0 0 1.50 0 . 0 0 2 . 50 0 . 0 0 0 . 0 0 8 . 0 5 1 0 . 3 0 1 1 . 05 1 0 . 0 5 9 . 3 0 10 .80 1 2 . 5 5 12 .80 1 2 . 5 5 13 . 8 0 15 .30 1 4 . 0 5 11 . 30 1 0 . 3 0 9 . 8 0 9 . 80 9 . 5 5 1 0 . 80 1 0 . 5 5 10 . 5 5 1 3 . 5 5 1 4 . 3 0 1 6 . 30 1 5 . 3 0 1 5 . 3 0 16 . 05 17 .30 18. 05 15 . 80 13 . 3 0 1 3 . 8 0 13. 30 12. 30 1 2 . 3 0 1 2 . 0 5 1 2 . 55 1 4 . 0 5 1 3 . 55 1 2 . 0 5 11 . 8 0 1 2 . 3 0 1 2 . 3 0 1 2 . 30 12 .30 1 1.55 11 . 30 1 1.80 14. 05 14. 80 14 . 0 5 13 .30 1 4 . 5 5 14. 55 1 4 . 0 5 1 2 . 3 0 10 . 05 1 0 . 5 5 1 0 . 30 1 0 . 3 0 11 . 3 0 13 .30 1 3 . 3 0 1 4 . 30 16 .80 1 5 . 0 5 14. 80 1 3 . 0 5 1 5 . 5 5 15. 30 1 5 . 5 5 15 .30 1 3 . 0 5 13 . 55 15 .80 1 8 . 3 0 2 0 . 05 2 1 . 5 5 2 1 . 80 2 4 . 5 5 25 . 8 0 2 5 . 5 5 2 4 . 8 0 2 4 . 05 2 2 . 8 0 2 3 . 0 5 2 4 . 05 2 4 . 3 0 2 4 . 30 2 4 . 0 5 25 . 0 5 2 7 . 0 5 2 5 . 8 0 2 2 . 5 5 2 3 . 5 5 2 2 . 3 0 2 3 . 30 2 3 . 8 0 2 3 . 05 2 3 . 3 0 23 . 8 0 2 4 . 3 0 2 4 . 3 0 2 4 . 30 2 3 . 8 0 2 3 . 3 0 2 0 . 30 2 0 . 30 2 1 . 05 2 0 . 55 20 . 0 5 2 1 . 0 5 2 2 . 0 5 2 0 . 05 1 8 . 8 0 1 8 . 8 0 1 9 . 30 1 9 . 5 5 1 9 . 55 1 9 . 8 0 20 . 0 5 2 0 . 3 0 2 0 . 3 0 1 8 . 80 17 .30 1 6 . 3 0 15 . 30 15 .80 16. 05 1 4 . 8 0 14 . 0 5 1 4 . 5 5 1 3 , 8 0 14. 55 13 .80 1 4 . 0 5 1 4 . 55 1 4 . 8 0 1 0 . 55 1 4 . 5 5 14 . 5 5 1 5 . 0 5 1 5 . 0 5 1 5 . 05 1 5 . 0 5 1 5 . 0 5 14. 05 14. 30 13. 30 1 2 . 5 5 12 . 0 5 1 1 . 05 9 . 80 10 . 80 10 . 80 0 . 7 1 . 2 1 6 . 5 4 7 . 1164 . 2 2 4 8 . 2 6 0 6 . 2 0 5 2 . 0 . 9 8 . 2 9 2 . 6 8 9 . 1438 . 3 0 5 0 . 2 5 9 8 . 2 0 4 5 . 0 . 3 9 . 9 9 . 2 6 8 . 5 7 0 . 9 9 5 . 1426. 1123 . 0 . 4 4 . 125 . 2 4 0 . 4 8 4 . 1035 i 1 2 3 9 . 9 7 5 . 0 . 3 4 . 1 0 3 . 2 0 1 . 4 7 6 . 9 0 7 . 9 15 . ' 7 2 1 . 149 . 0 . 0 1 8 7 0 . 0 2 0 5 0 . 0 2 2 8 0 . 0 2 9 8 0 . 0 2 3 7 0 . 0 1 6 0 0 . 0 2 7 9 0 . 0 18 2 0 0 . 7 9 5 . 5 7 0 . 6 1 2 0 . 7 1 5 2 . 2 4 8 . 4 1 1 0 . 3 145 .4 3 . 3 0 2 . 4 2 2 . 5 8 2 .41 2 . 47 1.93 2 . 8 6 3 . 0 0 Publications of G. S. Jamieson: 1) Jamieson, G. S. & H. D. Fisher. 1970. V i s u a l discriminations i n the harbour s e a l , Phoca v i t u l i n a , above and below water. V i s i o n Res., 10: H 7 5 - I I 8 0 . 2) Jamieson, G. S. & H. D. Fisher. 1971. The r e t i n a of the harbour se a l , Phoca v i t u l i n a . Can. J . Zool., 49(1) : 1Q-23. 3) Jamieson, G. S. 1971. The functional significance of corneal d i s t o r t i o n i n marine mammals. Can. J . Zool., 4-9(3): 421-423. 4) Jamieson, G. S. & H. D. Fis h e r . 1972. The pinniped eye - a review. In The Functional Anatomy of Marine Mammals. Ed. R. J . Harrison. V o l . 1. Academic Press, London. 5) Scudder, G.G.E. & G. S. Jamieson. 1972. The immature stages of Gerris (Hemiptera) i n B r i t i s h Columbia. J . Ent. Soc. B.C., 69: 72-79. 

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