Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Observations on a natural population of damselfly larvae. Pearlstone, Paul S.M. 1971

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1971_A6_7 P43.pdf [ 4.06MB ]
Metadata
JSON: 831-1.0101877.json
JSON-LD: 831-1.0101877-ld.json
RDF/XML (Pretty): 831-1.0101877-rdf.xml
RDF/JSON: 831-1.0101877-rdf.json
Turtle: 831-1.0101877-turtle.txt
N-Triples: 831-1.0101877-rdf-ntriples.txt
Original Record: 831-1.0101877-source.json
Full Text
831-1.0101877-fulltext.txt
Citation
831-1.0101877.ris

Full Text

OBSERVATIONS ON A NATURAL POPULATION OF DAMSELFLY LARVAE by PAUL S. M. PEARLSTONE B.Sc. The Unive r s i t y of Toronto, 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 t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA June, 1971 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of Brit ish Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of ^S^o/ c-g/^ The University of Brit ish Columbia Vancouver 8, Canada Date ^ <> 3 -7/1 I ABSTRACT A population of weed-dwelling damselfly larvae (Enallagma boreale) i n Marion Lake, B r i t i s h Columbia was sampled at regular i n t e r v a l s from June 30, 1961 to Ju l y 28, 1970 f o r the purpose of obtaining information regarding l i f e c ycle and feeding habits. The larvae sampled were measured, and t h e i r gut contents were determined. The Marion Lake population was found to consist of two overlapping, u n i v o l t i n e generations whose cycles d i f f e r e d by about two months. Adults have a long f l i g h t period i n the summer, and larvae overwinter i n mid-larval i n s t a r s . The food of the larvae consisted mainly of Cladocera and l a r v a l Chironomidae, although many types of prey were eaten. Cannibalism was not prevalent i n the population. The amount and type of food eaten v a r i e d both throughout the year and within d i f f e r e n t habitats i n the lake. It appears that the larvae feed predominantly during the daylight hours. v TABLE OF CONTENTS Page ABSTRACT v INTRODUCTION 1 THE SITE 3 TYPES AND DISTRIBUTION OF ODONATE LARVAE 4 SAMPLING METHODS 5 LIFE HISTORY 7 Results 7 L i f e cycle of E. boreale i n Marion Lake 7 Growth of the larvae 8 Discussion . 14 THE FOOD OF THE LARVAE 16 Methods of Analysis and Tabulation 16 Preparation and i d e n t i f i c a t i o n of prey remains . 17 Tabulation of food types 21 Results . 23 Laboratory observations 23 Food of the Marion Lake population 25 Diurnal feeding cycle 31 Discussion 34 SUMMARY 39 APPENDIX 41 LITERATURE CITED 51 i i LIST OF FIGURES Figure Page 1 Frequency histogram of head widths of E. boreale larvae from Marion Lake 9 2 Mean head widths (with 95% confidence i n t e r v a l s ) of samples of E. boreale i n Marion Lake over time . . . 12 3 Emergence of E. boreale i n Marion Lake 15 4 Examples of recognizable chitinous remains of prey organisms i n f a e c a l p e l l e t s examined 20 5 Means (with 95% confidence i n t e r v a l s ) of the t o t a l prey per E. boreale l a r v a i n samples over time i n Marion Lake 29 6 Mean number of cladocerans per l a r v a sampled over time 30 7 Mean number of prey per l a r v a f o r each of the desig-nated i n s t a r groups 32 8 Mean numbers of cladocerans per l a r v a sampled i n three habitat types 33 9 Indications of changing feeding patterns throughout the day by E. boreale larvae i n Marion Lake . . . . 36 i i i TABLES Table Page 1 Comparison of c a l c u l a t e d and observed head widths of Marion Lake damselfly larvae 11 2 A comparison of the "frequency of occurrence" with the "number" method of tab u l a t i n g the importance of prey items i n the guts of Marion Lake damselfly larvae 23 3 Food of damselfly larvae (Enallagma boreale) i n Marion Lake, B r i t i s h Columbia 27 i v - 1 -INTRODUCTION The Odonata are among the most ancient orders of i n s e c t s . F o s s i l remains i n d i c a t e that ancestors of modern dragonflies were present i n the Permian era, some 220 m i l l i o n years ago (Corbet, Longfield, and Moore, 1960). Through the course of time they have adapted to freshwater areas the world over, and have invaded climates ranging from t r o p i c a l through temperate to sub-arctic. Adult odonates, perhaps because of t h e i r b r i l l i a n t colours and powerful f l i g h t , have often been the subject of artwork, prose, and considerable s c i e n t i f i c i n v e s t i g a t i o n . The unique and f a s c i n a t i n g l a r v a l form, however, has not been well studied, e s p e c i a l l y i n the f i e l d . Since l a r v a l odonates are more numerous, generally longer l i v e d , and probably equally voracious i n t h e i r predatory habits as t h e i r a e r i a l adults, the impact of these larvae must be considerable i n many freshwater communities. Background Studies The basis f o r modern study of Odonata l i e s i n T i l l y a r d ' s c l a s s i c t r e a t i s e of 1917 which outl i n e s form, function, and habits of both adult and l a r v a l stages. Recently, Corbet (1962) has added to t h i s base with work which t i e s together many i s o l a t e d studies. As w e l l , Snodgrass (1954) has conducted a d e t a i l e d morphological examination of the odonate l a r v a , and - 2 -gives a c l e a r explanation of l a r v a l structures and t h e i r functions. F i e l d studies of odonate larvae are l i m i t e d although the l i f e h i s t o r i e s of many species have been published (see f o r example Corbet, 1957 or Macan, 1964). A number of authors have investigated food preferences of larvae; these studies w i l l be discussed l a t e r . The most thorough study to date of a f i e l d population of damselfly larvae i s the work of Lawton (1969, 1970a, b). The Present Study The work presented i n t h i s paper i s part of an integrated study of a freshwater community. Since 1962, an extended group of s c i e n t i s t s and students, under the d i r e c t i o n of Dr. Ian E f f o r d , Department of Zoology, Un i v e r s i t y of B r i t i s h Columbia, has been involved i n a study of Marion Lake, B r i t i s h Columbia. The work i s being c a r r i e d out as part of the Canadian co n t r i b u t i o n to the International B i o l o g i c a l Program. The major emphasis of the Marion Lake group has been to define and study the major energy pathways among organisms i n the lake (E f f o r d , 1969). When I joined the Marion Lake Project i n 1968, studies had been c a r r i e d out, or were i n progress, which delved i n t o aspects of the benthic organisms (Hargrave, 1969, 1970a, b, 1971; Hamilton, 1965; Neish, 1970), phytoplankton (Ef f o r d , 1967; Dickman, 1969; McQueen, 1969, 1970), and f i s h (Sandercock, 1969; Ware, 1971) i n the lake. Some knowledge had been obtained of the macrophytes (Davies, 1970) but v i r t u a l l y nothing was known of the - 3 -animals which inhabited the weedy areas. Preliminary observations l e d to the conclusion that, of the weed-dwelling animals, Odonata larvae ( e s p e c i a l l y one species, Enallagma boreale Selys) were a major component. This, along with the f a c t that not a great deal of work has been done on l a r v a l odonates, was the basis f o r the i n i t i a t i o n of t h i s study. The major emphasis throughout the work has been on aspects of the l i f e h i s t o r y and feeding of the larvae. Since t h i s work has been undertaken, some observations have been made of l a r v a l Trichoptera i n Marion Lake (Winterbourn, i n press). Other weed-dwelling animals ( l a r v a l Ephemeroptera and Plecoptera, as well as adult Hemiptera seem p a r t i c u l a r l y noteworthy) are p r a c t i c a l l y unknown. THE SITE The study s i t e was Marion Lake, a small c o a s t a l lake i n south-western B r i t i s h Columbia. The lake e l e v a t i o n i s approxi-mately 1000 feet (305 m) above sea l e v e l . Marion Lake l i e s i n a narrow, steep-sided v a l l e y . The lake i t s e l f i s narrow (approximately 800m by 200m) and shallow (maximum depth, 6m; average depth, 2.4m). The climate i s mild and wet (average annual r a i n f a l l , 240 cm) and the lake i s prone to seasonal f l o o d i n g . For a d e t a i l e d discussion of the g e o l o g i c a l , p h y s i c a l , and chemical features of the area, r e f e r to E f f o r d (1967). - 4 -TYPES AND DISTRIBUTION OF ODONATE LARVAE There are s i x known species of Odonata i n Marion Lake. These include the dragonflies (Anisoptera) Plathemis l y d i a Drury, Leucorrhina g l a c i a l i s Hagen, and Aeshna i n t e r r u p t a Walker and the damselflies (Zygoptera) Enallagma boreale Selys, Ischnura cervula Selys, and Lestes dryas Kirby. (The i d e n t i -f i c a t i o n of the l a t t e r three species was confirmed by Dr. P. S. Corbet to whom I am most g r a t e f u l . ) The f i r s t , two dragonflies mentioned have larvae which burrow i n the mud bottom. These were not seen i n my samples. The larvae of the other four species dwell predominantly among aquatic vegetation and could be captured with a sweep net. Enallagma Was by f a r the most common of the weed dwelling species. Very few larvae of Aeshna were seen. Of the 500 damselfly larvae examined during the course of the study, only 15 were i d e n t i f i e d as Ishnura and 6 as Lestes. Therefore, the remainder of t h i s paper w i l l concern i t s e l f only with the population of Enallagma. Much of the bottom of Marion Lake i s soft mud with sparse vegetation. In t h i s habitat Enallagma larvae were v i r t u a l l y absent. A bottom sampling program conducted by Mr. K. Tsumura which consisted of a s e r i e s of monthly Hargrave samples (Hargrave, 1969) taken between June 1968 and December 1969 pro-2 duced a t o t a l of only 51 larvae i n approximately 95 m of mud bottom ( t h i s can be compared with amphipods which number i n the order of 2000 per square metre i n the same samples — Mrs. A. Bryan, personal communication). - 5 -The larvae, therefore, are r e s t r i c t e d to areas of vegetation i n the lake. The majority of Enallagma larvae were c o l l e c t e d from sedge i n shallow water around the perimeter of the lake. Larvae were found here throughout the year. In the warmer months, larvae were also found i n patches of water l i l i e s (Nuphar variagatum), Potamogeton natans, and h o r s e t a i l s (Equisetum sp.). SAMPLING METHODS The larvae were sampled throughout the year f o r the purpose of obtaining information on t h e i r l i f e c y c l e , growth, and feeding. The t r i a l s and t r i b u l a t i o n s of achieving a quantitative sample of odonate larvae are well described by Macan (1964). He demonstrates that a sweep net i s by no means an unselective sampling method. More quantitative samplers (one.is described i n the same paper) have the disadvantage of not y i e l d i n g many animals and r e q u i r i n g much s o r t i n g time. Sweep nets must be c a l i b r a t e d , and the c a l i b r a t i o n w i l l change with vegetation type, l a r v a l s i z e , and l i k e l y , time of year. Macan's e f f o r t s i n Hodson's Tarn (surface area l e s s than 0.5 ha) and Lawton's l a t e r work (1969, 1970a) i n a small pond (surface area 93.1 nr1) l e d me to believe that a quantitative sampling program i n Marion Lake, which i s much larger (13.3 ha) than these other areas, and contains a greater v a r i e t y of raacro-phytes, would involve much time and was doomed to y i e l d r e s u l t s of questionable accuracy. I therefore chose to concentrate on^ - 6 -aspects of the l i f e h i s t o r y and food preferences of the larvae, rather than on areas which would require quantitative samples. C o l l e c t i o n of the larvae was c a r r i e d out with a c i r c u l a r sweep net, approximately 30 cm i n diameter, having a shallow f i b r e glass screen mesh with about 5 meshes to 1 cm. The net was sturdy enough to be swept through dense vegetation. This sampling method captured mainly the larger s i z e classes of larvae. At the outset of the program, I was mainly concerned with the food of the population and f e l t that the smaller larvae would not contribute greatly since they are present only f o r a short time during the year. Later, when my sampling began to y i e l d i n t e r e s t i n g r e s u l t s concerning the development and l i f e h i s t o r y of the population, I regretted the lack of information on the smaller i n s t a r s . However, I did not change the sampling methods.as I wished the r e s u l t s to be comparable over time. Because a shallow dip net was used, larvae could be c o l l e c t e d and i s o l a t e d immediately upon being removed from the water. This was important i n order to insure that the e n t i r e gut contents of each l a r v a was obtained. The larvae were held i n i n d i v i d u a l dishes of clean lake water and brought to the laboratory where each was measured and held u n t i l i t had emptied i t s gut (generally two to three days) before being returned to the lake. During the summer, l a s t i n s t a r larvae were provided with a twig as an access route to emerge. Those that d i d so were released. Faecal p e l l e t s were stored i n alcohol f o r l a t e r a n a l y s i s . - 7 -The date, time of day, and l o c a t i o n of capture i n the lake were noted f o r each animal. In the l a t e r part of the study, the type of vegetation i n which the l a r v a had been caught was also noted. Sampling was c a r r i e d out at i n t e r v a l s of approximately two weeks between June 30, 1969 and J u l y 28, 1970. LIFE HISTORY Results L i f e Cycle of E. boreale i n Marion Lake According to Whitehouse (1941), E. boreale i s on the wing longer than any other odonate i n B r i t i s h Columbia. In Marion Lake, i t indeed has a long f l i g h t period with f i r s t emergence i n 1970 being observed on May 5 and f l i g h t continuing u n t i l mid-September. The l i f e cycle i s completed i n one year; eggs are l a i d i n the summer i n stems of Nuphar plants (and occas i o n a l l y i n others) and the young larvae hatch out several weeks l a t e r . The larvae grow u n t i l l a t e f a l l , then overwinter i n mid-larval i n s t a r s . Growth resumes the following spring and the larvae emerge i n the summer. E. boreale would therefore be c l a s s i f i e d with regard to i t s l i f e c ycle as a "summer species" i n Corbet's d e f i n i t i o n (1959). There i s evidence from my sampling data that the popula-t i o n of E. boreale i n Marion Lake possesses two overlapping generations. This w i l l be discussed l a t e r . - 8 -Growth of the Larvae a) Instar c l a s s i f i c a t i o n . Since a large number of larvae had been c o l l e c t e d , i t was expected that a plo t of head width against number of larvae would exhi b i t d i s t i n c t peaks. It would thus be possible to designate the i n s t a r s i n the population. The peaks, however, were l e s s d i s t i n c t than had been expected (Figure 1). The v e r t i c a l l i n e s i n Figure 1 i n d i c a t e what appear to be the most l i k e l y i n s t a r d i v i s i o n s . These were a r r i v e d at by inspection of the f i g u r e by an un-biased observer. It has been known f o r some time that arthropod head widths increase from one i n s t a r to the next i n a regular geometric progression, a f a c t often r e f e r r e d to as Dyar's law (Dyar, 1890). The "growth f a c t o r " from one i n s t a r to the next should therefore be constant. The growth f a c t o r from one i n s t a r to the next i s the increase i n head width expressed as a f a c t o r of the smaller head width. Thus the growth f a c t o r (GF) can be defined as G F ( I - 1 to I) = HWI_ x where HWj = head width of i n s t a r I HWj_^= head width of i n s t a r preceding I. The average growth f a c t o r f o r the s i x possible i n s t a r s shown i n Figure 1 i s 1.24 (S.E. = 0.014). In addition, a method of numbering the i n s t a r s was devised. Mr. D. Proctor, who has hatched eggs of t h i s - 9 -Figure 1. Frequency histogram of head widths of E. boreale larvae from Marion Lake. Shown above i s the hypothesized d i v i s i o n of the population into i n s t a r s along with the "growth f a c t o r " from one i n s t a r to the next and the i n s t a r numbers (see Table 1 ) . 1.28 1.24 1.20 1.23 1.24 8 10 11 12 13 1.0 2.0 h e a d width in m m 3.0 4.0 - 10 -population i n the laboratory, informs me that the head width of the second i n s t a r i s 0.33 (the f i r s t i n s t a r , the s o - c a l l e d "prolarva" i s extremely short l i v e d and not usually seen). Taking t h i s as a s t a r t i n g point and using the obtained growth fact o r of 1.24, the expected mean head widths of successive i n s t a r s can be c a l c u l a t e d . These are shown i n the l e f t hand column of Table 1. The r i g h t hand column gives the mean head widths of the i n s t a r groups designated i n Figure 1. It appears, then, that the population has 13 i n s t a r s and the l a s t s i x of these have been sampled i n t h i s study. During the time that the larvae were held i n the laboratory, seven of them emerged into adults: two i n the month of May and f i v e i n Ju l y . The two larvae which emerged i n May had a mean head width of 3.78 mm (range: 3.76- 3.80 mm) while the f i v e which emerged i n Ju l y had a mean head width of 3.49 mm (range: 3.33- 3.68 mm). These values correspond roughly to the two peaks i n Figure 1 i n the range ascribed to i n s t a r 13. Several of the other i n s t a r ranges have two peaks as we l l . This i s the i n i t i a l evidence that there may be two generations of s l i g h t l y d i f f e r e n t s i z e s i n t h i s population. b) Larval growth i n the f i e l d . Although the sampling methods used i n t h i s study were not quantitative, they did sample a por t i o n of the population over time i n a consistent fashion. It i s possible, then, to use the data to observe the growth of the animals over time (Figure 2). Note the drop i n the average head width s i z e on Ju l y 19, 1969 and again on - 11 -Jul y 27, 1970. This i s the time of year when the new genera-t i o n enters the population being sampled ( i t i s not the time hatching since the samples do not include the small i n s t a r s ) . The growth of the population can then be observed by the increase i n average s i z e of the larvae with time. Instar Calculated Observed Number Head Width Head Width (mm) (mm) 2 0.33 3 0.41 4 0.51 5 0.63 6 0.78 7 0.98 8 1.21 1.23 9 1.50 1.58 10 1.86 1.96 11 2.31 2.36 12 2.86 2.90 13 3.56 3.60 Table 1. Comparison of ca l c u l a t e d and observed head widths of Marion Lake damselfly larvae. This system was used to obtain i n s t a r numbers f o r the s i x i n s t a r groups observed i n the study (see t e x t ) . - 12 -Figure 2. Mean head widths (with 95% confidence i n t e r v a l s ) of samples of E. boreale i n Marion Lake over time. The asterisked point i s a 1970 sample ins e r t e d f o r comparison (see t e x t ) . - 13 -One inconsistency i n the growth pattern i s apparent during the winter of 1969- 1970. (There i s also a gap i n the spacing of samples at t h i s time, since adverse weather made sampling most d i f f i c u l t . ) During t h i s period, the average s i z e of i n d i v i d u a l s i n the population appears to have decreased. There are a number of possible explanations f o r t h i s . F i r s t , i t may not be a decrease at a l l but simply a cessation of growth during the cold part of the winter. If there i s a drop i n s i z e , i t may be caused by a s e l e c t i v e d i e - o f f of larger larvae or perhaps by another generation of smaller larvae entering the sampled population at t h i s time. In order to obtain more information about the population during the winter, a large sample (65 larvae) was taken on December 13, 1970 during the period i n which the apparent drop i n s i z e had occurred the year before. This sample (see asterisked point i n Figure 2) had a very small mean head s i z e , comparable with the l a t e J u l y samples. The smaller average siz e was due not to a decline i n large larvae, but to a con-siderable number of small larvae appearing i n the sample (22% of the larvae were smaller than i n s t a r number 8 and larvae of i n s t a r s 5 and 6 were common). The suspicion therefore grows that the Marion Lake population of E. boreale has two generations. To check t h i s d i r e c t l y , I obtained emergence data of the population f o r the summers of 1969 and 1970 from Mr. D. Proctor (Figure 3). The small emergence i n l a t e May and early June of 1969 i s - 14 -undoubtedly due to poor weather conditions. The emergence was delayed u n t i l the following week. Taking t h i s into account, the pattern of emergence i s s i m i l a r f o r the two years. There i s an ea r l y emergence i n May - June tapering o f f i n l a t e June. Subsequently, a second emergence i s noted i n mid-July. Thus, a l l the data indicate that there are two genera-tions comprising the Marion Lake population of E. boreale. One emerges e a r l y i n the season i n May and June. The eggs l a i d hatch i n Ju l y and grow to spend the winter i n advanced l a r v a l i n s t a r s ( 8 - 1 3 ) . They continue growing i n the spring to emerge again i n May and June. The second generation emerges i n the l a t e summer i n Ju l y and the eggs hatch l a t e i n the season. These larvae enter the winter as junio r i n s t a r classes (mainly 5-6) and must grow through the following spring and summer to emerge i n July . Discussion The growth f a c t o r of 1.24 ca l c u l a t e d from the data i n Figure 1 compares favourably with published data f o r odonate larvae: Calvert (1934) obtained a value of 1.23 f o r an i n d i -v i d u a l of Anax Junius; Kormondy (1959) obtained 1.27, 1.27 and 1.26 f o r three dragonfly species; i n the only published data f o r damselflies, Lawton (1969) obtained a growth f a c t o r of 1.25 fo r a population of Pyrrhosoma nymphula Sulz. The i n s t a r c l a s s i f i c a t i o n presented therefore appears to be reasonable. - 15 -Figure 3. Emergence of E. boreale i n Marion Lake (unpublished data of D. Proctor). Each bar represents one week's accumulation of exuviae i n a s p e c i f i e d area of the lake. 140- n 100 'cn CD E CD u CD E 60 20 140 100 60 20 d m 1969 n P 1970 - 16 -This i s not the f i r s t observation of an odonate species with d i f f e r e n t l i f e cycles i n the same body of water. Macan (1964) found i n a population of Pyrrhosoma nymphula i n Hodson's tarn, England, that some i n d i v i d u a l s took two years to complete t h e i r l i f e c ycle while others required three. He hypothesized that the difference could be due to the type of vegetation on which the larvae were found, or the degree of crowding which the larvae experienced, or perhaps a combination of these f a c t o r s . My i n v e s t i g a t i o n s show no c o r r e l a t i o n between vege-t a t i o n type and the d i f f e r e n t generations of larvae i n the lake. It i s possible as well that the two generations may be two c l o s e l y r e l a t e d species. This can be tested by capturing adults from the e a r l y and l a t e emergences and examining them morphologically. THE FOOD OF THE LARVAE Methods of Analysis and Tabulation A s e r i e s of observations was c a r r i e d out on larvae feeding on l i v e prey i n the laboratory. Several conclusions were drawn about the a b i l i t y of the larvae to handle c e r t a i n prey types; however, i t became apparent that observations made i n an a r t i f i c i a l s e t t i n g could not reveal very much about the feeding of the lake population. More d i r e c t observations were required. Thus a considerable proportion of the e f f o r t involved i n t h i s study was devoted to obtaining an accurate sample of the food of the population. - 17 -Many authors have r e f e r r e d to the food of f i e l d populations of l a r v a l Odonata, but the majority of the studies are fragmentary — most cover only a portion of the year, and many are based on scattered samples containing few animals. A summary of the e x i s t i n g l i t e r a t u r e i s presented i n the appendix. The most comprehensive study of the food of a f i e l d population of odonate larvae to date i s the work of Lawton (1969, 1970b). The data presented here provide an i n t e r e s t i n g comparison to h i s . Preparation and I d e n t i f i c a t i o n of Prey Remains Faecal p e l l e t s of odonate larvae are enclosed i n a strong p e r i t r o p h i c membrane. They are therefore easy to handle and analyze without l o s i n g any of the contents. Dissection of the gut may permit i d e n t i f i c a t i o n of some undigested sof t bodied prey; however, t h i s method has several disadvantages. C o l l e c t i o n of the gut contents i s more d i f f i c u l t ; more time i s required; and the larvae must be k i l l e d . The f a e c a l p e l l e t s from each l a r v a c o l l e c t e d were stored i n a v i a l i n 100% ethanol. For analysis, the p e l l e t s were r e -moved from the alcohol and placed on a microscope s l i d e i n a drop of p o l y v i n y l lactophenol containing l i g n i n pink s t a i n . They were then teased apart under a binocular microscope (12X), covered with a cover s l i p , l a b e l l e d , and dried (either over-night i n a 60° C drying oven, or, i f t h i s were not a v a i l a b l e , f o r several days at room temperature). The s l i d e s could then be kept i n d e f i n i t e l y . - 18 -There are several d i s t i n c t advantages of t h i s method of analysis over others. The s l i d e s can be r e f e r r e d to again and again. This was found to be important, since as my a b i l i t y to recognize various pieces of prey improved, I re-analyzed many s l i d e s . D i f f i c u l t s l i d e s can be taken to experts f o r confirmation. Lignin pink i s a s e l e c t i v e s t a i n f o r c h i t i n , and since most of the animal remains i n the f a e c a l p e l l e t s are chitinous, the s t a i n f a c i l i t a t e s detection and recognition of parts; i t also allows an immediate dis c r i m i n a t i o n between animal and plant material. F i n a l l y , permanent s l i d e s are easier to analyze under high magnification. In order to discriminate small items, i t was necessary to analyze the f a e c a l material under a compound microscope at 100X with frequent checks of s p e c i f i c areas at 400X. Other studies, which have employed s i m i l a r analysis methods to t h i s one, but have used much lower magnifications, have probably overlooked a considerable number of prey items. The i d e n t i f i c a t i o n of the f a e c a l p e l l e t s was aided by the preparation of reference s l i d e s of i d e n t i f i e d organisms c o l l e c t e d among the weeds i n Marion Lake and prepared i n the same manner as the f a e c a l p e l l e t s . It was necessary to update t h i s f i l e of s l i d e s at d i f f e r e n t times of the year as new items appeared i n the f a e c a l p e l l e t s of larvae c o l l e c t e d . Methods of i d e n t i f i c a t i o n and counting of prey items are presented below. In addition, some of the i n d i c a t i v e chitinous remains f o r a number of prey types are shown (Figure 4). - 1 9 -a) Cladocerans. These were e a s i l y i d e n t i f i e d to species by the footpiece (postabdomen), which was unique f o r each species. Counting of footpieces also gave a t o t a l count of the number of each species, and t h i s could be roughly checked by observations of pieces of valves and thoracic legs. b) Chironomid larvae. These were i d e n t i f i e d by the head capsule, p a r t i c u l a r l y the labium and mandibles. It was impossible to i d e n t i f y species, but i n most cases the l a r v a could be c l a s s i f i e d (mainly by the p a r a l a b i a l plates) i n t o one of four groups: the subfamilies Tanypodinae and Ortho-cladinae, and the t r i b e s Chironomini and T a n y t a r s i n i of the subfamily Chironominae. The appearance of anal t u f t s and hooks gave an i n d i c a t i o n , although not quantitative, of the presence of chironomid larvae. c) Water mites. These could be i d e n t i f i e d by pieces of palps, chitinous appendages (sometimes with spines), and p a r t i c u l a r l y by the c h a r a c t e r i s t i c acetabular p l a t e s . In some species, the exoskeleton remained v i r t u a l l y i n t a c t . d) Oligochaetes. Annelid worms leave only one i d e n t i f i a b l e remain i n the f a e c a l p e l l e t . This i s the S-shaped chitinous chaeta which i s generally present i n considerable numbers. Although i t was not possible to determine the number of annelids i n the f a e c a l p e l l e t , the occurrence of annelid remains was rare enough that there was assumed to be only one. - 20 -Figure 4. Examples of recognizable chitinous remains of prey organisms i n the f a e c a l p e l l e t s examined, a. - d. Cladoceran postabdomens (foot p a r t s ) , a. Sida c r y s t a l l i n a Mtiller (10X). b. Alona  quadrangularis (10X). c. Eurycercus lamellatus (25X). d. Kurzia l a t i s s i m a (10X) with carapace and thoracic legs. e. Chironomid l a r v a (Orthocladinae) head capsule (10X). f . Centre, Tanypodinae chironomid l a r v a , head capsule (10X). g. Water mite palp (10X). h. Damselfly l a r v a , labium (10X). i . Piece of chitinous appendage of damselfly l a r v a containing " t r i d e n t spines" (10X). j . R o t i f e r , K e r a t e l l a cochlearis (25X). h - 21 -e) Mayflies and s t o n e f l i e s . There are a number of species of mayflies and s t o n e f l i e s i n the lake. Not enough i s known about them to permit t h e i r being i d e n t i f i e d to species from f a e c a l remains. It was necessary to lump them into one category. Detection was aided by the appearance of chitinous appendages, t a r s a l claws, compound eye parts, wing pads, and pieces of chitinous plates from the body w a l l . Sometimes mouth parts could be detected. When they were present, i d e n t i f i c a t i o n was easy; however they were often missing. f ) Ostracods. The heavy valves were generally nearly i n t a c t . g) R o t i f e r s . The hard l o r i c a appeared i n t a c t . Those types without a hard l o r i c a l e f t no recognizable remains. h) Damselfly larvae. These were recognized by mouth parts, claws, spined appendages, and often the presence of c h a r a c t e r i s t i c " t r i d e n t spines" mentioned i n T i l l y a r d (1917). Tabulation of Food Types There are a number of methods of i n d i c a t i n g the r e l a t i v e importance of food items i n an animal's d i e t . One could record weights, numbers, volume, or the frequency of occurrence of each food type. Lawton (1969) presents his data i n terms of both numbers and r e l a t i v e weights. Pritchard (1964) prefers to use the "frequency of occurrence" method — that i s , the per-centage of the t o t a l number of larvae sampled which contains at l e a s t one of a s p e c i f i c prey item. He claims that except - 22 -with "very small prey" t h i s method y i e l d s comparable r e s u l t s to others which record volume or numbers. Since t h i s study lent i t s e l f to both "numbers" and "frequency of occurrence" methods, i t seemed i n t e r e s t i n g to compare r e s u l t s from both (Table 2). In order to f a c i l i t a t e comparison of the two methods, a r a t i o of importance i s included f o r each, with the importance of the f i r s t prey item (Sida c r y s t a l l i n a ) set at 1.0. It can be noted from Table 2 that the two methods do not y i e l d s i m i l a r r e s u l t s , even i n the order of importance of the prey types. It i s d i f f i c u l t to assess which method i s more accurate i n i t s i n t e r p r e t a t i o n of prey importance; t h i s would depend upon one's i n t e r p r e t a t i o n of what i s important. However, the r e s u l t s of the comparison emphasize the necessity f o r care i n the presentation of t h i s type of information. It was decided to present the r e s u l t s of t h i s study i n terms of the number of prey consumed. This i s a basic observa-t i o n which i s easy to understand, and which lends i t s e l f to t r a n s l a t i o n by the reader into terms of importance by s i z e , weight, or a number of other f a c t o r s . - 23 -Prey Type Cladocerans Sida c r y s t a l l i n a Kurzia l a t i s s i m a Alona quadrannularis Chironomid larvae Mayfly and st o n e f l y larvae Water mites Frequency of Occurrence % Ratio of importance 27 40 17 37 9.4 5.3 1.0 1.5 0.63 1.4 0.35 0.20 Number of Prey % Ratio of importance 36 28 9.2 17 2.1 1.5 1.0 0.75 0.26 0.47 0.06 0.04 Table 2. A comparison of the "frequency of occurrence" (see text) with the "number" method of tab u l a t i n g the importance of prey items i n the guts of Marion Lake damselfly larvae. The r a t i o of importance gives a d i r e c t comparison of the two methods. Results Laboratory observations E. boreale larvae, when held i n c a p t i v i t y , w i l l s t r i k e i n d i s c r i m i n a t e l y at any object which i s ( i ) moving and ( i i ) within c e r t a i n s i z e l i m i t s . They w i l l s t r i k e at a moving needle point, but w i l l ignore a f r e s h l y k i l l e d cladoceran unless i t i s mechanically moved. The behavior of the larvae i n the laboratory i s so mechanical that i t was found to be - 2 4 -impossible to draw any conclusions as to what t h e i r food p r e f e r -ences might be under natural conditions. However, by observing the physical nature of the l a r v a l attack i n the laboratory, I was able to observe that some prey types, because of t h e i r mor-phology or behavior, could not be e a s i l y captured and/or held by the larvae. It seemed l o g i c a l to hypothesize that these prey could not be e a s i l y captured i n the f i e l d e i t h e r , and were therefore u n l i k e l y to show up i n considerable numbers i n the l a r v a l guts. The prey which were observed to be d i f f i c u l t to capture are mentioned i n the following paragraphs. Amphipods e l i c i t e d many s t r i k e s from the larvae, but were caught very infrequently. Even l a s t i n s t a r larvae required many s t r i k e s to catch small H y a l e l l a azteca. On numerous occasions, amphipods were observed to struggle free from the grasp of a l a r v a . It appears that the l a b i a l hooks of the l a r v a are not able to pierce the amphipod*s carapace. This may be one reason why the l a r v a l guts were never found to contain amphipods although they are one of the most common invertebrates i n Marion Lake. Large mayfly larvae, through t h e i r s i z e and agitated escape behavior, often avoided capture, although small larvae were frequently eaten. Molluscs and leeches were never observed to e l i c i t a s t r i k e from the damselfly larvae. Perhaps t h e i r slow movements do not a t t r a c t the attention of the larvae. A s u r p r i s i n g observation was that copepods, e s p e c i a l l y diaptomids, were not s u c c e s s f u l l y captured a good proportion of the time. The larvae appeared to have d i f f i c u l t y l i n i n g up the - 25 -prey p r i o r to the s t r i k e . This was perhaps due to the stop and go nature of the copepods' locomotion. Once a l a r v a has released a s t r i k e , he cannot a l t e r i t s d i r e c t i o n (Pritchard, 1965); therefore, an error i n alignment w i l l r e s u l t i n an unsuccessful s t r i k e . Food of the Marion Lake Population Of the t o t a l number of prey observed i n the f a e c a l p e l l e t s of 490 Enallagma larvae sampled between June 30, 1969 and J u l y 28, 1970 (Table 3), cladocerans and chironomid larvae comprised the bulk. Three species: Sida c r y s t a l l i n a , Kurzia  l a t i s s i m a , and Alona quadrangularis represented the great majority of cladocerans observed i n the samples. A l l three of these species are c l o s e l y associated with standing vegeta-t i o n i n the lake. Sida i s p a r t i c u l a r l y noted covering the surfaces of weeds and other stationary objects. The predominant group of chironomid larvae i n the gut samples was the Orthocladinae. Mr. V. J . MacCauley, who i s c u r r e n t l y studying the Chironomidae of Marion Lake, suggests that the Orthocladinae are the group most l i k e l y to be found i n shallow water. They would therefore have a greater tendency to be found among the weeds which are the prime habitat of the damselfly larvae. Since no a d d i t i o n a l information could be gained by subdividing the chironomids i n t o the four groups presented i n Table 3, they have been treated as one group i n the remaining discussion. Among the larger prey organisms, the most frequently seen were a large cladoceran (Eurycercus lamellatus), mayfly - 26 -and stonefly larvae, and water mites. Other groups made only minor contributions to the food of the larvae. Table 3 reveals several other i n t e r e s t i n g f a c t s about the food of the larvae. F i r s t , a r e l a t i v e l y large proportion (25%) of the larvae sampled had empty guts. Since i t takes a l a r v a somewhere i n the order of 12-24 hours to c l e a r i t s gut, even i n warm water, the frequent occurrence of empty guts suggests that the Marion Lake population does not have an abundance of a v a i l a b l e food. This suggestion i s supported by the f a c t that the average number of prey per animal sampled was only 4.1 (5.5 i f the larvae with empty guts are excluded), a much smaller number than larvae are observed to eat i n the laboratory, and s u b s t a n t i a l l y l e s s than the maximum of 42 prey found i n a singl e gut. The extent of cannibalism observed i n the Marion Lake population was l i m i t e d , even though large and small larvae were commonly captured i n the same sample. There were only 10 incidences of guts containing damselfly l a r v a l remains. The average number of prey per l a r v a sampled (Figure 5) shows not much more than a tendency to eat more i n the summer and f a l l than i n the winter and spring. This i s to be expected i n a temperate lake. Note that the 95% confidence i n t e r v a l s on t h i s data are often large, unusually large perhaps when i t i s taken into account that most of the samples are of a reasonable s i z e (20- 30 animals). This gives an i n d i c a t i o n of the high v a r i a b i l i t y i n numbers of prey taken. NJ CO CO NJ VD CO Js. *» o VD O t-3 NJ VD \-> cn O 00 \ NJ VD \ \ ~4 \ \ Cn > \ o^ cn \ \ \ \ •^ J O o c o o M \-> cn CO NJ vo NJ NJ cn 1—' M (—1 vo o i Ol NJ t—1 M NJ NJ Ch CO 00 co 1—1 -LO CO j—1 NJ H CO NJ M NJ Cn CO NJ Oi «J CO NJ NJ NJ NJ NJ cn cn M 1—1 00 I—1 M NJ cn M H cn M M NJ *>• - J Ul o 00 >U Cn o CO NJ co 00 t-> j£> cn M I—1 1—' cn NJ cn \D VD NJ t—1 cn co NJ NJ Co NJ NJ NJ NJ NJ NJ CO co NJ o VD *» o cn O O Ul Ln o o NJ (—1 NJ co H NJ CO M NJ NJ h-1 Cn O VD H •^1 o Ch \ NJ VD Ch \ \ \ \ \ \ \ \ \ \ VD \ \ \ co CO NJ I- 1 M M VD \ CO - J \ \ \ \ \ \ O O \ Ch \ \ \ -J \ \ \ Ch VO Ch Ch Ch o o o O O o Ch Ch Ch vo VD VD VD VD VD VD M CO CO NJ ui 00 NJ CO •Ci 00 Ul 00 . NJ M 1—1 (—1 NJ NJ (—1 H 1—1 t-1 l—1 CO (—1 M oo Cn o cn Cn CO o Ch NJ CO NJ (-• ~J 00 1—1 00 H Cn M h-1 CO M \-> H M M 1—' CO 1—1 NJ NJ M H H M VD NJ M o Ch Ch O Ch CO Ul Co M Ul O Ul NJ 00 NJ NJ CO co CO CO Co VD CO Ul 1—1 CO NJ h-1 NJ M h-* NJ o o VD CO CO CO VD Ch NJ vo cn I—1 1—' M 00 M Ul co CO h-1 M 1—1 >£> (-" M M I -J M co Cn *> oo Ch O NJ Ch o NJ o o O u> CO CO M Ch Ul M r-1 1—1 M 1—1 00 NJ VD CO \ Ch VD CO o Ch Ch VD CO NJ NJ VD N J CO NJ CO Ch VD Ch CO Ul Ch Number of animals sampled Sampling date Number wit h empty guts Tanypodinae O r t h o c l a d i n a e C h i r o n o m i n i T a n y t a r s i n i Other TOTAL CHIRONOMIDS • Sid a c r y s t a l l i n a A l e n a quadrangular!s K u r z i a l a t i s s i m a E urycercus l a m e l l a t u s Ac a n t h o l e be r i s  c u r v i r o s t r i s Other TOTAL CLAD0CERA NJ CO Ul NJ CO O CO NJ O NJ VO NJ M I—1 I—1 ifc> I- 1 *>. Ch VO CO co cn NJ ch o^ I—' I—1 I—1 I—» NJ NJ NJ NJ NJ Ch Ul H VO NJ VD Ch NJ OJ U l Ch 0 ^ CO 00 1^ CO VD —J CO cn ui Ch o Co NJ 00 iC* co t—' VD NJ N J Ch CO O Co R o t i f e r s Ostracods Da m s e l f l y l a r v a e M a y f l i e s and S t o n e f l i e s Water mites A n n e l i d worms Copepods U n i d e n t i f i e d prey TOTAL PREY 1-3 o H - H cr o 0 P» H H CD c 3 < 3 co cr ti fD • H -0 • 0 OJ' o I—1 9> h-1 P o fD 2^ H i 3 PJ w I-i cr 3 p - 0 CO (+ i-i (D H - (D I- 1 cn PJ hh n> - 28 -Since, unlike most of t h e i r prey, the damselfly larvae are present and active throughout the year, they would be ex-pected to u t i l i z e d i f f e r e n t sources of food at d i f f e r e n t times. A good example of t h i s pattern can be seen i n the predation on the three major cladoceran prey species (Figure 6). Sida and Alona are present i n the lake only i n the warm season. Kurzia appears to inhabit the lake throughout the year. Although no formal studies have been c a r r i e d out with the Kurzia population i n the lake, i t would appear from casual observations that t h e i r numbers increase during the warm months and remain at a r e l a t i v e l y low l e v e l i n the winter and spring. Despite t h i s , Figure 6 shows that they are taken by Enallagma larvae to a considerable extent during the colder periods. Thus there may be some mechanism by which the larvae e x p l o i t prey populations not e n t i r e l y according to t h e i r abundance when food i s scarce. Within the s i z e range of larvae examined, there appears to be no d i s t i n c t i o n between the type of prey eaten by d i f f e r -ent s i z e s of larvae. There i s a s l i g h t i n d i c a t i o n that larger larvae eat larger prey items, but t h i s i s not s t a t i s t i c a l l y s i g n i f i c a n t . Lawton (1969, 1970b) has argued that the very small larvae ( i n s t a r 2) do have a d i f f e r e n t diet because they cannot handle many of the prey items eaten by the l a t e r i n s t a r s . My study did not deal with the e a r l y i n s t a r s and therefore cannot comment upon t h i s observation. There i s a d e f i n i t e i n d i c a t i o n , however, that the la r g e r larvae eat more than the small ones, as would be ex-pected. The average number of prey items per l a r v a increases - 29 -Figure 5. Means (with 95% confidence i n t e r v a l s ) of the t o t a l prey per E. boreale l a r v a i n samples over time i n Marion Lake. - 30 -Figure 6. Mean number of cladocerans per l a r v a sampled over time. s. « Sida c r y s t a l l i n a . k. = Kurzia  l a t i s s i m a . a. = Alona quadrangularis. - 31 -l i n e a r l y with l a r v a l s i z e (Figure 7), although the weight increase of the l a r v a i s , of course, exponential. The f i n a l observation concerning the food of the Marion Lake population i s that some types of prey are captured pre-dominantly i n one habitat within the lake. Unfortunately, the s p e c i f i c habitat i n which samples were taken was noted only during the l a t t e r h a l f of the sampling program (January 17 -J u l y 28, 1970). However, the data do show some strong trends. For example, Sida c r y s t a l l i n a i s observed to be captured i n f a r greater numbers i n i t s preferred habitat of water l i l i e s than i n areas of e i t h e r sedge or h o r s e t a i l (Figure 8a). Kurzia  l a t i s s i m a , the other major cladoceran prey species, shows a much more va r i e d pattern of where i t i s captured (Figure 8b). In addition, the number of prey items per l a r v a d i f f e r s within d i f f e r e n t habitats. During the spring and summer of 1970, when the vegetation i n a l l three of the major habitats had emerged i n the lake, larvae sampled i n water l i l y beds contained on the average more than twice as many prey as those found i n sedges and h o r s e t a i l s . Diurnal feeding cycle Since the time of day at which samples were taken was noted, i t i s possible to examine whether any patterns of feed-ing occur throughout the day. Several complications a r i s e , however. F i r s t , samples were taken only throughout the day-l i g h t hours. Second, since animals were brought into the laboratory and allowed to evacuate t h e i r guts, the exact time - 32 -Figure 7 . Mean number of prey per l a r v a for each of the designated i n s t a r groups. Larvae smaller than i n s t a r 8 were treated as one group. Line was f i t t e d by inspection. - 33 -Figure 8. Mean numbers of cladocerans per l a r v a sampled i n three habitat types. 8a gives data f o r Sida  c r y s t a l l i n a , 8b f o r Kurzia l a t i s s i m a . 1= water l i l i e s . s= sedges, h= h o r s e t a i l s . - 34 -at which a l a r v a ingested a p a r t i c u l a r prey item cannot be determined. Therefore, i n d i r e c t methods of examining feeding patterns must be employed. Changes i n the percentage of animals with empty guts w i l l give i n d i c a t i o n s of changes i n the feeding behaviour of the population. In addition, changes i n the number of prey items i n the guts of the re s t of the population w i l l provide s i m i l a r information. Throughout the daylight hours, the percentage of animals having empty guts decreases while the number of prey per non-empty gut increases (Figure 9) . It appears, then, that the Marion Lake population feeds predominantly during the daylight hours. During the night i t feeds at a lower rate or not at a l l . Discussion The disadvantage of a f a e c a l p e l l e t analysis i s that i t does not permit detection of prey items which leave no recognizable hard remains. In Marion Lake the most l i k e l y prey of t h i s type would be molluscs (which might be taken on occasion without t h e i r s h e l l s being ingested), leeches, f l a t -worms, soft-bodied r o t i f e r s , and, i n the spring, small tadpoles. During the course of the sampling program, no mollusc s h e l l s were ever found i n f a e c a l p e l l e t s . In the laboratory, larvae were never seen to consume molluscs or leeches, even when they were the only prey a v a i l a b l e . Damselfly larvae have - 35 -been known to eat flatworms (see f o r example, Davies, 1967), and these might be a s i g n i f i c a n t portion of the d i e t . The rare occurrence of hard-bodied r o t i f e r s i n the f a e c a l p e l l e t s leads me to suspect that soft-bodied r o t i f e r s may not be of major importance e i t h e r ; however, t h i s cannot be known f o r c e r t a i n since the types and r e l a t i v e numbers of r o t i f e r s i n -habiting the weedy areas are not known. Observations have been made of large Enallagma larvae consuming small tadpoles i n Marion Lake. However, i t appears that the tadpoles are taken only during the f i r s t week following t h e i r hatching. After t h i s time, they grow too large to be handled by even the largest larvae. Most observers of odonate larvae i n the laboratory have found, as I did, that the animals w i l l s t r i k e at a great v a r i e t y of s t i m u l i . For example, larvae w i l l s t r i k e at moving cardboard discs (Pritchard, 1965) and moving l i g h t spots (Etienne and Howland, 1964) as well as at moving prey items. If the prey can be handled and i s palatable to the predator, i t w i l l be eaten. For t h i s reason, the only i n s i g h t which could be gained from my laboratory observations into the feed-ing a c t i v i t i e s of the natural population was, as has previously been mentioned, that c e r t a i n prey items could not e a s i l y be captured, even i n a confined area. It i s not s u r p r i s i n g to note that these same prey items were not r e g u l a r l y captured i n the f i e l d e i t h e r . This observation points out the f o l l y of studies which compare data on possible prey species i n an area with those on what a predator a c t u a l l y captures, arguing - 36 -Figure 9. Indications of changing feeding patterns through-out the day by E. boreale larvae i n Marion Lake. 9a. Mean number of prey organisms per l a r v a sampled (includes only larvae with something i n t h e i r guts). 9b. Mean percentage of empty guts among larvae sampled. Both l i n e s are regression l i n e s and the slopes of each d i f f e r s i g n i f i c a n t l y from zero ( 5 % of l e v e l of s i g n i f i c a n c e ) . 4-6 6-8 a.m. 8-10 10-12 12-2 time of day 2-4 4-6 p.m. 6-11 - 37 -that the predator i s a c t i v e l y " s e l e c t i n g " c e r t a i n prey items. In many cases, the s e l e c t i o n process may be simply an i n a b i l i t y to capture c e r t a i n organisms. Although cladocerans appear to be the most important prey, previous work (Lawton, 1970b) has shown that on a dry weight basis, chironomid larvae are much more important than cladocerans. Therefore, the chironomids probably are a more important food item than t h e i r numbers would suggest. I measured the dry weights of several prey types, but found the weights too v a r i a b l e to be of any use, p a r t l y since within any of the prey categories i n Table 3, the s i z e of i n d i v i d u a l animals v a r i e s immensely. The remains present i n the f a e c a l p e l l e t s were often so fragmented, that i t was impossible to obtain accurate s i z e measurements. Most other studies of odonate larvae also f i n d that cannibalism i n the f i e l d i s minimal (Chutter, 1961; Lawton, 1969; Macan, 1964) although there are records of large dragon-f l y larvae preying upon smaller damselfly larvae (Ross, 1967, P r i t c h a r d , 1964). The study which does report high degrees of cannibalism (Fischer, 1961) i s unique i n that the species observed i s one which inhabits temporary ponds and therefore i s subject to severe crowding conditions. A l l of my observations on the larvae of E. boreale lead me to the conclusion that they are opportunistic feeders, capable of handling large numbers of prey when a v a i l a b l e , but able to survive f o r long periods without eating ( c e r t a i n l y , i n the laboratory, larvae can be held without food f o r several - 38 -months without s u f f e r i n g any apparent i l l e f f e c t s ) . This has been observed by many inv e s t i g a t o r s (Pritchard, 1964; Lawton, 1969 and others). The apparent s e l e c t i o n of Kurzia during the winter months may be r e a l . However, i t must be kept i n mind that since much of the standing vegetation dies back during the winter, the Kurzia population may be just as concentrated i n the weeds at t h i s time of year, as i t i s i n the summer. The f a c t that the food of the larvae sampled varies considerably i n both composition and t o t a l number of prey depending on the habitat, emphasizes that caution must be taken, i n analyzing the food of such a population, to sample a l l of the major habitats. Otherwise, considerable errors may be made i n estimating the number and importance of prey items. As has been noted before, the food of the larvae appears to d i r e c t l y r e f l e c t the quantity of a v a i l a b l e prey within the habitat. L i t t l e i s known about the diurnal feeding cycle of odonate larvae. Corbet (1962), r e l y i n g on the observations of Robert (1958) and others who noted greater a c t i v i t y of dragonfly larvae at night, suggests that t h i s type of anisopteran l a r v a (the type which r e l i e s upon i t s antennae f o r prey recognition) may feed p r e f e r e n t i a l l y at night. The large dragonfly larvae which are predominantly v i s u a l preda-tors have been observed to crawl about on the bottom at night as i f they were " f e e l i n g " f o r food (Paulian and Serfaty, 1944). The Coenagrionidae have a small number of ommatidia i n t h e i r - 39 -compound eyes (Ando, 1957) and have therefore not been thought of as v i s u a l predators. However, since Enallagma feeds during the day i n Marion Lake, there may be a s i g n i f i c a n t v i s u a l com-ponent to i t s searching behaviour. In addition, the f a c t that the population appears to eat very l i t t l e during the night lends some suspicion to the i n t e r p r e t a t i o n of the night crawling of v i s u a l species as predatory a c t i v i t y . SUMMARY 1. A population of l a r v a l damselflies (Enallagma boreale) i n Marion Lake, B r i t i s h Columbia was sampled throughout the year and data concerning i t s l i f e c y c l e , growth rates, and feeding were obtained. 2. The larvae inhabited the weedy areas of the lake almost e x c l u s i v e l y . Few were found on the open mud. 3. The annual cycle of the population was observed to consist of two overlapping generations. 4. The food of the population was mainly cladocerans and chironomid larvae, although a large number of prey types were noted. 5. Laboratory observations showed that many of the common species i n the lake which d i d not appear i n the damselfly guts could not be e a s i l y captured or handled by the larvae. 6. A high proportion (25%) of the larvae sampled had empty gut s. 7. Cannibalism among the Marion Lake population was n e g l i g i b l e . - 40 -8. The diet of the larvae changes throughout the year, and also v a r i e s from one habitat to another within the lake. 9. Large larvae do not have a d i f f e r e n t diet from smaller ones (excluding the very small larvae which were not observed) but they consume more prey. 10. Larvae appear to feed predominantly during the daylight hours. APPENDIX THE FOOD OF NATURAL POPULATIONS OF LARVAL ODONATA This appendix i s an accumulation of published data ( a l l that I am aware of) concerning the food of Odonata larvae under natural conditions. The references are l i s t e d i n chrono-l o g i c a l order, and have been summarized as f a r as possible under standard headings so that the reader may make comparisons e a s i l y . A missing heading s i g n i f i e s that I have been unable to locate that p a r t i c u l a r information i n the reference. The heading "Food tabulation method" s p e c i f i e s which method (% frequency of occurrence or % of t o t a l numbers) was used to tabulate the food items observed i n the larvae examined. These two methods do not always y i e l d d i r e c t l y comparable r e -s u l t s (Table 2), hence the reason f o r i n c l u d i n g t h i s heading. For a d e s c r i p t i o n of these two t a b u l a t i o n methods, see page 21. In summarizing many of the studies, I have condensed the data by combining groups into larger c l a s s i f i c a t i o n s and by omitting prey items present i n very small numbers. - 42 -Reference: Needham and Hart, 1901. This e a r l y reference does not include s p e c i f i c observa-t i o n s . It does, however, out l i n e some general trends regarding the food of odonate larvae. These are: Agrionidae: preference f o r Entomostraca and mayfly nymphs. Vegetation dwellers eat backswimmers (Notonecta), water-boatmen (Corixa), small crustaceans such as Asellus and Allorchestes, t h i n - s h e l l e d molluscs l i k e Physa, coleopter-ous and dipterous larvae, and the "younger or weaker members of t h e i r own order". Reference: Lyon, 1915. Study side: C a s c a d i l l a Creek, New York, U.S.A. Species observed: 14 species of anisopteran and zygopteran larvae. Number of larvae examined: 36 Method of examination: Dissection of gut. % with empty guts: 25 Food ta b u l a t i o n method: % of t o t a l number of prey. Observations: Food type % Chironomid larvae 54 Crustacea (Amphipods, Cladocerans, Ostracods, and Copepods) 12 Odonata (mainly Zygoptera) 11 Ephemeroptera 7 Hemiptera nymphs 5 Other s i g n i f i c a n t prey — Hydrachnida. - 43 -Reference: Warren, 1915. Study s i t e : Various l o c a t i o n s on the i s l a n d of Oahu, Hawaii. Species observed: Anax .-Junius Drury and Pantala flavescens Fabr. Number of larvae examined: 41 Anax, 294 Pantala. Method of examination: Dissection of gut. % with empty guts: 25. Food ta b u l a t i o n method: % frequency of occurrence. Observations: Food type % Diptera Chironomid larvae 44 Mosquito larvae and pupae 4 Crustacea Cypris sp. 29 Protozoa Euglena sp. 8 Cleoptera Dytiscidae 5 Molluscs 4 Hymenoptera Ants 3 Other s i g n i f i c a n t prey — Odonata nymphs, tadpoles. - 44 -Reference: Wilson, 1920. Study s i t e : Ponds near F a i r p o r t , Iowa, U.S.A. Species observed: Anax Junius Drury, L i b e l l u l a luctuosa Burmeister, Erythemis s i m p l i c i c o l l i s Say, Pachydiplax longipennis Burmeister, and several species of Zygoptera tabulated as one group. Number of larvae examined: 50 of each anisopteran species and 50 zygopterans. Method of examination: Dissection of gut. Food ta b u l a t i o n method: % of frequency of occurrence. Observations: Food type Molluscs - Planorbis sp. Physa sp. Crustacea- Bosmina sp. Cypris sp. Simocephalus sp. Unident. Entomostraca Diaptomus sp. Unident. copepods C r a y f i s h Odonata - Zygoptera larvae Ischnura v e r t i c a l i s Enallagma sp. Lestes sp. Others Anisoptera larvae Diptera - Ceratopogon larvae Chironomid larvae Mosquito larvae Simulid larvae A..j . L. 1. E.s. P. 1 • Zygop. 30 64 44 8 24 32 12 32 8 16 6 16 12 8 24 12 70 4 40 30 10 4 4 8 16 10 42 52 30 40 — 2 2 8 12 4 4 8 28 30 22 - 2 4 -24 6 4 2 6 4 10 2 — 12 4 8 10 — 22 30 2 4 4 26* 8 2 — — 4 6 12 — 4 — — 10 6 16 — 2 — 2 12 — — 16 8 18 (Cont'd) * This number may be an overestimate since i t i s the sum of values from three anisopteran species. - 45 -Food type A..j. L. 1. E.s. P. 1. Zygop. Hemiptera - Corixa sp. adult 28 8 6 2 -Beetles - Dytiscus larvae 20 16 4 4 Haplid adults 16 6 16 24 Ephemeroptera larvae 54 18 14 8 12 Algae - Filaments 14 14 64 2 50 Desmids 18 8 4 26 Oedogonium sp. - 12 6 28 Other s i g n i f i c a n t prey — Diatoms, Daphnia sp., Stonefly larvae. N.B. Many other studies have found algae i n the guts of l a r v a l Odonata; however, t h i s i s the only reference which suggests that algae are sometimes taken v o l u n t a r i l y and not merely by accident. - 46 -Reference: Chutter, 1961. Study s i t e : Jukskei River, South A f r i c a (a p o l l u t e d stream). Species observed: Pseudoagrion salisburyense Ris. C o l l e c t i o n method: Sweep net. Number of larvae examined: 147 Method of examination: Dissection of the foregut (small larvae of head width le s s than 0.9 mm. were omitted). Food tabulation method: % of t o t a l number of prey. Observations: % (by i n s t a r * ) Food type F F - l F-2 F-3 F-4 F-5 F-6 F-7 F-8 Chironimidae Psychoda sp.** Ephemeroptera (Baetidae) Cyclops spp. Chydorinae Oligochaeta* * * 76 58 84 60 59 48 42 18 9 7 1 — 1 5 3 - - -7 4 16 1 mm 3 4 — 5 15 15 37 18 22 1 21 — 5 3 3 5 29 9 5 12 - 27 18 24 13 29 57 Other s i g n i f i c a n t prey — Macrothrix sp, * "F" i s the f i n a l i n s t a r , ' F - l ' i s the penultimate, and so on. ** Important i n the spring. *** Available throughout the year, but eaten mainly i n winter. - 47 -Reference: Macan, 1964. Study s i t e : Hodson's tarn, England. Species observed: Pyrrhosoma nymphula Sulzer and Enallagma cyathigerum Charpentier. C o l l e c t i o n method: Mainly sweep net; some with a quantitative sampler described i n the paper. Number of larvae examined: Pyrrhosoma 192, Enallagma 142. Method of examination: Probably d i s s e c t i o n of gut. % with empty guts: Pyrrhosoma 58, Enallagma 75. Observations: Major prey of both species was chironomids and entomostracans although "the range of animals eaten was wide". Reference: Pr i t c h a r d , 1964. Study s i t e : A number of ponds, ditches, and marshes i n northern Alberta, Canada. Species observed: Ten species of anisopteran larvae. C o l l e c t i o n method: Sweep net. Number of larvae examined: 801 Method of examination: Analysis of f a e c a l p e l l e t s . % with empty guts: None. Food tabulation method: % frequency of occurrence. Observations: (pooled data from a l l l a r v a e ) . - 48 -Food type %* Chironomid larvae 52 Small crustacea (Cladocera and Ostracoda) 21 Coleoptera larvae 14 Molluscs 10 Other s i g n i f i c a n t prey — Ceratopogonid larvae, C u l i c i n e larvae, Trichoptera larvae, Gammaridae, and Hydracarina. * These percentages were transposed from a graph and may be s l i g h t l y inaccurate. Reference: Staddon and G r i f f i t h s , 1967. Study s i t e : Pen-ffordd-goch pond near Blaenavon, Wales. Species observed: Aeshna .juncea L. Number of larvae examined: 36; 16 on Oct. 12, 1965, and 20 on Nov. 9, 1965. Method of examination: Analysis of f a e c a l p e l l e t s . Food t a b u l a t i o n method: % frequency of occurrence. Observations: % Food source Food type Oct. 12 Nov. 9 T e r r e s t r i a l Hemiptera: Aphididae 50 15 Diptera: Cyclorrhapha adults 37 5 Hymenoptera: Rhizarcha sp. 31 -Aquatic Hemiptera: Corixidae adults 37 30 nymphs 12 15 Coleoptera: Dytiscidae larvae 19 20 Trichoptera: Phryganeidae larvae 56 35 Diptera: Chironomidae larvae 69 50 Cyclorrhapha larvae 19 -Acarina: Hydrozetes l a c u s t r i s Michael 25 15 - 49 -Reference: Fischer, 1967. Study s i t e : Three ponds i n Poland. Species observed: Lestes sponsa L. Number of larvae examined: 475; breakdown by ponds i s given below. Method of examination: Dissection of gut. Food ta b u l a t i o n method: % of t o t a l number of prey. Observations: Pond 1 Pond 2 Pond 3 Mansfeldova Zmijowa Tun Zoldanka (3 c o l l e c t i o n s (8 c o l l e c t i o n s (8 c o l l e c t i o n s 75 animals, 200 animals, 200 animals, Apr. 24-May 14, Apr. 24-July 5, Apr. 24-July 20, 1964) 1964) 1964)  Food type Copepoda Cladocera Ostracoda Hydracarina Ephemeroptera Diptera, Tendipedidae 48 49 27 39 2 3 29 23 60 1 4 12 Reference: Lawton, 1969. Study s i t e : A pond at Braside, England. Species observed: Pyrrhosoma nymphula Sulz. C o l l e c t i o n method: Sweep net. Number of larvae examined: 452. Method of examination: Analysis of f a e c a l p e l l e t s . Food ta b u l a t i o n method: % of t o t a l number of prey. Observations: - 50 -Food type % Ostracods 45 Chironomids 24 Simocephalus vetulus 9 Copepods 9 Chydorus sp. 4 Other s i g n i f i c a n t prey: Chaoberus sp., Cloendipterum sp., Dytiscidae, Hyracarina, Oligochaeta. Reference: G r i f f i t h s , 1970. Study s i t e : A bog i n northern Norway ( i n Chara beds). Species observed: Aeshna ,-juncea L., Aeshna caerula Strom, and Somatochlora a r c t i c a Zett. Number of larvae examined: A . j . 37, A.c. 29, S.a. 8. Method of examination: Di s s e c t i o n of gut. Food t a b u l a t i o n method: % of t o t a l number of prey. Observations: Food type: Cladocera Daphnia longispina  Bosmina coregoni  Eurycercus lamellatus  Alonopsis elongata Diptera Chaoberus sp. larvae Chironomid larvae Trichoptera larvae Orbatid mites S.a. 8 20 48 16 8 A.c. 5 19 31 26 4 11 A..j. 1 1 62 5 1 21 6 3 N.B. These samples were taken i n July . In August, Trichopt larvae became a more important food item. - 51 -LITERATURE CITED Ando, H. (1957). A comparative study on the development of omnatidia i n Odonata. S c i . Rep. Tokyo Kyoiku Daig. (B) 8: 174-216. (Reference from Corbet, 1962.) Calvert, P. P. (1934). The rates of growth, l a r v a l develop-ment and seasonal d i s t r i b u t i o n of dragonflies of the genus Anax (Odonata: Aeshnidae). Proc. Amer. P h i l . Soc. 73: 1-70. Chutter, F. M. (1961). Certain aspects of the morphology and ecology of the nymphs of several species of Pseudo-agrion Selys (Odonata). Arch. Hydrobiol. 57: 430-463. Corbet, P. S. (1957). The l i f e h i s t o r y of the Emperor Dragonfly Anax imperator Leach (Odonata: Aeshnidae). J . Anim. Ec o l . 26: 1-60. . (1958). Temperature i n r e l a t i o n to seasonal development of B r i t i s h dragonflies (Odonata). Proc. Xth Int. Congr. Ent., Montreal 2: 755-757. . (1962). A Biology of Dragonflies. Chicago: Quadrangle Books. Corbet, P. S., C. Longfield, and N. M. Moore. (1960) Dragon-f l i e s . London: C o l l i n s . Davies, G. S. (1970). P r o d u c t i v i t y of macrophytes i n Marion Lake, B r i t i s h Columbia. J . F i s h Res. Bd. Canada. 27: 71-81. Davies, R. W. (1967). A study of predators of t r i c l a d s by  means of a s e r o l o g i c a l technique. Ph.D. Thesis, Univer-s i t y of Wales. Dickman, M. (1969). Some e f f e c t s of lake renewal i n phyto-plankton p r o d u c t i v i t y and species composition. Limnol. and Oceanogr. 14: 660-666. Dyar, H. G. (1890). The number of molts of Lepidopteran larvae. Psyche. 5: 420-422. Ef f o r d , I. E. (1967). Temporal and s p a t i a l differences i n phytoplankton p r o d u c t i v i t y i n Marion Lake, B r i t i s h Columbia. J . F i s h . Res. Bd. Canada. 24:2283-2307. - 52 -. (1969). I I . Lakes 1. America. Energy t r a n s f e r i n Marion Lake, B r i t i s h Columbia; with p a r t i c u l a r reference to f i s h feeding. Verh. Internat. Verein. Limnol. 17: 104-108. Etienne, A. and H. Howland. (1964). E l i c i t a t i o n of s t r i k e s by projected images and l i g h t spots. Experimentia 20: 152-153. Fischer, Z. (1961). Cannibalism among the larvae of the dragonfly Lestes nympha Selys. Ekologia Polska S e r i a B VII: 33-39. . (1967). Food composition and food preference i n larvae of Lestes sponsa (L.) i n a s t a t i c water environment. Polskie Arch. Hydrobiol. 14: 59-71. G r i f f i t h s , D. (1970). Observations on the food of dragonfly nymphs from a bog water i n north Norway. Ent. Mon. Mag. 103: 226-230. Hamilton, A. L. (1965). An analysis of a freshwater benthic  community with s p e c i a l reference to the Chironomidae. Unpublished Ph.D. t h e s i s , U n i v e r s i t y of B r i t i s h Columbia. Hargrave, B. T. (1969). Epibenthic a l g a l production and community r e s p i r a t i o n i n the sediments of Marion Lake. J . F i s h . Res. Bd. Canada. 26: 2003-2026. . (1970a). D i s t r i b u t i o n , growth, and seasonal abundance of H y a l l e l a azteca (Amphipoda) i n r e l a t i o n to sediment micro-f l o r a . J . F i s h . Res. Bd. Canada. 27: 685-699. . (1970b). The e f f e c t of a deposit-feeding amphipod on the metabolism of benthic microf l o r a . Limnol. and Oceanogr. 15: 21-30. . (1971). An energy budget f o r a deposit-feeding amphi-pod. Limnol. and Oceanogr. 16: 99-103. Kormondy, E. J . (1959). The systematics of Tetragoneuria, based on e c o l o g i c a l , l i f e h i s t o r y , and morphological evidence (Odonata: C o r d u l i i d a e ) . Misc. Publ. Mus. Zool. Univ. Mich. 107: 1-79. Lawton, J . H. (1969). Studies on the e c o l o g i c a l energetics of the damselfly larvae (Odonata: Zygoptera). Ph.D. t h e s i s , University of Durham, England. . (1970a). A population study on larvae of the damsel-f l y Pyrrhosoma nymphula (Sulzer) (Odonata: Zygoptera). Hydrobiologia 36: 33-52. - 53 -. (1970b). Feeding and food energy a s s i m i l a t i o n i n larvae of the damselfly Pyrrhosoma nymphula (Sulz) (Odonata: Zygoptera). J . Anim. E c o l . 39: 669-689. Lyon, Mary B. (1915). The ecology of the dragonfly nymphs of C a s c a d i l l a Creek (Odon.). Ent. News 26: 1-15. Macan, T. T. (1964). The Odonata of a moorland fishpond. Int. Revue ges. Hydrobiol. 49: 325-360. McQueen, D. J . (1969). Reduction of zooplankton standing stocks by predaceous Cyclops bicuspidatus thomasi i n Marion Lake, B r i t i s h Columbia, J . F i s h . Res. Bd. Canada 26: 1605-1618. . (1970). Grazing rates and food s e l e c t i o n i n Diaptomus oregonensis (Copepoda) from Marion Lake, B r i t i s h Columbia. J . F i s h . Res. Bd. Canada. 27: 13-20. Needham, J . G. and C. A. Hart. (1901). The dragon-flies (Odonata) of I l l i n o i s , with descriptions of the immature stages. Part I. Petaluridae, Aeschnidae, and Gomphidae. 111. St. Lab. Nat. H i s t . 6: 1-94. Neish, I. C. (1970). A comparative analysis of the feeding  behavior of two salamander populations i n Marion Lake, B r i t i s h Columbia. Unpublished Ph. D. t h e s i s , U n i v e r s i t y of B r i t i s h Columbia. Paulian, R. and A. Serfaty. (1944). Le rhythme nycthemeral des larves d'Aeschnes. B u l l . Mus. H i s t . Nat., P a r i s . 16: 442-447. Prit c h a r d , G. (1964). The prey of larvae (Odonata: Anisoptera) i n ponds i n northern Alberta. Canadian J . Zool. 42: 785-800. . (1965). Prey capture by dragonfly larvae (Odonata: Anisoptera). Canadian J . Zool. 43: 271-289. Robert, P. A. (1958). Les l i b e l l u l e s (odonates). Neuchatel: Delachaux et n i e s t l e S. A. Ross, Q. E. (1967). The e f f e c t of d i f f e r e n t naiad and prey  de n s i t i e s on the feeding behavior of Anax .Junius (Drury) naiads. M. Sc. Thesis, C o r n e l l University. Sandercock, K. F. (1969). Energy budget of kokanee and r a i n - bow trout i n Marion Lake. Unpublished Ph. D. Thesis, Uni v e r s i t y of B r i t i s h Columbia. - 54 -Snodgrass, R. E. (1954). The dragonfly l a r v a . Sraithson. Misc. C o l l . 123: 1-38. Staddon, B. W. and D. G r i f f i t h s . (1967). Some observations on the food of Aeshna juncea (L.) nymphs (Odonata) with p a r t i c u l a r reference to Corixidae (Hemiptera). Ent. Mon. Mag. 103: 226-230. T i l l y a r d , R. J . (1917). The Biology of Dragonflies (Odonata). Cambridge U. Press, Cambridge. Ware, D. M. (1917). The predatory behavior of rainbow trout (Salmo g a i r d n e r i ) . Unpublished Ph. D. t h e s i s , U n i v e r s i t y of B r i t i s h Columbia. Warren, A l f r e d . (1915). A study of the food habits of Hawaiian dragonflies. College of Hawaii Pub. B u l l . 3: 1.45. Whitehouse, F. C. (1941). B r i t i s h Columbia dragonflies (Odonata) with notes on d i s t r i b u t i o n and habits. Amer. Midi. Natur. 26: 488-557. Wilson, C. B. (1920). Dragonflies and damselflies i n r e l a t i o n to pondfish culture, with a l i s t of those found near F a i r -port, Iowa. B u l l . Bureau F i s h . U. S. 36, Document 882: 181-264. Winterbourn, M. J . ( i n press). The l i f e h i s t o r i e s and trophic r e l a t i o n s h i p s of the Trichoptera of Marion Lake, B r i t i s h Columbia. Canadian J . Zool. 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0101877/manifest

Comment

Related Items