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Population study of waterstriders (Gerridae: Hemiptera) in Marion Lake, B.C. Maynard, Kathleen Jennifer 1969

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A POPULATION STUDY OF WATERSTRIDERS (GERRIDAE:HEMIPTERA) IN MARION LAKE, B. C. by KATHLEEN JENNIFER MAYNARD B.Sc. (Honours), University of B r i t i s h Columbia, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of Zoology We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August, 1969 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 of 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 C olumbia, I a g r e e 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 Study. I f u r t h e r a g r e e 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 u rposes 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 of 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 not 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 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 A B S T R A C T This study consisted i n part of general observations of the biology of four species of waterstrider (Gerris buenoi, incurvatus, n o t a b i l i s , and remiges, i n Marion Lake, B. C , and i n part of an experimental manipulation of density and food supply i n repl i c a t e d populations of penned G. n o t a b i l i s . Gerrids have a generation time of one year; adults overwinter, lay t h e i r eggs i n the spring, and then die; nymphs become adults i n about two months. In the natural population storms and food shortage probably caused the greatest mortality. In the penned populations survival of nymphs was inversely proportional to spring density of adults, and d i r e c t l y proportional to food supply. In low density pens fewer nymphs hatched but r e l a t i v e l y more sur-vived; i n high density pens more nymphs hatched but r e l a t -i v e l y fewer survived; thus f a l l numbers were much the same (within food treatments) regardless of i n i t i a l density. Increased density also lowered survival of adults i n both food treatments, and, i n turn, low adult survival enhanced the expectation of survival of nymphs, i n the unfed pens only. However these e f f e c t s were unimportant compared with the d i r e c t e f f e c t s of density and food on nymph sur-v i v a l . Most of the nymphs died during the f i r s t stadium, probably owing to cannibalism by older nymphs and parents i n the fed pens, and to both starvation and cannibalism i n the unfed pens. In view of the flu c t u a t i n g food supply to which i i i gerrids are subject, t h e i r opportunistic method of feeding and concomitant c a n n i b a l i s t i c behaviour i s probably of sel e c t i v e advantage to the i n d i v i d u a l , as well as being a p o t e n t i a l population regulating mechanism. i v TABLE OF CONTENTS Abstract i i Table of contents i v L i s t of tables v L i s t of figures v i Acknowledgments v i i INTRODUCTION 1 MATERIALS AND METHODS _ 4 OBSERVATIONS AND RESULTS 13 BIOLOGY 13 EXPERIMENTAL RESULTS 18 DISCUSSION 32 CONCLUSIONS 39 REFERENCES 41 APPENDIX 42 V LIST OF TABLES I. Measurements of hind leg length. 5 II. I n i t i a l numbers of Gerris n o t a b i l i s placed i n experimental pens, and feeding rates. 9 I I I . Reproduction data for Gerris n o t a b i l i s . 12 IV. Seasonal occurrence of l i f e stages i n Marion Lake, 1968. 14 V. Average number of nymphs becoming adults (production) i n each basic treatment. 23 VI. Average number of nymphs becoming adults depending on s u r v i v a l of parents and laying of a second batch of eggs. 24 VII. Weighted means of expectation of survival of nymphs in each basic treatment. 25 VIII. Expectation of survival of nymphs depending on s u r v i v a l of parents and laying of a second batch of eggs. 27 IX. Analysis of variance of regressions of survivorship curves of nymphs. 29 X. Expectation of sur v i v a l of parents under the three density treatments. 31 v i LIST OF FIGURES 1. Method of t r e a t i n g census data as one cohort . 10 2. Mean numbers i n cohort (nymphs entering the population) i n the s ix basic treatments. 20 3. Changes i n the t o t a l number of nymphs of a l l i n s t a r s for the s ix basic treatments. 20 4. Deaths of f i r s t and second i n s t a r nymphs expressed as a number dying i n a c e r t a i n week d iv ided by the t o t a l number of o lder nymphs a l i v e at the beginning of the week. 21 5. Estimated body volume of a l l nymphs and immature adu l t s . 21 6. Mean surv ivorship curves of nymphs. 2 8 7. S u r v i v a l of adu l t s . 30 ACKNOWLEDGMENTS I would l i k e to thank Dr. Dennis Chitty for his help and advice during t h i s study. My work benefitted from h e l p f u l c r i t i c i s m by Dr. R. Harger, Dr. G. Scudder, Dr. R. L i l e y , and Dr. J . Stimson. The f i e l d experiments could not have been set up without the help of R. Black, Pat Bright, B. Seghers, K. Tsumura, and my husband, Rick. R. Black and N. Gil b e r t helped me greatly during the writing of the thesis; without t h e i r encouragement the work might have continued i n d e f i n i t e l y . I was supported by a bursary and scholarship from the National Research Council of Canada. 1 INTRODUCTION There are two approaches to the study of "population regulation". One i s to examine s p e c i f i c natural populations to see what factors i n the environment are influencing reproduction, mortality, emigration, and immigration. It has been claimed that starvation, predation, disease, parasitism, and c l i m a t i c catastrophes have arrested increase i n numbers by a f f e c t i n g one or more of these pro-cesses. Thus t h i s f i r s t approach t e l l s us what actually happens to a p a r t i c u l a r species i n a p a r t i c u l a r environ-ment, but i t does not necessarily help us to predict what w i l l happen to some population at some future time. It i s for t h i s reason that some workers prefer another approach, and ask "what w i l l happen to a population i f , by chance, a l l environmental factors remain favourable for • increase?" Through experimental manipulation they attempt to bring about such favourable s i t u a t i o n s , and observe the con-sequences at both the i n d i v i d u a l and population l e v e l . Chitty (1964, 1967) uses the second approach. He states his hypothesis as generally as possible: " A l l species of animals have a form of behaviour that can prevent unlimited increase i n population density". I attempted to see whether t h i s was true i n a population of waterstriders, Gerris n o t a b i l i s . According to Brinkhurst 2 (1966) i t i s u n l i k e l y that under normal circumstances predation, disease, or weather l i m i t gerrid populations; however> since gerrids feed o p p o r t u n i s t i c a l l y on s o f t -bodied insects accidentally trapped on the surface f i l m , I suspected that numbers i n at least some populations could be l i m i t e d by food supply. Since i t i s useless to t r y to test Chitty's hypothesis on a population which i s starving, I planned to add excess food to a penned pop-ul a t i o n . This treatment should remove the most important l i m i t i n g factor, so that numbers would continue to r i s e unless checked by some physiological or behavioural mech-anism which caused death or emigration, or i n h i b i t e d reprod-uction. I suspected that cannibalism might be a form of behaviour which could cause a mortality d i r e c t l y proport-io n a l to density, or inversely proportional to food supply, or both. If t h i s were so then exceptionally high and exceptionally low density populations, enclosed i n pens, should converge towards d i f f e r e n t absolute levels depend-ing on food supply. Eisenburg (1966) attempted to demonstrate the existence of density-dependent regulation of numbers by a l t e r i n g densities of iso l a t e d units of a natural population of pond s n a i l . Although he observed no changes i n adult mort-a l i t y , the density of young sn a i l s i n unaltered and altered units appeared to converge over a four-month period, appar-ently owing to changes i n fecundity and survivorship of 3 young. However, his statement that the "only variable i n t h i s system was adult density" cannot be substantiated, because by changing numbers of adults i n his pens, he altered both the frequency of i n t r a s p e c i f i c contacts and the food supply per i n d i v i d u a l s n a i l . Thus his alleged convergence may have been caused either by food l i m i t a t i o n or by some form of behaviour, or a combination of both. To separate the e f f e c t s of density ( i . e . frequency of encounters) and food supply, I designed a f a c t o r i a l exper-iment using three density and two food l e v e l s , the food being added i n proportion to i n i t i a l density. At the end of a generation, t h i s experiment yielded six groups of adults, each exposed to a d i f f e r e n t treatment since they hatched; three at d i f f e r e n t densities with excess food, the other three at similar densities with only limited resources of food. In addition to manipulating food and density i n the experimental pens I planned to determine the important factors influencing the natural population of gerrids i n Marion Lake. In essence, then, I wanted to f i n d out from the nat-u r a l population "what does happen", and from an experimental manipulation of food supply "what would happen" to the behavioural and numerical c h a r a c t e r i s t i c s of the group, given favourable environmental conditions. 4 MATERIALS AND METHODS The study area was a small lake i n the University of B r i t i s h Columbia Research Forest near Haney, B. C. (described by Efford, 1967). The animals studied were waterstriders of the genus Gerris. The project required that I be able to recognize the nymphs and adults of a l l four species of Gerris i n Marion Lake. From laboratory-raised nymphs I made accurate drawings and measurements of the fourth and f i f t h i nstars of G. remiges Say and a l l f i v e i n s t a r s of the other three species, G. notabjlis Drake and Hottes, G. incurvatus Drake and Hottes, and G. buenoi Kirkaldy (Table 1). (Only a few samples of each in s t a r were taken because i n Brinkhurst 1s (1959) description of nymphs of B r i t i s h gerrids the v a r i a t i o n i n limb length i s only ±.05 mm for f i r s t i n s t a r nymphs to ±.15 mm for f i f t h i n s t a r s . Since the differences between d i f f e r e n t instars of the same species and between corresponding instars of d i f f e r e n t species i s greater than that, I thought i t unnecessary to have a large sample size.) From the measurements and drawings I constructed a key to the nymphs of the four species (Appendix). During the winter of 1967-68 I kept adults of a l l four species i n the laboratory, where I observed courtship, mating, egg-laying, and feeding behaviour. I also attempted to r a i s e the nymphs to maturity; however owing to lack of proper f a c i l i t i e s few nymphs survived. Nevertheless, I 5 TABLE I. Measurements of hind leg length, mm. F = femur, T i = t i b i a , Ta = tarsus. Stadium G. buenoi G. incurvatus G. n o t a b i l i s G. remiges F T i Ta F T i Ta F T i Ta F T i Ta 1 o.5 o.4 0.3 o.7 0.5 0.3 1.2 .75 .55 2 1.0 0.6 0.4 1.0 0.6 0.4 1.6 1.0 0.7 3 1.5 0.8 0.5 1.9 1.1 0.7 3.1 1.8 1.0 4 2.3 1.4 0.8 2.9 1.5 0.9 4.5 3.0 1.3 4.0 3.5 1.5 5 4.0 1.5 1.0 8.0 5.3 2.0 6.0 5.3 2.0 6 found that G. n o t a b i l i s was probably the best species for experimental study because i t had highest nymph s u r v i v a l , was largest, and adapted i t s e l f most e a s i l y to confinement. During the summer of 1968 I made general f i e l d obser-vations of the biology of the four species i n Marion Lake. I noted d e t a i l s of the preferred habitats of each species by observing and recording occurrences i n various parts of the lake. I recorded dates and places when eggs and nymphs of various stages were found and observed courtship and o v i p o s i t i o n . I also used a mark and recapture (Lincoln Index) method to determine the numbers of G. n o t a b i l i s present i n a part of the lake known as Productivity Bay, and to determine the extent of movements around the lake. To investigate the e f f e c t s of food l e v e l per i n d i v -idual and i n i t i a l density on s u r v i v a l of nymphs I used a 2x3 f a c t o r i a l design (fed and unfed; three densities of adults—one, three, and nine pairs) with four r e p l i c a t e s . The enclosures for each experimental unit were pens, 122 cm by 244 cm by 46 cm, constructed out of .9 cm plywood. I bolted a 10 cm s t r i p of 2.5 cm thick styrofoam a l l around the inside at the base of the pen, which buoyed up the pen so that the sides were 30 cm out of the water. Nylon screen covered the bottom, a roof 30 cm wide sheltered one end, and a fringe of p l a s t i c (7 to 10 cm wide) was stapled around the top of the inside wall to prevent gerrids from climbing out. I anchored the pens i n groups 7 of two i n Productivity Bay, a sheltered and shallow part of the lake which supported very large numbers of G. not- a b i l i s . To each pair of pens I randomly assigned a density, and designated one member of each pair as a "fed" pen. In each pen I also anchored several small pieces of styrofoam to be used as re s t i n g areas. For the reasons noted above I decided to use G. not-a b i l i s as the experimental animal. A l l gerrids used i n the experiment were c o l l e c t e d i n Marion Lake, most of them i n Productivity Bay. They were co l l e c t e d between 16 and 25 May 196 8, and held i n the laboratory, where some of the females l a i d eggs. I marked the o r i g i n a l adults on the pro-notum with a spot of fluorescent paint (red for females; green for males), and glued t h e i r wings down with a dab of n a i l p o l i s h to prevent them f l y i n g away. I made no attempt to stop the o f f s p r i n g f l y i n g away at the end of the obser-vations on nymph s u r v i v a l . Since the experiment was begun with a standard age s t r u c t u r e — a l l a d u l t s — a n d concluded when t h i s state was again reached, only differences i n su r v i v a l of nymphs were important i n the r e s u l t i n g l i f e tables. Because some females l a i d eggs while being held i n the laboratory, I had to place these laboratory-hatched nymphs i n the pens as well. I placed the gerrids i n the pens on 12 June, with gravid and empty females allocated proportionately within treatment r e p l i c a t e s . I placed laboratory-hatched f i r s t i n s t a r nymphs i n each pen i n 8 proportion to the number of adults, 2 0 for each non-gravid female (Table 2). I censused each pen once a week by extending a long pole with a sheet of aluminum at the end to the far end of the pen and drawing i t slowly towards me. In t h i s way I c o l l e c t e d e s s e n t i a l l y a l l the insects present into a small area so that I could tabulate them by instars on a blood c e l l counter. The census was found to be very accurate for second i n s t a r and older nymphs; however I probably missed some f i r s t i n s t a r nymphs, so that they were under-represented i n the census. I added food (vestigial-winged Drosophila) every other day to "fed" pens—the amount of food was proportional to the i n i t i a l density of adults. Drosophila were anaes-thetized and then measured i n a standard b o t t l e — l o w density pens got about 2 cc of f l i e s , medium about 7 cc, and high about 21 cc every two days. Unfortunately, s u f f i c i e n t stocks of v e s t i g i a l Drosophila were d i f f i c u l t to keep going i n the fl u c t u a t i n g temperature of the f i e l d labor-atory, and for t h i s reason i t was at times impossible to add the planned amount of food. However, as w i l l be shown, the "fed" pens were apparently not limited by food. The weekly census yielded a table showing the numbers of nymphs a l i v e i n each instar each week. In order to analyze these figures I decided to treat a l l nymphs i n a pen as onecohort (the method i s shown i n Figure 1){ 9 TABLE I I . I n i t i a l numbers of G. n o t a b i l i s placed i n experimental pens, and feeding rate of the fed pens. Density Number Adults Number F i r s t Instar Feeding Rate ( I n i t i a l ) Nymphs/Non-gravid? (per 2 days) Low Medium High 2 6 18 20 60 180 2 cc 7 cc 21 cc 10 Figure 1. Method of t r e a t i n g census data as one cohort . Large numbers are the numbers of each i n s t a r , at each census. Numbers over l i n e s ind ica te numphs changing from one stadium to the next. Underlined numbers rep-resent deaths. d^ i s the t o t a l number of deaths during the stadium; 1 i s the number of nymphs enter ing the stadium. The t o t a l number of deaths (313) plus the number of adults s u r v i v i n g (14) i s the s ize of the cohort (327). The 1 f igures were used to ca l cu la t e a l i f e table for each pen. I assumed that a l l nymphs molted once between census per-iods , unless the census data obviously indicated otherwise. This assumption i s i n turn based on others , that a l l s tadia las ted about the same length of time, and were constant both between i n d i v i d u a l s and over the season. In f a c t , under constant laboratory condi t ions , a l l s tadia except the f i f t h were less than one week (Table 3) . I a lso assumed that the d i f ference between the numbers i n one i n s t a r at a census per iod and the numbers i n the next i n s t a r at the subsequent census represented deaths. The t o t a l morta l i ty plus the number of nymphs becoming adults gave the t o t a l number i n the cohort . Using the numbers of nymphs a l i v e at the beginning of each i n s t a r , from hatching to f i n a l molt , I computed a l i f e table for each pen which gave p r o b a b i l i t i e s of s u r v i v a l of each i n s t a r , proport ion a l i v e at the beginning of each i n s t a r , and average expect-a t ion of further l i f e at hatching (Les l ie e_t a l , 1955) . This method of t r e a t i n g the census data obscured any changes i n s u r v i v a l rates over the summer; i t also minimized the estimate of m o r t a l i t y , since any nymphs hatching and dying between censuses were missed, and the census f igures under-represented f i r s t i n s t a r nymphs. However, because a l l treatments were analysed the same way, comparison of pens i s p o s s i b l e . 12 TABLE I I I . Reproduction data for Gerris n o t a b i l i s Duration of copular p a i r i n g Time between mating and laying Time for laying Development time: Incubation F i r s t stadium Second stadium Third stadium Fourth stadium F i f t h stadium Total Mean number of eggs per female per two weeks, i n lab (+S.E.) n=7 8-10 min. 2-3 hours 10 min./15 eggs 8 days 6 days 5 days 5 days 5- 7 days 6- 10 days 35-41 days 93 ± 16 'A3 OBSERVATIONS AND RESULTS BIOLOGY The l i t e r a t u r e contains no information on the biology of.the four Gerris species i n Marion Lake, and for t h i s reason I include a few observations on t h e i r natural his t o r y . Table 4 shows seasonal occurrence of the various l i f e stages of each species. G. remiges (11.4-16 mm, apterous) i s primarily a stream-dwelling species but f a i r l y large numbers of both nymphs and adults were observed throughout the summer i n open water at the i n l e t end of the lake. However, eggs were not found i n the lake. Individuals seem to be p o s i t i v e l y thigmotactic when confined i n a small area, but casual observations suggest that they space themselves out i n the l i n e a r environment of the streams. G. incurvatus and G. buenoi are small species (8-10 mm, polymorphic) and look much a l i k e ; the nymphs of the two species are also very s i m i l a r . They apparently occupy i d e n t i c a l habitats and yet seem to be coexisting i n almost equal numbers. I was unable to d i s t i n g u i s h between t h e i r egg masses—both were l a i d i n rows along the under edge of w a t e r - l i l y and Potomogeton leaves, each egg slanted at a 45 degree angle to the edge. Both species were r a r e l y found i n open water; t h e i r preferred habitat was patches of l i l y pad and scummy Potomogeton beds. They were very abundant i n Productivity Bay at the beginning of the summer, but very scarce by August. Both species are rather 14 TABLE IV. Seasonal occurrence of l i f e stages i n Marion Lake, 1968. Stage Gv remiges G . n o t a b i l i s G . incurvatus G . buenoi Mature adults May 3 - Oct 12 May 3 - oct 12 Eggs May 15 - Aug 8 May 3 - oct 12 Nymphs 1 2 3 4 5 Immature June 8 - Aug 2 8 June 4 - Sept 1 June 16 - Sept 6 June 10 - Sept 7 June 28 - Sept 14 June 13 - Sept 12 July 4 - Sept 20 June 21 - Sept 20 July 2 - Sept 2 0 July 12 - Oct 1 July 2 - Sept 2 9 adults July 9 on July 18 on July 8 on 15 gregarious; more than 10 adults on one l i l y pad was a regular occurrence. G. not a b i l i s , . the largest species (15-20 mm) was the easiest to keep i n the laboratory, and i t s nymphs were e a s i l y distinguished i n the f i e l d . Individuals were apparently dispersed within t h e i r preferred h a b i t a t — shallow water thick with Potomogeton, l i l y pads, and grasses. Eggs were l a i d i n neat rows along the under edge of these leaves, each egg perpendicular to the edge of the l e a f . When f i r s t l a i d the eggs are white, but as they develop they change f i r s t to yellow (with v i s i b l e red eye spots) and then to black. After hatching the grayish egg cases remain. Incubation time varies depend-ing on .the water temperature—two to three weeks i n May and eight to ten days i n August (Table 3). Although most eggs are f e r t i l e , a few f a i l to hatch, and of those nymphs that do hatch many f a i l to break through the surface f i l m and dry themselves o f f . Although both laboratory and f i e l d observations indicated that individuals are normally spaced out, they are opportunistic feeders, and I often observed up to eight adults feeding on one damselfly. The importance of t e r r i t o r i a l behaviour therefore needs to be investigated further. The courtship behaviour of G. n o t a b i l i s i s i n t e r e s t i n g . Males approach other conspecifics apparently indiscrimin-antly as to sex, "bouncing" or "vibrating" up and down on t h e i r long legs. If the intruder i s another male, a 16. f ight may ensue, with the combatants jumping around on the water locked together, each apparently attempting to pierce the other with h i s s t y l u s . I f the approached insect i s a recept ive female she w i l l stop moving and allow the male to mount her; i f she i s not recept ive she w i l l skate away immediately, or wait u n t i l the male attempts -to mount her before moving away. Copular p a i r i n g las t s about ten min-utes i n undisturbed animals i n the laboratory . In court -ship in terac t ions the antennae seem to be important, for t h e i r movement increases at these times. Struggl ing insects trapped by surface tension are apparently located both by s ight and by zeroing i n on the source of the r i p p l e s by means of rap id darts which over-shoot the target l i k e dampening o s c i l l a t i o n s . Dead or motionless food organisms are apparently located by means of chemical sense organs on the antennae. Predators are probably an unimportant cause of mort-a l i t y i n Marion Lake. Stomach contents of trout and s a l -amanders seldom contain gerr ids (Tsumura, pers . comm.). A Water Spider was placed for two days i n a container with several nymphs of a l l stages and a l l the nymphs survived. A red water mite (Elyais) was found as a paras i te on a l l four species from mid-July u n t i l September. As many as four mite larvae were found attached to nymphs of a l l stages. Several infested nymphs were brought into the laboratory (with uninfested nymphs of corresponding ins tars 17 as controls) and su r v i v a l u n t i l the mites dropped o f f was 100% i n both groups. Weather caused some mortality, e s p e c i a l l y i n f i r s t to t h i r d i n s t a r nymphs. During a very severe wind and r a i n -storm nymphs and adults were observed c l i n g i n g to the sides of the pens well above the water l i n e , and to the lee side of l i l y pads. However, many unpenned animals were blown into the water where they were battered about, and afte r the storm I saw f l o a t i n g and submerged bodies. Adults and older nymphs were less susceptible to such mortality. The adult population i n Productivity Bay was estimated by simple Lincoln Index to be about 400 animals (in June), a density of about six per ten square feet. Other areas i n the lake had much lower densities of G. n o t a b i l i s . I did not obtain an accurate estimate of the size of the f a l l population because.the adults l e f t the lake i n mid-August, e a r l i e r than I expected. However numbers i n August appeared to be of the same order of magnitude as i n June. The density of G. n o t a b i l i s i n Productivity Bay corresponded to "high" densities i n the experimental pens. Natural food supplies were probably greater than i n the "unfed" pens, but were c e r t a i n l y not i n excess, so that the natural conditions of food per i n d i v i d u a l probably corresponded to those i n the medium density unfed pens. In the laboratory I observed cannibalism many times; IS i t has also been reported by Brinkhurst (1966) . Nymphs which are molting are p a r t i c u l a r l y susceptible, as are f i r s t and second instar nymphs. I also observed adults and f i f t h i n s t a r nymphs i n the pens attacking early i n s t a r nymphs. The l a t t e r often escaped, but many were caught and eaten. EXPERIMENTAL RESULTS Eff e c t s of Treatments on Population Size Figure 2 shows that the more adults i n a pen, the more nymphs they produced over the summer. In t h i s case the number of nymphs hatching was not related to adult food supply; however t h i s does not rule out possible e f f e c t s of food supply before and immediately after hiber-nation. Figure 3 shows changes i n t o t a l numbers of nymphs and immature adults a l i v e through the summer, for each treatment. The unfed pens reached a lower peak and dropped more rapidly than the fed pens. Because of the technical d i f f i c u l t y of adding f l i e s , I wanted to f i n d out whether the fed pens were ever short of food. In addition I wanted to fi n d out whether starv-ation and cannibalism affected a l l instars equally, or i f there was se l e c t i v e mortality of younger nymphs during food shortage. I plotted the r a t i o of the number of deaths of f i r s t and second in s t a r nymphs to the t o t a l number of older nymphs (Figure 4) for weeks three to s i x , when t o t a l numbers were highest. During t h i s time there was a d i f f -19. erence i n the pattern of mortality i n fed and unfed pens. In the fed pens the r a t i o of young nymphs dying to the t o t a l number of older nymphs remained constant, while i n the unfed pens the r a t i o was high during the t h i r d and fourth week when density was highest (and there was least food per i n d i v i d u a l ) . This r e s u l t for unfed pens indicates one or both of the following p o s s i b i l i t i e s : (1) young nymphs are more susceptible to starvation, or (2) there i s sele c t i v e cannibalism on young nymphs during times of food shortage. In the fed pens, the fact that the r a t i o does not change indicates that there i s probably no food shortage. For further evidence that my fed pens were never short of food, I assumed f i r s t that a plo t of t o t a l bio-mass i n food-limited pens should l e v e l out at the same time as t o t a l numbers, while i f mortality i s not associated with food shortage then biomass should continue to increase when t o t a l numbers l e v e l out. Since I had no dry weights of nymphs, I made two d i f f e r e n t estimates of r e l a t i v e biomass: (1) From the r e l a t i v e lengths of hind leg of each in s t a r (Table 1) I estimated the r e l a t i v e biomass of each i n s t a r . (2) From tables showing the r e l a t i o n between t o t a l body length and several other measurements (includ-ing body width and hind leg length) (Matsuda 1961a and b) I estimated the body volume of each i n s t a r . Figure 5 shows the p l o t of average t o t a l body volume per treatment; the p l o t of t o t a l hind leg length was very s i m i l a r . 20 Figure 2. Mean numbers i n cohort (nymphs entering the population) i n the s i x basic treatments (n=4) . Lines represent standard e r r o r . U = unfed, F = fed treatments. Figure 3. Changes i n the t o t a l number of nymphs of a l l i n s t a r s f o r the s i x basic treatments. Each point i s the mean of four r e p l i c a t e s . O Low density A Medium density ° High density — Fed — U n f e d 21 Figure 4. Deaths of f i r s t and second i n s t a r nymphs expressed as number dying i n a c e r t a i n week divided by the t o t a l number of older nymphs a l i v e at the beginning of the week. The greatest changes i n numbers of nymphs occurred during weeks three to s ix when numbers began to f a l l . Each point i s the mean of four r e p l i c a t e s . o Low density Fed A Medium density Unfed a High density Figure 5 . Estimated body volume of a l l nymphs and immature adults i n the s i x basic treatments (n=4). The t o t a l numbers began to f a l l i n the t h i r d week. o Low density Fed A Medium density Unfed n High density 22 In unfed pens (medium and high^density) body volume remained at a low l e v e l u n t i l the n inth week when i t i n -creased as fourth and f i f t h i n s t a r s became a d u l t s . The low density unfed pen increased markedly i n volume even a f ter numbers began to f a l l . A l l three fed pens increased even more r a p i d l y i n volume even a f t er t o t a l numbers began to f a l l , u n t i l the s i x t h week i n low and medium dens i ty , and n inth week i n high densi ty pens, and then l e v e l l e d o f f . Table 5 shows the mean number of nymphs becoming adults ("production") for the s ix treatments. Within feeding treatments, the f i n a l dens i t i e s were not s i g n i f i c a n t l y d i f f e r e n t . Between feeding treatments, numbers i n fed pens converged to a s i g n i f i c a n t l y higher l e v e l than that i n unfed pens. Table 6 shows that i n t h i s experiment, production i n both fed and unfed pens was unaffected by s u r v i v a l of parents . However, i n fed pens where at l eas t one female • survived to lay a second batch of eggs, production was s l i g h t l y h igher . In unfed pens t h i s had no e f f ec t on pro-duction . Ef fec t s of Treatments on Expectat ion of S u r v i v a l Table 7 shows that expectation of s u r v i v a l i s higher with added food, and lower with increase i n i n i t i a l densi ty of parents . Highest expectations occurred i n fed pens of low and medium dens i ty . Animals i n the low densi ty unfed pens had s i g n i f i c a n t l y bet ter expectations than those i n the high densi ty fed pens. 23 TABLE V. Average number of nymphs becoming adults ("production") i n each basic treatment. n=4 Food Treatments Fed (tS.E.) • Unfed (tS.E.) Density Low 12.25 ± 4.6 4.25 ± 1.1 Medium 7.5 t.86 2.5 t.29 High 16.25 i5.19 3.75±1.1 Analysis of Variance Source df Mean square Total Density Food Interaction Remainder 23 2 1 2 18 41.7 51.5 433.5 28.5 20.3 2.536 21.35 1.40 F2,18 = 6 ' 0 1 at p = .01 TABLE VI. Average number of nymphs becoming adults depending on sur v i v a l of parents and laying of a second batch of eggs. Sample sizes of each treatment are i n parentheses Food Treatments Fed Unfed Parents (3) 14*10.58 (5) 2.6 ±.548 present t=1.076 t=1.338 . Parents (5) 7.4 £1.14 (3) 4.6 ±2.51 absent Second batch (6) 15.6+7.85 (3) 2.6 ±.570 l a i d t=2.168 t=1.141 Second batch (6) 8.3 ± 2.53 (5) 3.8±2.17 not l a i d t n c. 7df = 2.365 p l l d f = 2.201 25 TABLE VII. Weighted means of expectation of sur v i v a l of nymphs i n each basic treatment (n=4). Weighted variance i s i n parentheses. Units of expectation of survival are stadia. Food Treatments Density Low Medium High Fed 2.58 (.006) 1.66 (.003) 1.26 (.001) Unfed 1.54 (.0058) 0.91 (.001) 0.71 (.0003) Analysis of Variance Source df Mean square F Total 23 0.5295 Density 2 2.8433 26.49 Food 1 4.1967 39.1 Interaction 2 0 .1821 . 1.697 Remainder 18 0.1073 at p=.01 2.6 In addition to the food/density treatments, another variable was unexpectedly introduced into the experiment. In some pens parents died immediately a f t e r laying the f i r s t batch of eggs, i n others at least one female survived and l a i d a second batch of eggs, and i n others they survived but l a i d no more eggs. I decided to see i f either the presence of parents, or the i n c l u s i o n i n the cohort of nymphs hatched into an environment of older nymphs, affected expectation of further l i f e between food treatments. Because these data were non-orthogonal ( i . e . straight means of parents present/absent were mixed up with dens-i t y e f f e c t s ) , I took differences between weighted means of expectations i n presence and absence of parents, sep-arately for each density, and then took a weighted average so that e f f e c t s of density on expectation would not a f f e c t the r e s u l t . Table 8 shows the re s u l t s of t h i s analysis under both food treatments. In unfed pens i n which parents died immediately aft e r laying eggs, expectation of s u r v i v a l was better than i n unfed pens i n which parents survived well. S i m i l a r l y where a second batch of eggs was not l a i d the expectation for the whole cohort was higher. In fed pens there was no difference i n expectation with either presence or absence of parents or a second batch of nymphs. Figure 6 shows mean survivorship curves for each of the six basic treatments. From these i t appears that the difference i n the intercepts of four of the six curves i s l a r g e l y caused by the differences i n slope (survival) 27 TABLE V I I I . Expectation of s u r v i v a l of nymphs depending on s u r v i v a l of parents and lay ing of a second batch of eggs. Units of expectation of s u r v i v a l are s t a d i a . Treatment (Sample s ize Mean.difference i n expect-i s i n parentheses) a t ion taken between density treatments (variance) oodf Unfed Parents absent (3) minus parents present (5) 0.23 (.0059) 2.99 Second batch not l a i d (5) minus second batch l a i d (3) 0.604 (.0046) 8.80 Fed Parents absent (5) minus parents present (3) 0.036 (.011) 0.34 Second batch not l a i d (6) minus second batch l a i d (6) 0.016 (.004) 0.25 t n_ oodf =1.960 p=. 05 28 I 2 3 4 5 A STADIUM Figure 6. Mean survivorship curves of nymphs for the six basic treatments (n=4). o Low density A Medium density o High density Fed Unfed 29 TABLE IX. Analysis of variance of regressions of survivorship curves of nymphs (Figure 6) for six basic treatments (n=4). Curves are compared from second stadium onwards, Treatment Log slope Log intercept (iS.E.) (+S.E.) Unfed Low density -.5289 + .0564 .3331 ±.2392 Medium density -.7187 + .0564 .0641 ± .2392 High density -.5511 ±.0564 1.2663 ±.2392 Fed Low density -.4150 ± .0564 .6825 ± .2392 Medium density -.4965 ±.0564 .3737 ±.2392 High density -.4439 ±.0564 .2642 ±.2392 Total -.5257 ± .0230 .0342 ±.0977 Analysis of Variance Source Pooled regression (within blocks) Regression between blocks Curvature Blocks adjusted regression Remainder df 5 1 5 107 Mean square 66.320541 0.461489 0. 066588 16.702817 0.127163 3.629 0.523 131.35 F5,107 3 , 3 4 at p=.01 30 4 6 8 10 FOOD ^act-io DENSITY Figure 7. S u r v i v a l of a d u l t s . There i s no d i f ference i n s u r v i v a l between males and females or between fed and unfed treatments. The expectation of s u r v i v a l under the three dens i t i e s (Table 9) are s i g n i f i c a n t l y d i f f e r e n t . (p=.05) Sex: Males Food: Fed Density: Low Females Unfed Medium High 31 TABLE X. Expectation of s u r v i v a l of parents under the three density-treatments. Numbers of adults i n each density treatment are i n parentheses. Units of expectation of s u r v i v a l are weeks. I n i t i a l density of Expectation ( + S .E . ) t oodf adults Low (16) 4.812 ± . 6 9 9 1.269 Medium (48) 3.812 ± . 3 6 1 2.807 High (144) 2.646 ± . 2 0 5 = 1.96 32' during the f i r s t time i n t e r v a l (hatching to f i r s t molt). In addition, i t appears that the three fed treatments and the unfed low density treatment had less steep slopes (better survival) a f t e r the f i r s t molt than the unfed medium and high density treatments, although t h i s may not be b i o l o g i c a l l y s i g n i f i c a n t . Table 9 shows an analysis of variance of the six regressions from the beginning of the second i n s t a r . There i s a s i g n i f i c a n t difference between both slope and intercept of the six curves. Most of the variance i s i n intercept, however, and i s caused by the difference i n slope during the f i r s t i n s t a r . Figure 7 shows that there was no difference i n sur-v i v a l of parents between fed and unfed pens, or between sexes. However, expectation of survival decreased as i n i t i a l density increased (Table 10). DISCUSSION The general aim of t h i s study was to f i n d out what happens to caged population which i s unlimited by food and subject to few environmental hazards. More s p e c i f i c -a l l y , I wanted to f i n d out whether some sort of behaviour would act i n proportion to density to lower survival of immature or mature insects. I interpreted my r e s u l t s i n the l i g h t of unquantified observations of interactions amongst gerrids i n both f i e l d and laboratory, which indicate that cannibalism i s a common occurrence, even where food i s abundant. The g e r r i d s 1 opportunis t i c method of feeding means that they unse lec t ive ly attack any helpless so f t -bodied insect which they encounter. Older nymphs can e a s i l y subdue younger ones, and nymphs of any age are helpless while mol t ing . I t i s obvious then, that as density r i s e s , the frequency of encounters between hungry and helpless conspeci f ics also r i s e s . I assume that food was not l i m i t i n g i n the fed pens because biomass continued to increase when numbers l e v e l l e d out (Figure 5) , whereas th i s d id not occur to the same extent i n the unfed pens. However, the pens containing the low density unfed treatments showed a s i m i l a r trend , which may indicate that i n these pens the food per i n d i v -i d u a l exceeded maintenance requirements. E i t h e r cann iba l -ism o:r a higher morta l i ty amongst younger nymphs, or both, could account for t h i s phenomenon. The re su l t s ind icate that mor ta l i t y var ied with the s u r v i v a l rate of parents and density of nymphs. Ef fec t s of Surv iva l of Parents The e f fects of parental s u r v i v a l on expectation of l i f e and production were d i f f e r e n t i n the two food t r e a t -ments. Where food was short , production was unaffected by parenta l s u r v i v a l or a second batch of nymphs (Table 6 ) , i n d i c a t i n g that food was the main l i m i t i n g f a c t o r . How-ever, s u r v i v a l of the whole cohort was poorer i f at l eas t one female l a i d a second c lu tch than i f a l l adults died soon af ter l ay ing the f i r s t batch of eggs ( T a b l e 8 ) . This poor su r v i v a l was due to the fact that almost none of the second batch of nymphs survived; t h i s agrees with Brinkhurst's (1966) observation that "as the parent generation increases i n abundance, so does the abundance of the f i r s t crop of nymphs. Survival of the second crop of nymphs i s inversely dependent on the abundance of the f i r s t . " Where food was i n excess, survival of nymphs was unaffected by s u r v i v a l of parents (Table 8), although the laying of a second batch of eggs seemed to increase production s l i g h t l y (Table 6). Apparently food abundance reduced cannibalism of older nymphs on younger ones to a s l i g h t degree. Thus where food was unlimited there were two opposing forces. In low density pens parents were more l i k e l y to survive (Figure 7),.and t h i s lowered expectation of survival of nymphs (Table 8); but i f at least one female l a i d a second batch of eggs production (Table 6) was increased. In high density pens, parents were less l i k e l y to survive and lay a second batch of eggs. This meant both a smaller cohort and lower prod-cti o n than i n low density pens, i n spite of better exp-ectation of s u r v i v a l of nymphs. Thus survival of parents seems to influence autumn numbers i n two c o n f l i c t i n g ways—the decrease i n expectation caused by parental cannibalism i s o f f s e t by the increase i n the size of the cohort, so that production i s increased very s l i g h t l y . E f f e c t s of Density of Nymphs The e f f e c t s of density on survival of parents, and, in turn, on sur v i v a l of nymphs, are complex, and probably not as important as interactions among developing nymphs. Under natural conditions food was probably i n short supply for the gerrid population, at least for part of the season. Conditions i n the medium-density unfed pens probably corr-esponded to those i n the natural population, and data from the unfed pens indicate that the main mortality factor producing observed f a l l densities was starvation. In these pens food available to each i n d i v i d u a l was inversely proportional to density, so that changes i n nymph survival were probably due to changes i n numbers of nymphs starving. From a l l three i n i t i a l densities production was the same (Table 5 ) because of the inverse c o r r e l a t i o n between density and expectation of sur v i v a l (Table 7 ) . Better s u r v i v a l at low densities was apparently due to reduced competition for food.. Poorer s u r v i v a l at high densities could be attributed to starvation as well as to increased cannibalism. As food supply diminishes i n r e l a t i o n to numbers of nymphs, the chances that a hungry nymph w i l l meet a susceptible nymph increase. In the fed pens production to f a 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 among the three i n i t i a l d e nsities. The inverse r e l a t i o n s h i p between expectation of sur v i v a l and i n i t i a l density, even though food was not l i m i t i n g , indicates that mortality was caused by an i n t r a s p e c i f i c i n t e r a c t i o n other than competition for food. The main ef f e c t s of density on survival were probably due to cannibalism amongst the nymphs. The survivorship curves (Figure 6) show that d i f f e r e n t i a l mortality of nymphs occurs during the f i r s t i n s t a r . The highest rates of loss are i n the unfed pens, the lowest i n the fed pens. In both food treatments su r v i v a l rates during the f i r s t stadium are inversely proportional to density. The only exception i s that the low density unfed treatment had better s u r v i v a l during the f i r s t stadium than the high density fed treatment, i n d i c a t i n g that food was probably not c r i t i c a l i n the former, and that density was so high i n the l a t t e r that nymphs were encountering each other more frequently than food. In the fed pens, cannibalism was probably d i r e c t l y proportional to density. However i n natural populations t h i s simple re l a t i o n s h i p i s probably somewhat modified. Cannibalism may be expected to increase at a constant rate i n proportion to density u n t i l food becomes l i m i t i n g , and then i t s rate of increase with density w i l l increase. This w i l l be due both to increased numbers of susceptible (starving) nymphs and to increased density of nymphs i n r e l a t i o n to food. Istock (1966) came to a similar conclusion, that, i n the laboratory, l a r v a l cannibalism of whirlygig beetles (Gyrinidae) i s geared to food supply and increases as food becomes scarce. He suggested that such a mechanism i s adaptive i n that i t f a c i l i t a t e s the e f f i c i e n t e x p l o i t -a t ion of a f l u c t u a t i n g food supply. "If the food supply remains constant or decreases, cannibalism i n t e n s i f i e s . If the food supply suddenly increases , cannibalism decl ines as the increas ing i n d i v i d u a l food requirements of the growing larvae are s a t i s f i e d by the expanded food supply." Thus i n G e r r i s , cannibalism may be a form of behaviour which can prevent unl imited increase i n population dens i ty . Perhaps the tendency to cannibal ize var ies with the cond-i t i o n s under which the nymphs are r a i s e d . I t would be i n t e r e s t i n g to quantify the aggressive behaviour of nymphs and adults ra i sed under d i f f e r e n t condit ions of crowding and competition for food. C h i t t y ' s hypothesis pred ic t s that i n d i v i d u a l s taken from expanding (low density) populations w i l l be less aggressive (or have less tendency to cannibal ize) and be more v iab le than those taken from s tat ionary or d e c l i n i n g (high density) populat ions . Differences i n v i a b i l i t y of g e r r i d eggs are e a s i l y quant i f i ed because they are e a s i l y v i s i b l e and the stage at which development stopped can be t o l d at a glance by the. colour of the egg. Differences i n aggressive behaviour and cannibalism should also be quant i f iab le because the insects are conspicuous, and because sucked out c u t i c l e s of cannibal ized nymphs can be c o l l e c t e d and counted. T e r r i t o r i a l behaviour i s another poss ib le mechanism of r e g u l a t i o n , at l east i n the non-gregarious species 38 such as G. n o t a b i l i s . My observations suggest that adult males have defended areas (immediately surrounding a resti n g place) and that females are free to move anywhere. I think that the aggressiveness of the male (and therefore the size of h i s ' t e r r i t o r y ) may depend upon time elapsed since mating or time since feeding, or both. It would be i n t e r e s t i n g to determine the eff e c t s of these two variables (which are probably to some extent dependent on density) on a c t i v i t y , existence of t e r r i t o r i a l i t y , extent of t e r r i t o r y , reactions to strange males, and surv i v a l of nymphs. Another aspect of regulation of density i n gerrids which I have not discussed i s emigration of adults. In G. n o t a b i l i s a l l adults are f u l l y winged and able to f l y — perhaps crowding stimulates f l i g h t , both i n mature adults s e t t l i n g i n a breeding area i n the spring and i n immature adults entering the population i n summer and f a l l . A l -though I terminated my experiments when a l l nymphs became adults, I noticed that immature adults disappeared rather quickly from some pens, es p e c i a l l y the high food pens where f a l l densities were higher. By determining the important s t i m u l i causing f l i g h t i n these insects some additional l i g h t might be thrown on population regulating mechanisms. CONCLUSIONS The greatest mortality to the natural population of Gerris n o t a b i l i s i n Marion Lake was probably caused by storms and food shortage. In the experimental pens abundant shelter rendered the e f f e c t s of storms less important, and i n the fed pens food was probably not l i m i t i n g . Where extra food was provided numbers of nymphs were higher than where i t was not, but owing to the increased frequency of i n t r a s p e c i f i c contacts at high densities, cannibalism acted to reduce the numbers of nymphs. Where food was i n short supply, starvation and concomitant cannibalism were appar-ently higher where i n i t i a l density of parents (and therefore of nymphs) was higher. Brinkhurst (1966) thought that cannibalism by the f i r s t crop of nymphs on the second crop was an important "density-dependent" cause of mortality; although he had no experimental data he found that when the entire f i r s t crop was accidentally k i l l e d , s u r v i v a l of the second crop was very high. Thus my re s u l t s support his conclusions. However, my study does not cover a complete year, and a l l I can do i s recognize some of the processes that a f f e c t numbers: I cannot say what regulates them. It i s possible that the high production correlated with abund-ant food i s o f f s e t by greater mortality the following spring. At any rate, i t appears that cannibalism i s a form of behav-iour which can play some part i n the regulation of insect numbers. In addition, natural populations of gerrids are 40 subject to a fl u c t u a t i n g food supply, and t h i s , combined with t h e i r opportunistic method of feeding, makes cannib-alism an adaptive form of behaviour, which has the further consequence of l i m i t i n g the abundance of the population i n summer * 41 REFERENCES Brinkhurst, R.O. 1959. A description of the nymphs of B r i t i s h Gerris species. Proc. R. ent. Soc. Lond. (A) 34: 130-136. Brinkhurst, R.O. 1966. Population dynamics of the large pond-skater Gerris najas Degeer (Hemiptera-Heteroptera). J. Anim. Ecol. 35: 13-25. Chitty, D. 1964. Animal numbers and behaviour. In Fish and W i l d l i f e : A memorial to W.J.K. Harkness. 41-53. Chitty, D. 1967. Natural selection of self-regulatory behaviour. Proc. ec o l . Soc. Aust. 2: 51-78. Efford, I.E. 1967. Temporal and s p a t i a l differences i n phytoplankton productivity i n Marion Lake, B.C. J. F i s h . Res. Bd. Canada 24: 2283-2307. Eisenberg, R.M. 1966. The regulation of density i n a natural population of the pond s n a i l , Lymnaea elodes. Ecology 47: 889-905. Istock, C A . 1966. D i s t r i b u t i o n , coexistence, and comp-e t i t i o n of whirlygig beetles. Evolution 20: 211-234. L e s l i e , P.H., J.S. Tener, M. Vizoso, and H. Chitty. 1955. The longevity and f e r t i l i t y of the Orkney vole, Microtus orcadensis, as observed i n the laboratory. Proc. zool. Soc. Lond. 125: 115-125. Matsuda, R. 1961. Studies i n r e l a t i v e growth i n Gerridae (Hemiptera-Heteroptera). J . Kansas ent. Soc. 34: 5-17. Matsuda, R. 1961. Studies of r e l a t i v e growth i n Gerridae (Hemiptera-Heteroptera). Ann. ent. Soc. Amer. 54: 578-598. 42 APPENDIX A KEY TO GERRIS NYMPHS FOUND IN MARION LAKE To Instar 1. Pronotum more than one-third length of mesonotum. 1st instar Pronotum less than one-third length of mesonotum...2 2. Hind margin of mesonotum straight 2nd inst a r Hind margin of mesonotum concave . 3 3. Hind wing buds do not extend onto tergum 2. 3rd instar Hind wing buds do extend onto tergum 2 4 4. Tarsus of hind leg more than one-quarter length of femur 4 th 3 i n s t a r Tarsus of hind leg less than one-quarter length of femur 5th instar To Species F i r s t Instar ( n o t a b i l i s , buenoi, incurvatus) 1. Abdominal terga 3 to 7 without paired dark markings G. n o t a b i l i s Abdominal terga with paired dark markings 2 2. Hind leg less than 1.4 mm long G. buenoi Hind leg more than 1.4 mm long . G. incurvatus Second Instar ( n o t a b i l i s , buenoi, incurvatus) 1. Mesonotum with l i g h t and dark longitudinal s t r i p e s ; hind leg more than 3 mm long....G. n o t a b i l i s 43 Mesonotum without dark l o n g i t u d i n a l s t r i p e s ; hind leg less than 2.5 mm long 2 2. Body colour dark; mesonotum with paired round l i g h t markings enclos ing a dark s p o t . . . . . G . incurvatus Body colour l i g h t ; mesonotum with paired dark spots G. buenoi Th ird Instar ( n o t a b i l i s , buenoi, incurvatus) 1. General body colour pale with dark s t r ipes running length of body G. n o t a b i l i s Body colour dark, mesonotum with paired l i g h t patches enclos ing a dark spot. 2 2. Hind leg less than 3 mm G. buenoi Hind leg more than 3.5 mm G. incurvatus Fourth Instar ( n o t a b i l i s , buenoi, incurvatus , remiges) 1. General body colour pale with dark s tr ipes running length of body; hind leg more than 8 .5 mm long G. n o t a b i l i s Body colour dark . .2 2. Hind leg more than 8.5 mm long G. remiges Hind leg less than 6.0 mm long 3 3. Mesonotum with large paired pale patches containing t r i a n g u l a r dark patch; hind leg more than 5 mm long. G. incurvatus Mesonotum with pale arrowshaped markings; hind - . leg less than 5 mm l o n g . . . G. buenoi f th Instar ( n o t a b i l i s , buenoi, incurvatus , remiges) Hind leg more than 15 mm long; abdominal terga with no markings G. n o t a b i l i s At l eas t some abdominal terga with pale markings. . . . 2 Hind leg more than 11 mm long; paired "hook-shaped" markings on mesothorax G. remiges Hind leg less than 10 mm long; mesothoracic markings not hook-shaped 3 Hind leg less than 7 mm long; mesonotum with "arrowshaped" markings G. buenoi Hind leg more than 7 mm long G. incurvatus 

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