OBSERVATIONS ON A NATURAL POPULATION
OF DAMSELFLY LARVAE
by
PAUL S. M. PEARLSTONE
B.Sc. The University of Toronto, 1967
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
in the Department
of
ZOOLOGY
We accept this thesis 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 British 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 British Columbia Vancouver 8, Canada
Date ^ <> 3 -7/1 I ABSTRACT
A population of weed-dwelling damselfly larvae
(Enallagma boreale) in Marion Lake, British Columbia was sampled at regular intervals from June 30, 1961 to July 28,
1970 for the purpose of obtaining information regarding life cycle and feeding habits. The larvae sampled were measured, and their gut contents were determined.
The Marion Lake population was found to consist of two overlapping, univoltine generations whose cycles differed by about two months. Adults have a long flight period in the summer, and larvae overwinter in mid-larval instars.
The food of the larvae consisted mainly of Cladocera and larval Chironomidae, although many types of prey were eaten. Cannibalism was not prevalent in the population.
The amount and type of food eaten varied both throughout the year and within different habitats in 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
Life cycle of E. boreale in Marion Lake 7
Growth of the larvae 8
Discussion . 14
THE FOOD OF THE LARVAE 16
Methods of Analysis and Tabulation 16
Preparation and identification 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
ii 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 intervals) of
samples of E. boreale in Marion Lake over time ... 12
3 Emergence of E. boreale in Marion Lake 15
4 Examples of recognizable chitinous remains of prey
organisms in faecal pellets examined 20
5 Means (with 95% confidence intervals) of the total
prey per E. boreale larva in samples over time in
Marion Lake 29
6 Mean number of cladocerans per larva sampled over
time 30
7 Mean number of prey per larva for each of the desig•
nated instar groups 32
8 Mean numbers of cladocerans per larva sampled in
three habitat types 33
9 Indications of changing feeding patterns throughout
the day by E. boreale larvae in Marion Lake .... 36
iii TABLES
Table Page
1 Comparison of calculated and observed head widths
of Marion Lake damselfly larvae 11
2 A comparison of the "frequency of occurrence" with
the "number" method of tabulating the importance
of prey items in the guts of Marion Lake damselfly
larvae 23
3 Food of damselfly larvae (Enallagma boreale) in
Marion Lake, British Columbia 27
iv - 1 -
INTRODUCTION
The Odonata are among the most ancient orders of insects.
Fossil remains indicate that ancestors of modern dragonflies were present in the Permian era, some 220 million 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 tropical through temperate
to sub-arctic.
Adult odonates, perhaps because of their brilliant colours and powerful flight, have often been the subject of artwork, prose, and considerable scientific investigation. The unique and fascinating larval form, however, has not been well
studied, especially in the field. Since larval odonates are more numerous, generally longer lived, and probably equally voracious in their predatory habits as their aerial adults, the
impact of these larvae must be considerable in many freshwater communities.
Background Studies
The basis for modern study of Odonata lies in Tillyard's classic treatise of 1917 which outlines form, function, and habits of both adult and larval stages. Recently, Corbet (1962) has added to this base with work which ties together many
isolated studies. As well, Snodgrass (1954) has conducted a
detailed morphological examination of the odonate larva, and - 2 - gives a clear explanation of larval structures and their functions.
Field studies of odonate larvae are limited although the life histories of many species have been published (see for example Corbet, 1957 or Macan, 1964). A number of authors have investigated food preferences of larvae; these studies will be discussed later. The most thorough study to date of a field population of damselfly larvae is the work of Lawton
(1969, 1970a, b).
The Present Study
The work presented in this paper is part of an integrated study of a freshwater community. Since 1962, an extended group of scientists and students, under the direction of Dr. Ian Efford,
Department of Zoology, University of British Columbia, has been involved in a study of Marion Lake, British Columbia. The work is being carried out as part of the Canadian contribution to the
International Biological Program. The major emphasis of the
Marion Lake group has been to define and study the major energy pathways among organisms in the lake (Efford, 1969).
When I joined the Marion Lake Project in 1968, studies had been carried out, or were in progress, which delved into aspects of the benthic organisms (Hargrave, 1969, 1970a, b, 1971;
Hamilton, 1965; Neish, 1970), phytoplankton (Efford, 1967;
Dickman, 1969; McQueen, 1969, 1970), and fish (Sandercock, 1969;
Ware, 1971) in the lake. Some knowledge had been obtained of the macrophytes (Davies, 1970) but virtually nothing was known of the - 3 - animals which inhabited the weedy areas.
Preliminary observations led to the conclusion that, of the weed-dwelling animals, Odonata larvae (especially one
species, Enallagma boreale Selys) were a major component. This, along with the fact that not a great deal of work has been done on larval odonates, was the basis for the initiation of this
study. The major emphasis throughout the work has been on aspects of the life history and feeding of the larvae.
Since this work has been undertaken, some observations have been made of larval Trichoptera in Marion Lake (Winterbourn,
in press). Other weed-dwelling animals (larval Ephemeroptera and Plecoptera, as well as adult Hemiptera seem particularly noteworthy) are practically unknown.
THE SITE
The study site was Marion Lake, a small coastal lake in
south-western British Columbia. The lake elevation is approxi• mately 1000 feet (305 m) above sea level. Marion Lake lies in a narrow, steep-sided valley. The lake itself is narrow
(approximately 800m by 200m) and shallow (maximum depth, 6m; average depth, 2.4m). The climate is mild and wet (average annual rainfall, 240 cm) and the lake is prone to seasonal flooding. For a detailed discussion of the geological,
physical, and chemical features of the area, refer to Efford
(1967). - 4 -
TYPES AND DISTRIBUTION OF ODONATE LARVAE
There are six known species of Odonata in Marion Lake.
These include the dragonflies (Anisoptera) Plathemis lydia
Drury, Leucorrhina glacialis Hagen, and Aeshna interrupta
Walker and the damselflies (Zygoptera) Enallagma boreale Selys,
Ischnura cervula Selys, and Lestes dryas Kirby. (The identi• fication of the latter three species was confirmed by Dr. P. S.
Corbet to whom I am most grateful.) The first, two dragonflies mentioned have larvae which burrow in the mud bottom. These were not seen in 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 far 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 identified as Ishnura and 6 as Lestes. Therefore, the remainder of this paper will concern itself only with the population of Enallagma.
Much of the bottom of Marion Lake is soft mud with sparse vegetation. In this habitat Enallagma larvae were virtually absent. A bottom sampling program conducted by Mr. K.
Tsumura which consisted of a series of monthly Hargrave samples
(Hargrave, 1969) taken between June 1968 and December 1969 pro- 2 duced a total of only 51 larvae in approximately 95 m of mud bottom (this can be compared with amphipods which number in the order of 2000 per square metre in the same samples — Mrs. A.
Bryan, personal communication). - 5 -
The larvae, therefore, are restricted to areas of vegetation in the lake. The majority of Enallagma larvae were
collected from sedge in shallow water around the perimeter of
the lake. Larvae were found here throughout the year. In the warmer months, larvae were also found in patches of water lilies
(Nuphar variagatum), Potamogeton natans, and horsetails
(Equisetum sp.).
SAMPLING METHODS
The larvae were sampled throughout the year for the purpose of obtaining information on their life cycle, growth,
and feeding. The trials and tribulations of achieving a
quantitative sample of odonate larvae are well described by
Macan (1964). He demonstrates that a sweep net is by no means
an unselective sampling method. More quantitative samplers
(one.is described in the same paper) have the disadvantage of not yielding many animals and requiring much sorting time.
Sweep nets must be calibrated, and the calibration will change with vegetation type, larval size, and likely, time of year.
Macan's efforts in Hodson's Tarn (surface area less than 0.5 ha)
and Lawton's later work (1969, 1970a) in a small pond (surface
area 93.1 nr1) led me to believe that a quantitative sampling program in Marion Lake, which is much larger (13.3 ha) than
these other areas, and contains a greater variety of raacro-
phytes, would involve much time and was doomed to yield results
of questionable accuracy. I therefore chose to concentrate on^ - 6 -
aspects of the life history and food preferences of the larvae, rather than on areas which would require quantitative samples.
Collection of the larvae was carried out with a circular
sweep net, approximately 30 cm in diameter, having a shallow fibre 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 size classes of
larvae. At the outset of the program, I was mainly concerned with the food of the population and felt that the smaller
larvae would not contribute greatly since they are present only for a short time during the year. Later, when my sampling began
to yield interesting results concerning the development and life history of the population, I regretted the lack of information on the smaller instars. However, I did not change the sampling methods.as I wished the results to be comparable over time.
Because a shallow dip net was used, larvae could be collected and isolated immediately upon being removed from the water. This was important in order to insure that the entire gut contents of each larva was obtained. The larvae were held
in individual dishes of clean lake water and brought to the
laboratory where each was measured and held until it had emptied its gut (generally two to three days) before being returned to the lake. During the summer, last instar larvae were provided with a twig as an access route to emerge. Those that did so were released. Faecal pellets were stored in alcohol for later analysis. - 7 -
The date, time of day, and location of capture in the
lake were noted for each animal. In the later part of the
study, the type of vegetation in which the larva had been
caught was also noted.
Sampling was carried out at intervals of approximately
two weeks between June 30, 1969 and July 28, 1970.
LIFE HISTORY
Results
Life Cycle of E. boreale in Marion Lake
According to Whitehouse (1941), E. boreale is on the wing longer than any other odonate in British Columbia. In
Marion Lake, it indeed has a long flight period with first
emergence in 1970 being observed on May 5 and flight continuing until mid-September.
The life cycle is completed in one year; eggs are laid
in the summer in stems of Nuphar plants (and occasionally in
others) and the young larvae hatch out several weeks later.
The larvae grow until late fall, then overwinter in mid-larval
instars. Growth resumes the following spring and the larvae
emerge in the summer. E. boreale would therefore be classified with regard to its life cycle as a "summer species" in Corbet's
definition (1959).
There is evidence from my sampling data that the popula•
tion of E. boreale in Marion Lake possesses two overlapping
generations. This will be discussed later. - 8 -
Growth of the Larvae
a) Instar classification. Since a large number of larvae had been collected, it was expected that a plot of head width against number of larvae would exhibit distinct peaks.
It would thus be possible to designate the instars in the population. The peaks, however, were less distinct than had been expected (Figure 1). The vertical lines in Figure 1 indicate what appear to be the most likely instar divisions.
These were arrived at by inspection of the figure by an un• biased observer.
It has been known for some time that arthropod head widths increase from one instar to the next in a regular geometric progression, a fact often referred to as Dyar's law
(Dyar, 1890). The "growth factor" from one instar to the next should therefore be constant. The growth factor from one instar to the next is the increase in head width expressed as a factor of the smaller head width. Thus the growth factor
(GF) can be defined as
GF = (I - 1 to I) HWI_ x where HWj = head width of instar I
HWj_^= head width of instar preceding I.
The average growth factor for the six possible instars shown in Figure 1 is 1.24 (S.E. = 0.014).
In addition, a method of numbering the instars was devised. Mr. D. Proctor, who has hatched eggs of this - 9 -
Figure 1. Frequency histogram of head widths of E. boreale
larvae from Marion Lake. Shown above is the
hypothesized division of the population into
instars along with the "growth factor" from one
instar to the next and the instar numbers (see
Table 1). 1.28 1.24 1.20 1.23 1.24
8 10 11 12 13
1.0 2.0 3.0 4.0 head width in mm - 10 - population in the laboratory, informs me that the head width of the second instar is 0.33 (the first instar, the so-called
"prolarva" is extremely short lived and not usually seen).
Taking this as a starting point and using the obtained growth factor of 1.24, the expected mean head widths of successive instars can be calculated. These are shown in the left hand column of Table 1. The right hand column gives the mean head widths of the instar groups designated in Figure 1. It appears, then, that the population has 13 instars and the last six of these have been sampled in this study.
During the time that the larvae were held in the laboratory, seven of them emerged into adults: two in the month of May and five in July. The two larvae which emerged in May had a mean head width of 3.78 mm (range: 3.76- 3.80 mm) while the five which emerged in July had a mean head width of 3.49 mm
(range: 3.33- 3.68 mm). These values correspond roughly to the two peaks in Figure 1 in the range ascribed to instar 13.
Several of the other instar ranges have two peaks as well. This is the initial evidence that there may be two generations of slightly different sizes in this population.
b) Larval growth in the field. Although the sampling methods used in this study were not quantitative, they did sample a portion of the population over time in a consistent fashion. It is possible, then, to use the data to observe the growth of the animals over time (Figure 2). Note the drop in the average head width size on July 19, 1969 and again on - 11 -
July 27, 1970. This is the time of year when the new genera• tion enters the population being sampled (it is not the time hatching since the samples do not include the small instars).
The growth of the population can then be observed by the increase in average size 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 calculated and observed head widths
of Marion Lake damselfly larvae. This system was
used to obtain instar numbers for the six instar
groups observed in the study (see text). - 12 -
Figure 2. Mean head widths (with 95% confidence intervals)
of samples of E. boreale in Marion Lake over time.
The asterisked point is a 1970 sample inserted for
comparison (see text).
- 13 -
One inconsistency in the growth pattern is apparent during the winter of 1969- 1970. (There is also a gap in the spacing of samples at this time, since adverse weather made sampling most difficult.) During this period, the average size of individuals in the population appears to have decreased.
There are a number of possible explanations for this. First, it may not be a decrease at all but simply a cessation of growth during the cold part of the winter. If there is a drop in size, it may be caused by a selective die-off of larger larvae or perhaps by another generation of smaller larvae entering the sampled population at this 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 in which the apparent drop in size had occurred the year before. This sample (see asterisked point in Figure 2) had a very small mean head size, comparable with the late July samples. The smaller average size was due not to a decline in large larvae, but to a con• siderable number of small larvae appearing in the sample (22% of the larvae were smaller than instar number 8 and larvae of instars 5 and 6 were common).
The suspicion therefore grows that the Marion Lake population of E. boreale has two generations. To check this directly, I obtained emergence data of the population for the summers of 1969 and 1970 from Mr. D. Proctor (Figure 3). The small emergence in late May and early June of 1969 is - 14 - undoubtedly due to poor weather conditions. The emergence was delayed until the following week. Taking this into account, the pattern of emergence is similar for the two years. There is an early emergence in May - June tapering off in late June.
Subsequently, a second emergence is noted in mid-July.
Thus, all the data indicate that there are two genera• tions comprising the Marion Lake population of E. boreale.
One emerges early in the season in May and June. The eggs laid hatch in July and grow to spend the winter in advanced larval instars (8-13). They continue growing in the spring to emerge again in May and June. The second generation emerges in the late summer in July and the eggs hatch late in the season. These larvae enter the winter as junior instar classes
(mainly 5-6) and must grow through the following spring and summer to emerge in July.
Discussion
The growth factor of 1.24 calculated from the data in
Figure 1 compares favourably with published data for odonate larvae: Calvert (1934) obtained a value of 1.23 for an indi• vidual of Anax Junius; Kormondy (1959) obtained 1.27, 1.27 and
1.26 for three dragonfly species; in the only published data for damselflies, Lawton (1969) obtained a growth factor of 1.25 for a population of Pyrrhosoma nymphula Sulz. The instar classification presented therefore appears to be reasonable. - 15 -
Figure 3. Emergence of E. boreale in Marion Lake (unpublished
data of D. Proctor). Each bar represents one week's
accumulation of exuviae in a specified area of the
lake. 140- n 1969 100
60 20 n 'cn d P CD E CD u CD E 140
100 1970
60
20 m - 16 -
This is not the first observation of an odonate species with different life cycles in the same body of water. Macan
(1964) found in a population of Pyrrhosoma nymphula in Hodson's tarn, England, that some individuals took two years to complete their life cycle 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 factors. My investigations show no correlation between vege• tation type and the different generations of larvae in the lake.
It is possible as well that the two generations may be two closely related species. This can be tested by capturing adults from the early and late emergences and examining them morphologically.
THE FOOD OF THE LARVAE
Methods of Analysis and Tabulation
A series of observations was carried out on larvae feeding on live prey in the laboratory. Several conclusions were drawn about the ability of the larvae to handle certain prey types; however, it became apparent that observations made in an artificial setting could not reveal very much about the feeding of the lake population. More direct observations were required. Thus a considerable proportion of the effort involved in this study was devoted to obtaining an accurate sample of the food of the population. - 17 -
Many authors have referred to the food of field populations of larval 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 existing literature is presented in the appendix.
The most comprehensive study of the food of a field population of odonate larvae to date is the work of Lawton
(1969, 1970b). The data presented here provide an interesting comparison to his.
Preparation and Identification of Prey Remains
Faecal pellets of odonate larvae are enclosed in a strong peritrophic membrane. They are therefore easy to handle and analyze without losing any of the contents. Dissection of the gut may permit identification of some undigested soft bodied prey; however, this method has several disadvantages.
Collection of the gut contents is more difficult; more time is required; and the larvae must be killed.
The faecal pellets from each larva collected were stored in a vial in 100% ethanol. For analysis, the pellets were re• moved from the alcohol and placed on a microscope slide in a drop of polyvinyl lactophenol containing lignin pink stain.
They were then teased apart under a binocular microscope (12X), covered with a cover slip, labelled, and dried (either over• night in a 60° C drying oven, or, if this were not available, for several days at room temperature). The slides could then be kept indefinitely. - 18 -
There are several distinct advantages of this method of analysis over others. The slides can be referred to again and again. This was found to be important, since as my ability to recognize various pieces of prey improved, I re-analyzed many slides. Difficult slides can be taken to experts for confirmation. Lignin pink is a selective stain for chitin, and since most of the animal remains in the faecal pellets are chitinous, the stain facilitates detection and recognition of parts; it also allows an immediate discrimination between animal and plant material. Finally, permanent slides are easier to analyze under high magnification. In order to discriminate small items, it was necessary to analyze the faecal material under a compound microscope at 100X with frequent checks of specific areas at 400X. Other studies, which have employed similar analysis methods to this one, but have used much lower magnifications, have probably overlooked a considerable number of prey items.
The identification of the faecal pellets was aided by the preparation of reference slides of identified organisms collected among the weeds in Marion Lake and prepared in the same manner as the faecal pellets. It was necessary to update this file of slides at different times of the year as new items appeared in the faecal pellets of larvae collected.
Methods of identification and counting of prey items are presented below. In addition, some of the indicative chitinous remains for a number of prey types are shown (Figure 4). - 19 -
a) Cladocerans. These were easily identified to species by the footpiece (postabdomen), which was unique for each species. Counting of footpieces also gave a total count of the number of each species, and this could be roughly checked by observations of pieces of valves and thoracic legs.
b) Chironomid larvae. These were identified by the head capsule, particularly the labium and mandibles. It was impossible to identify species, but in most cases the larva could be classified (mainly by the paralabial plates) into one of four groups: the subfamilies Tanypodinae and Ortho- cladinae, and the tribes Chironomini and Tanytarsini of the subfamily Chironominae. The appearance of anal tufts and hooks gave an indication, although not quantitative, of the presence of chironomid larvae.
c) Water mites. These could be identified by pieces of palps, chitinous appendages (sometimes with spines), and particularly by the characteristic acetabular plates. In some species, the exoskeleton remained virtually intact.
d) Oligochaetes. Annelid worms leave only one identifiable remain in the faecal pellet. This is the S-shaped chitinous chaeta which is generally present in considerable numbers. Although it was not possible to determine the number of annelids in the faecal pellet, 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 in the faecal pellets examined,
a. - d. Cladoceran postabdomens (foot parts),
a. Sida crystallina Mtiller (10X). b. Alona
quadrangularis (10X). c. Eurycercus lamellatus
(25X). d. Kurzia latissima (10X) with carapace
and thoracic legs. e. Chironomid larva
(Orthocladinae) head capsule (10X). f. Centre,
Tanypodinae chironomid larva, head capsule (10X).
g. Water mite palp (10X). h. Damselfly larva,
labium (10X). i. Piece of chitinous appendage of
damselfly larva containing "trident spines" (10X).
j. Rotifer, Keratella cochlearis (25X).
h - 21 -
e) Mayflies and stoneflies. There are a number of species of mayflies and stoneflies in the lake. Not enough is known about them to permit their being identified to species from faecal remains. It was necessary to lump them into one category. Detection was aided by the appearance of chitinous appendages, tarsal claws, compound eye parts, wing pads, and pieces of chitinous plates from the body wall. Sometimes mouth parts could be detected. When they were present, identification was easy; however they were often missing.
f) Ostracods. The heavy valves were generally nearly intact.
g) Rotifers. The hard lorica appeared intact. Those types without a hard lorica left no recognizable remains.
h) Damselfly larvae. These were recognized by mouth parts, claws, spined appendages, and often the presence of characteristic "trident spines" mentioned in Tillyard (1917).
Tabulation of Food Types
There are a number of methods of indicating the relative importance of food items in an animal's diet. One could record weights, numbers, volume, or the frequency of occurrence of each food type. Lawton (1969) presents his data in terms of both numbers and relative weights. Pritchard (1964) prefers to use the "frequency of occurrence" method — that is, the per• centage of the total number of larvae sampled which contains at least one of a specific prey item. He claims that except - 22 - with "very small prey" this method yields comparable results to others which record volume or numbers.
Since this study lent itself to both "numbers" and
"frequency of occurrence" methods, it seemed interesting to compare results from both (Table 2). In order to facilitate comparison of the two methods, a ratio of importance is included for each, with the importance of the first prey item
(Sida crystallina) set at 1.0. It can be noted from Table 2 that the two methods do not yield similar results, even in the order of importance of the prey types. It is difficult to assess which method is more accurate in its interpretation of prey importance; this would depend upon one's interpretation of what is important. However, the results of the comparison emphasize the necessity for care in the presentation of this type of information.
It was decided to present the results of this study in terms of the number of prey consumed. This is a basic observa• tion which is easy to understand, and which lends itself to translation by the reader into terms of importance by size, weight, or a number of other factors. - 23 -
Frequency of Prey Type Occurrence Number of Prey
% Ratio of % Ratio of importance importance
Cladocerans Sida crystallina 27 1.0 36 1.0 Kurzia latissima 40 1.5 28 0.75 Alona quadrannularis 17 0.63 9.2 0.26 Chironomid larvae 37 1.4 17 0.47 Mayfly and stonefly larvae 9.4 0.35 2.1 0.06 Water mites 5.3 0.20 1.5 0.04
Table 2. A comparison of the "frequency of occurrence" (see
text) with the "number" method of tabulating the
importance of prey items in the guts of Marion Lake
damselfly larvae. The ratio of importance gives a
direct comparison of the two methods.
Results
Laboratory observations
E. boreale larvae, when held in captivity, will strike indiscriminately at any object which is (i) moving and (ii) within certain size limits. They will strike at a moving needle point, but will ignore a freshly killed cladoceran unless it is mechanically moved. The behavior of the larvae in the laboratory is so mechanical that it was found to be - 24 - impossible to draw any conclusions as to what their food prefer• ences might be under natural conditions. However, by observing the physical nature of the larval attack in the laboratory, I was able to observe that some prey types, because of their mor• phology or behavior, could not be easily captured and/or held by the larvae. It seemed logical to hypothesize that these prey could not be easily captured in the field either, and were therefore unlikely to show up in considerable numbers in the larval guts. The prey which were observed to be difficult to capture are mentioned in the following paragraphs.
Amphipods elicited many strikes from the larvae, but were caught very infrequently. Even last instar larvae required many strikes to catch small Hyalella azteca. On numerous occasions, amphipods were observed to struggle free from the grasp of a larva.
It appears that the labial hooks of the larva are not able to pierce the amphipod*s carapace. This may be one reason why the larval guts were never found to contain amphipods although they are one of the most common invertebrates in Marion Lake.
Large mayfly larvae, through their size and agitated escape behavior, often avoided capture, although small larvae were frequently eaten.
Molluscs and leeches were never observed to elicit a strike from the damselfly larvae. Perhaps their slow movements do not attract the attention of the larvae.
A surprising observation was that copepods, especially diaptomids, were not successfully captured a good proportion of the time. The larvae appeared to have difficulty lining up the - 25 - prey prior to the strike. This was perhaps due to the stop and go nature of the copepods' locomotion. Once a larva has released a strike, he cannot alter its direction (Pritchard,
1965); therefore, an error in alignment will result in an unsuccessful strike.
Food of the Marion Lake Population
Of the total number of prey observed in the faecal pellets of 490 Enallagma larvae sampled between June 30, 1969 and July 28, 1970 (Table 3), cladocerans and chironomid larvae comprised the bulk. Three species: Sida crystallina, Kurzia latissima, and Alona quadrangularis represented the great majority of cladocerans observed in the samples. All three of these species are closely associated with standing vegeta• tion in the lake. Sida is particularly noted covering the surfaces of weeds and other stationary objects.
The predominant group of chironomid larvae in the gut samples was the Orthocladinae. Mr. V. J. MacCauley, who is currently studying the Chironomidae of Marion Lake, suggests that the Orthocladinae are the group most likely to be found in 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 additional information could be gained by subdividing the chironomids into the four groups presented in Table 3, they have been treated as one group in 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 interesting facts about the food of the larvae. First, a relatively large proportion
(25%) of the larvae sampled had empty guts. Since it takes a larva somewhere in the order of 12-24 hours to clear its gut, even in warm water, the frequent occurrence of empty guts suggests that the Marion Lake population does not have an abundance of available food. This suggestion is supported by the fact that the average number of prey per animal sampled was only 4.1 (5.5 if the larvae with empty guts are excluded), a much smaller number than larvae are observed to eat in the laboratory, and substantially less than the maximum of 42 prey found in a single gut.
The extent of cannibalism observed in the Marion Lake population was limited, even though large and small larvae were commonly captured in the same sample. There were only
10 incidences of guts containing damselfly larval remains.
The average number of prey per larva sampled (Figure 5) shows not much more than a tendency to eat more in the summer and fall than in the winter and spring. This is to be expected in a temperate lake. Note that the 95% confidence intervals on this data are often large, unusually large perhaps when it is taken into account that most of the samples are of a reasonable size (20- 30 animals). This gives an indication of the high variability in numbers of prey taken. NJ CO CO NJ co NJ NJ Co NJ NJ NJ NJ NJ NJ CO co NJ VD CO *» o o o cn O O Ul Ln o o NJ Number of O *» VD Js. VD VD animals sampled
-3 1 NJ \-> 1 t VD cn (— NJ co H NJ CO M NJ NJ h- CO CO Sampling O 00 \ NJ \ Cn O H •^1 o \ NJ \ o \ ~4 \ \ Cn \ \ \ \ \ \ \ \ \ \ \ \ \ date VD VD 1 Ch VD Ch > \ o^ cn \ co CO NJ I- M M \V D -J \ \ \ •^J \ \ \ \ \ \ \V D \C O \ \ Ch O o \ \ \ Ch c o o o o o O O o O O vo Ch VD M -J Ch VO Ch Ch Ch VD Ch Ch Ch VD VD VD Number with M VD VD VD \-> cn CO CO CO NJ ui 00 NJ CO •Ci 00 Ul 00 . NJ CO NJ empty guts
NJ vo NJ NJ cn 1—' M 1—1 (—1 NJ NJ NJ Tanypodinae
1 1 1 1 1 M (— (— H 1— t- l— CO (—1 vo oi Ol NJ M oo Cn o cn Cn CO o NJ t—1 Ch VD Orthocladinae M NJ NJ Ch CO NJ (-• ~J 00 1—1 00 Chironomini CO H 00 co Cn M h-1 CO M Tanytarsini 1—1 - LO CO j—1 NJ H \-> M H M 1—' Other
CO
1 NJ M NJ CO 1— NJ NJ M H H NJ TOTAL Cn CO NJ Oi o M VD NJ M Ch Ch O Ch CO CO CHIRONOMIDS • C«OJ NJ NJ NJ Ul Co M Ul NJ Ch NJ NJ cn cn M O Ul NJ 00 VD Sida crystallina 1—1 1 00 I— M NJ NJ CO co Alena quadrangular!s NJ cn M H CO CO Co CO Ul CO
1 cn 1 M M NJ *>• 1— CO NJ h- VNDJ M h-* Ch -J Ul o 00 >U Cn NJ o o CO CO CO NJ vo cn Kurzia latissima o VD I—1 VD Ch CO NJ co 1—' M 00 M Ul 00 Eurycercus lamellatus t-> Ac anthole be r i s j£> cn co CO h-1 M c urvirostri s M 1 I— 1 1— >£> (-" Other 1—' cn NJ M M I-J cn M co Cn oo O NJ o CO TOTAL CLAD0CERA 1 *> NJ t— \cDn VD NJ o o O u> COh CO M Ch Ul M Ch Ul Ch
NJ 1 CO r- 1—1 M 1—1 00 NJ Rotifers
Ul NJ Ostracods
1 1 M I— I— ifc> I-1 Damselfly larvae
NJ NJ *>. Ch co cn ch 1 1 o^ I—' I— I— I—» Mayflies and Stoneflies
CO O NJ NJ Water mites VO CO
NJ NJ Annelid worms
CO NJ Copepods
NJ NJ Unidentified prey O NJ NJ OJ Ch H VD Ch 00 CO —J cn o 00 iC* Ul Ch Ul 0^ Co NJ O VO VO CO ^1 CO ui Ch t—' VD VD NJ co NJ Ch CO Co 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 Hi 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 their prey, the damselfly larvae are present and active throughout the year, they would be ex• pected to utilize different sources of food at different times.
A good example of this pattern can be seen in the predation on the three major cladoceran prey species (Figure 6). Sida and
Alona are present in the lake only in the warm season. Kurzia appears to inhabit the lake throughout the year. Although no formal studies have been carried out with the Kurzia population in the lake, it would appear from casual observations that their numbers increase during the warm months and remain at a relatively low level in the winter and spring. Despite this,
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 exploit prey populations not entirely according to their abundance when food is scarce.
Within the size range of larvae examined, there appears to be no distinction between the type of prey eaten by differ• ent sizes of larvae. There is a slight indication that larger larvae eat larger prey items, but this is not statistically significant. Lawton (1969, 1970b) has argued that the very small larvae (instar 2) do have a different diet because they cannot handle many of the prey items eaten by the later instars.
My study did not deal with the early instars and therefore cannot comment upon this observation.
There is a definite indication, however, that the larger larvae eat more than the small ones, as would be ex• pected. The average number of prey items per larva increases - 29 -
Figure 5. Means (with 95% confidence intervals) of the
total prey per E. boreale larva in samples
over time in Marion Lake.
- 30 -
Figure 6. Mean number of cladocerans per larva sampled
over time. s. « Sida crystallina. k. = Kurzia
latissima. a. = Alona quadrangularis.
- 31 -
linearly with larval size (Figure 7), although the weight
increase of the larva is, of course, exponential.
The final observation concerning the food of the Marion
Lake population is that some types of prey are captured pre•
dominantly in one habitat within the lake. Unfortunately, the
specific habitat in which samples were taken was noted only
during the latter half of the sampling program (January 17 -
July 28, 1970). However, the data do show some strong trends.
For example, Sida crystallina is observed to be captured in
far greater numbers in its preferred habitat of water lilies
than in areas of either sedge or horsetail (Figure 8a). Kurzia
latissima, the other major cladoceran prey species, shows a
much more varied pattern of where it is captured (Figure 8b).
In addition, the number of prey items per larva differs within different habitats. During the spring and summer of
1970, when the vegetation in all three of the major habitats
had emerged in the lake, larvae sampled in water lily beds
contained on the average more than twice as many prey as those
found in sedges and horsetails.
Diurnal feeding cycle
Since the time of day at which samples were taken was
noted, it is possible to examine whether any patterns of feed•
ing occur throughout the day. Several complications arise,
however. First, samples were taken only throughout the day•
light hours. Second, since animals were brought into the
laboratory and allowed to evacuate their guts, the exact time - 32 -
Figure 7. Mean number of prey per larva for each of the
designated instar groups. Larvae smaller than
instar 8 were treated as one group. Line was
fitted by inspection.
- 33 -
Figure 8. Mean numbers of cladocerans per larva sampled
in three habitat types. 8a gives data for Sida
crystallina, 8b for Kurzia latissima. 1= water
lilies. s= sedges, h= horsetails.
- 34 - at which a larva ingested a particular prey item cannot be determined. Therefore, indirect methods of examining feeding patterns must be employed. Changes in the percentage of animals with empty guts will give indications of changes in the feeding behaviour of the population. In addition, changes in the number of prey items in the guts of the rest of the population will provide similar 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 it feeds at a lower rate or not at all.
Discussion
The disadvantage of a faecal pellet analysis is that it does not permit detection of prey items which leave no recognizable hard remains. In Marion Lake the most likely prey of this type would be molluscs (which might be taken on occasion without their shells being ingested), leeches, flat- worms, soft-bodied rotifers, and, in the spring, small tadpoles.
During the course of the sampling program, no mollusc shells were ever found in faecal pellets. In the laboratory, larvae were never seen to consume molluscs or leeches, even when they were the only prey available. Damselfly larvae have - 35 - been known to eat flatworms (see for example, Davies, 1967), and these might be a significant portion of the diet. The rare occurrence of hard-bodied rotifers in the faecal pellets leads me to suspect that soft-bodied rotifers may not be of major importance either; however, this cannot be known for certain since the types and relative numbers of rotifers in• habiting the weedy areas are not known. Observations have been made of large Enallagma larvae consuming small tadpoles in Marion Lake. However, it appears that the tadpoles are taken only during the first week following their hatching.
After this time, they grow too large to be handled by even the largest larvae.
Most observers of odonate larvae in the laboratory have found, as I did, that the animals will strike at a great variety of stimuli. For example, larvae will strike at moving cardboard discs (Pritchard, 1965) and moving light spots
(Etienne and Howland, 1964) as well as at moving prey items.
If the prey can be handled and is palatable to the predator, it will be eaten. For this reason, the only insight which could be gained from my laboratory observations into the feed• ing activities of the natural population was, as has previously been mentioned, that certain prey items could not easily be captured, even in a confined area. It is not surprising to note that these same prey items were not regularly captured in the field either. This observation points out the folly of studies which compare data on possible prey species in an area with those on what a predator actually captures, arguing - 36 -
Figure 9. Indications of changing feeding patterns through•
out the day by E. boreale larvae in Marion Lake.
9a. Mean number of prey organisms per larva
sampled (includes only larvae with something in
their guts). 9b. Mean percentage of empty guts
among larvae sampled. Both lines are regression
lines and the slopes of each differ significantly
from zero (5% of level of significance). 4-6 6-8 8-10 10-12 12-2 2-4 4-6 6-11 a.m. time of day p.m. - 37 - that the predator is actively "selecting" certain prey items.
In many cases, the selection process may be simply an inability to capture certain 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 their numbers would suggest.
I measured the dry weights of several prey types, but found the weights too variable to be of any use, partly since within any of the prey categories in Table 3, the size of individual animals varies immensely. The remains present in the faecal pellets were often so fragmented, that it was impossible to obtain accurate size measurements.
Most other studies of odonate larvae also find that cannibalism in the field is minimal (Chutter, 1961; Lawton,
1969; Macan, 1964) although there are records of large dragon• fly larvae preying upon smaller damselfly larvae (Ross, 1967,
Pritchard, 1964). The study which does report high degrees of cannibalism (Fischer, 1961) is unique in that the species observed is one which inhabits temporary ponds and therefore is subject to severe crowding conditions.
All 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 available, but able to survive for long periods without eating (certainly, in the laboratory, larvae can be held without food for several - 38 - months without suffering any apparent ill effects). This has been observed by many investigators (Pritchard, 1964; Lawton,
1969 and others).
The apparent selection of Kurzia during the winter months may be real. However, it must be kept in mind that since much of the standing vegetation dies back during the winter, the Kurzia population may be just as concentrated in the weeds at this time of year, as it is in the summer.
The fact that the food of the larvae sampled varies considerably in both composition and total number of prey depending on the habitat, emphasizes that caution must be taken, in analyzing the food of such a population, to sample all of the major habitats. Otherwise, considerable errors may be made in estimating the number and importance of prey items.
As has been noted before, the food of the larvae appears to directly reflect the quantity of available prey within the habitat.
Little is known about the diurnal feeding cycle of odonate larvae. Corbet (1962), relying on the observations of Robert (1958) and others who noted greater activity of dragonfly larvae at night, suggests that this type of anisopteran larva (the type which relies upon its antennae for prey recognition) may feed preferentially at night. The large dragonfly larvae which are predominantly visual preda• tors have been observed to crawl about on the bottom at night as if they were "feeling" for food (Paulian and Serfaty, 1944).
The Coenagrionidae have a small number of ommatidia in their - 39 - compound eyes (Ando, 1957) and have therefore not been thought of as visual predators. However, since Enallagma feeds during the day in Marion Lake, there may be a significant visual com• ponent to its searching behaviour. In addition, the fact that the population appears to eat very little during the night lends some suspicion to the interpretation of the night crawling of visual species as predatory activity.
SUMMARY
1. A population of larval damselflies (Enallagma boreale) in
Marion Lake, British Columbia was sampled throughout the
year and data concerning its life cycle, growth rates,
and feeding were obtained.
2. The larvae inhabited the weedy areas of the lake almost
exclusively. 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 in the lake which did not appear in the damselfly
guts could not be easily 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 negligible. - 40 -
8. The diet of the larvae changes throughout the year, and
also varies from one habitat to another within the lake.
9. Large larvae do not have a different 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 is an accumulation of published data
(all that I am aware of) concerning the food of Odonata larvae under natural conditions. The references are listed in chrono• logical order, and have been summarized as far as possible under standard headings so that the reader may make comparisons easily. A missing heading signifies that I have been unable to locate that particular information in the reference.
The heading "Food tabulation method" specifies which method (% frequency of occurrence or % of total numbers) was used to tabulate the food items observed in the larvae examined.
These two methods do not always yield directly comparable re• sults (Table 2), hence the reason for including this heading.
For a description of these two tabulation methods, see page 21.
In summarizing many of the studies, I have condensed the data by combining groups into larger classifications and by omitting prey items present in very small numbers. - 42 -
Reference: Needham and Hart, 1901.
This early reference does not include specific observa• tions. It does, however, outline some general trends regarding the food of odonate larvae. These are:
Agrionidae: preference for Entomostraca and mayfly
nymphs. Vegetation dwellers eat backswimmers
(Notonecta), water-boatmen (Corixa), small
crustaceans such as Asellus and Allorchestes,
thin-shelled molluscs like Physa, coleopter•
ous and dipterous larvae, and the "younger
or weaker members of their own order".
Reference: Lyon, 1915.
Study side: Cascadilla 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 tabulation method: % of total 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 significant prey — Hydrachnida. - 43 -
Reference: Warren, 1915.
Study site: Various locations on the island 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 tabulation 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 significant prey — Odonata nymphs, tadpoles. - 44 -
Reference: Wilson, 1920.
Study site: Ponds near Fairport, Iowa, U.S.A.
Species observed: Anax Junius Drury, Libellula luctuosa
Burmeister, Erythemis simplicicollis 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 tabulation method: % of frequency of occurrence.
Observations:
Food type A..j . L. 1. E.s. P. 1 • Zygop.
Molluscs - Planorbis sp. 30 64 44 8 24 Physa sp. 32 12 32 8 16 Crustacea- Bosmina sp. 6 16 12 8 24 Cypris sp. 12 70 4 40 30 Simocephalus sp. 10 4 4 8 16 Unident. Entomostraca 10 42 52 30 40 Diaptomus sp. — 2 2 8 12 Unident. copepods 4 4 8 28 30 Crayfish 22 2 4 Odonata - Zygoptera larvae - - Ischnura verticalis 24 6 4 2 Enallagma sp. 6 4 10 2 — Lestes sp. 12 4 8 10 — Others 22 30 2 4 4 Anisoptera larvae 26* 8 2 — — Diptera - Ceratopogon larvae 4 6 12 — 4 Chironomid larvae — — 10 6 16 Mosquito larvae — 2 — 2 12 Simulid larvae — — 16 8 18
(Cont'd)
* This number may be an overestimate since it is 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 significant prey — Diatoms, Daphnia sp., Stonefly larvae.
N.B. Many other studies have found algae in the guts of larval
Odonata; however, this is the only reference which suggests
that algae are sometimes taken voluntarily and not merely
by accident. - 46 -
Reference: Chutter, 1961.
Study site: Jukskei River, South Africa (a polluted stream).
Species observed: Pseudoagrion salisburyense Ris.
Collection method: Sweep net.
Number of larvae examined: 147
Method of examination: Dissection of the foregut (small larvae
of head width less than 0.9 mm. were omitted).
Food tabulation method: % of total number of prey.
Observations:
% (by instar*)
Food type F F-l F-2 F-3 F-4 F-5 F-6 F-7 F-8
Chironimidae 76 58 84 60 59 48 42 18 9 Psychoda sp.** 7 1 — 1 5 3 - Ephemeroptera - - (Baetidae) 7 4 16 1 mm Cyclops spp. 3 4 — 5 15 15 37 18 22 Chydorinae 1 21 — 5 3 3 5 29 9 Oligochaeta* * * 5 12 - 27 18 24 13 29 57 Other significant prey — Macrothrix sp,
* "F" is the final instar, 'F-l' is the penultimate, and so on.
** Important in the spring.
*** Available throughout the year, but eaten mainly in winter. - 47 -
Reference: Macan, 1964.
Study site: Hodson's tarn, England.
Species observed: Pyrrhosoma nymphula Sulzer and Enallagma
cyathigerum Charpentier.
Collection method: Mainly sweep net; some with a quantitative
sampler described in the paper.
Number of larvae examined: Pyrrhosoma 192, Enallagma 142.
Method of examination: Probably dissection 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: Pritchard, 1964.
Study site: A number of ponds, ditches, and marshes in
northern Alberta, Canada.
Species observed: Ten species of anisopteran larvae.
Collection method: Sweep net.
Number of larvae examined: 801
Method of examination: Analysis of faecal pellets.
% with empty guts: None.
Food tabulation method: % frequency of occurrence.
Observations: (pooled data from all larvae). - 48 -
Food type %*
Chironomid larvae 52 Small crustacea (Cladocera and Ostracoda) 21 Coleoptera larvae 14 Molluscs 10
Other significant prey — Ceratopogonid larvae, Culicine larvae, Trichoptera larvae, Gammaridae, and Hydracarina.
* These percentages were transposed from a graph and may be slightly inaccurate.
Reference: Staddon and Griffiths, 1967.
Study site: 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 faecal pellets.
Food tabulation method: % frequency of occurrence.
Observations:
Food source Food type Oct. 12 % Nov. 9
Terrestrial 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 lacustris - Michael 25 15 - 49 -
Reference: Fischer, 1967.
Study site: Three ponds in Poland.
Species observed: Lestes sponsa L.
Number of larvae examined: 475; breakdown by ponds is given
below.
Method of examination: Dissection of gut.
Food tabulation method: % of total number of prey.
Observations:
Pond 1 Pond 2 Pond 3 Mansfeldova Zmijowa Tun Zoldanka (3 collections (8 collections (8 collections 75 animals, 200 animals, 200 animals, Apr. 24-May 14, Apr. 24-July 5, Apr. 24-July 20, Food type 1964) 1964) 1964)
Copepoda 48 27 23 Cladocera 49 39 60 Ostracoda 2 Hydracarina 1 Ephemeroptera 3 4 Diptera, Tendipedidae 29 12
Reference: Lawton, 1969.
Study site: A pond at Braside, England.
Species observed: Pyrrhosoma nymphula Sulz.
Collection method: Sweep net.
Number of larvae examined: 452.
Method of examination: Analysis of faecal pellets.
Food tabulation method: % of total number of prey.
Observations: - 50 -
Food type %
Ostracods 45 Chironomids 24 Simocephalus vetulus 9 Copepods 9 Chydorus sp. 4
Other significant prey: Chaoberus sp., Cloendipterum sp.,
Dytiscidae, Hyracarina, Oligochaeta.
Reference: Griffiths, 1970.
Study site: A bog in northern Norway (in Chara beds).
Species observed: Aeshna ,-juncea L., Aeshna caerula Strom,
and Somatochlora arctica Zett.
Number of larvae examined: A.j. 37, A.c. 29, S.a. 8.
Method of examination: Dissection of gut.
Food tabulation method: % of total number of prey.
Observations:
Food type: S.a. A.c. A..j.
Cladocera Daphnia longispina 8 5 1 Bosmina coregoni 19 1 Eurycercus lamellatus 20 31 62 Alonopsis elongata 48 26 5 Diptera Chaoberus sp. larvae 4 1 Chironomid larvae 16 11 21 Trichoptera larvae 6 Orbatid mites 8 3
N.B. These samples were taken in July. In August, Trichopt
larvae became a more important food item. - 51 -
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