Abstract

Dragonflies and damselflies have evolved into some of the best equipped hunters among the invertebrates. The adult Odonata catch smaller insects on the wing and are so highly adapted as aerial predators that they no longer use their legs for walking, although they are able to cling to plants and to some extant to climb about them (Evans 1968). Dragonflies are fairly unique among insects in relying almost entirely upon their compound eyes for tracking and capturing prey. Experiments have shown that removal of antennae from dragonflies does not effect their navigation or capture of prey (Evans 1968). The legs of Odonata members are covered with spines that form a basket to capture prey. The larvae of Odonata are specialized predators as well as the adults. The larvae possess a unique prey-catching lip that can be thrusted forward and bring prey back to the mouth with terminal hooks .

     Odonata species are designed for a predatory life style. Both the adults and larvae have evolved important feeding behaviors to accompany their physical attributes. One of the major activities of adult Odonata is foraging (Mayhew 1994). When not engaged in territorial behaviors or mating, the winged adults forage great distances to prey on insects. In the water, the larvae are voracious predators consuming multitudes of insects and even small fishes (Evans 1968). The majority of Odonata larvae are sit and wait predators, where as the adults actively search out and capture prey. As generalist predators, feeding behavior is limited by feeding opportunities, environmental factors, metabolism and influence of competing activities (Mayhew 1994). This paper will focus on how the different morphologies of adults and larvae relate to feeding behaviors, prey selection, and foraging activities in Odonata.

Introduction

The Odonata, commonly referred to as dragonflies and damselflies, make up a relatively small order of insects, comprising about 5000 species and 23 families world-wide. The order is divided into three suborders, the Zygoptera (damselflies), the Anisoptera (dragonflies), and the Anisozygoptera, a small group not found in North America that is intermediate in appearance between the other two groups (Danks 1997).  The species within Odonata are among the most ancient of insects, and thus exhibit many primitive features, which reflect their specialized predatory lifestyle (Westfall and May 1996).  The chewing mouthparts with sharply toothed mandibles and maxillae, independently movable wings and enlarged compound eyes are all specialized for feeding in adults. The larvae are also well equipped for prey capture with a modified labium that can be thrust forward to grasp prey. The physical specialization’s in Odonata for capturing prey influence the feeding behavior. The adults are well adapted  for active foraging, catching insects in flight, and having large feeding ranges. The larvae have evolved specialization’s for aquatic lifestyles, where they use cryptic coloration and ambush their prey.

     Feeding behavior can be broken down into several components. Locating food and selecting what to eat are usually the first items looked at when examining feeding behavior. Capturing  the prey item and consuming it are integral parts as well (Alcock 1998). Within the components of feeding behavior, other behaviors can be looked at. How an organism processes the food, where feeding takes place, solitary verses group feeding strategies and frequency of feeding all can be discussed within the context of animal behavior.

Discussion

  When examining feeding behavior in Odonata, the role of specialized morphology should be discussed. The adult possesses physical characteristics that enable these winged predators to capture their prey. The mouthparts of Zygopterans comprises the mandibles, which are stout, wedge-shaped, and heavily sclerotized, the inner bearing four basal molar teeth. Three sharp incisor teeth are connected to the inner bearing and the maxillae each have five or six terminal teeth  (Westfall and May 1996). Adult Anisopterans have a similar chewing mouthpart system that effectively devours their prey.

      All adult members of Odonata have large, complex,  compound eyes.  The eyes of Zygoptera are less specialized then Anisopterans. The eyes in Anisoptera respond well to jerky movements and to small moving objects (Westfall and May 1996). Studies have been done that show strong evidence for eyesight as being the most prevalent of the senses while catching prey. The facets in the eyes face upwards, downwards, forward and even backwards (Evans 1968). The remarkable eyesight allows the flying adults to detect and capture other winged prey.   The spiny legs are used to form a basket that captures flying prey, or the enlarged mouthparts grab their prey in flight (Merrit and Cummins 1996).
 
    Immature Odonata differ greatly in morphology from the adults and are entirely aquatic. The mandibles and maxillae are generally similar to those of the adult, but the upper labium is highly modified for prey capture. It consists of a doubly hinged arm, folded beneath the head and covers the other mouthparts at rest. The modified labium is capable of being thrust forward by hemolymph pressure coupled with release of muscular tension to grasp prey with a pair of toothed lateral lobes (Westfall and May 1996).  Feeding behavior  in the larval stage has been described in three phases: (1) detecting prey and adjusting position in relation to it; (2) ejecting the labium to grasp the prey; (3) employing the mandibles to devour the prey (Merrit and Cummins 1996). The labium, when extended, is approximately as long as the fore legs. The labium is designed for grasping and holding the prey while the maxillae and mandibles cut and manipulate it for ingestion. The labial mechanism requires enough room to reach out for prey and is occasionally entangled in algae and other plants (Merrit and Cummins 1996). The morphology of adults and larval Odonata play a very important role in prey selection.

     Prey selection in Odonata can be again broken up into adult and larvae stage. The terrestrial adults prey upon various invertebrates. A study in Siberia by G.A. Sukhacheva of 35 species of dragonflies revealed a diverse prey selection. The study found that at the beginning of the season a majority of dragonflies feed on small dipterans. However, with the rise in population sizes of larger prey items were also included in their diet. The study also indicated a preference by dragonflies for Chironomids over mosquitoes that was associated with the different behaviors of mosquitoes and Chironomids during daylight hours. Marked differences in the time of swarming of mosquitoes and Chironomids also affected dragonfly foraging (Sukhacheva 1996).

     Mosquitoes comprise an important part of the adults diet, but also prey on midges, small flies, butterflies, bees and even other dragonflies (Nolnar and Lavigne 1979). J. Rolff and C. Kroger studied cannibalism in adult Odonata. They found that in adult Zygoptera,  cannibalism occurred only during periods of cold weather, and hypothesized that intraspecific predation is a switch in food selection due to bad weather condition (Rolff and Kroger 1997). As generalist predators, the adults prey selection appears to be dependent on prey availability.
 
     The larvae of Odonata play an important role in aquatic food chains. They prey upon any aquatic animal of appropriate size that they can capture. Prey items include invertebrates, fish and other aquatic vertebrates (Merrit and Cummins 1996). Laboratory studies have shown that despite their varied diet, they often show selective tendencies towards prey species and prey size. A study of Enallagma cyathigerum  revealed that they select small or large mosquito larvae in proportion close to the predicted maximum net rate of energy intake (Westfall and May 1996).  Prey selection often fits into classical models  of optimal diet. A study of diet  in Anisopteran larvae from Australia by J.H. Hawking and T.R. New, correlated abundance of prey reflected abundance in the habitat. Differences between species  reflect habitat specificity or segregation rather than prey selection (Hawking and New 1995). Dominant prey species in this study were Diptera: Chrironomidae larvae and Ephemeroptera larvae. Chironomid larvae were found to have conspicuous movements, convenient size and could be captured easily. Prey selection is probably most dependent on abundance and accessibility (Hawkins and New 1995). J.M. Baird and M.L. May studied prey selectivity in pachydiplax sp. , and found that they discriminate  against very small prey, < 1 mm length (0.01 mg dry mass). Large prey may, however, be costly to obtain due to handling time being longer for larger prey . Their studies showed that Pachydiplax can simultaneously search for and handle small prey (Baird and May 1997).  There is a well defined relationship between dietary components and habitats for Odonata larva.
 
     Detecting prey varies from adults to larval stages in Odonata. The adults main sensory organs for detecting prey are their eyes. The active foraging hunters have to rely on their eyesight for prey detection. The removal of antennae have shown experimentally to have little effect on navigation or prey capture. The reliance on eyesight is shown by their hunting style. Hunting in mid flight for these insects is more dependent on their vision then smell or touch (Evans 1968).  The aquatic existence of the larvae has brought about different prey detection mechanisms.

      Larvae of damselflies detect prey primarily by using their antennae, but in some species vision plays a major role (Westfall and May 1996). Studies by Gordon Pritchard at the University of Calgary, Alberta, found that dragonfly larvae will not strike at crushed insects even when hungry. It can be assumed that odor for prey recognition is not used by the larvae (Evans 1968). In many cases, touch and vision play a major role. The larvae of Libellua if kept in total darkness with various prey will eventually capture most of them. Green darner larvae, however, if kept in darkness are able to catch nothing even for a period of days. Experiments with the green darner show that with their eyes blacked out they soon starve to death, and need both eyes to orientate properly to their prey (Evans 1968). The majority of larvae are sit and wait predators while others actively pursue their prey. Merrit and Cummin’s classify Odonata larvae as sprawlers or climbers. The sprawlers are active hunters and have longer legs. Numerous setea give them a hairy appearance and particles of mud and silt adhere to them for camouflage. The setae also serve as detectors for prey. Climbers hide in vegetation where the stalk their prey, halting  their pursuit when the prey stops moving. Some species have varying forms of coloration that blends them into the surroundings (Merrit and Cummins 1996).  Vision is very important for the larvae. In later instars, the larvae fixate prey at a distance of a few centimeters then visual track or stalk the prey until it is in range of the labium (Westfall and May 1996).  Larvae of all species have anterior areas with small angles where binocular overlap of the fields from both eyes. These areas of high acuity focus near the point where prey are grasped  during labial extension, and represent an adaptation for accurate orientation of the strike (Westfall and May 1996). Hawkin’s and New found that the majority of Zygoptera larvae collected at different daylight hours  had  food in their guts, and that 11% collected had empty guts (Hawkins and New 1995). Aggression in larvae Zygopterans sometimes leads to cannibalism. Predation is one of the principal factors affecting population and community structure of Odonata (Westfall and May 1996).
 
      Foraging constitutes one of the major activities of adult Odonata. In two species of Japanese Odonata, it was found that they consumed 10-12% of their own weight in food each day (Westfall and May 1996). Hyliophilic species feed more rapidly because of high body temperature. Zygoptera feed either on aerial prey or by gleaning small insects from foliage. Swarm feeding is rare in Zygoptera, but occurs occasionally (Westfall and May 1996). Foraging behavior is affected by seasonal changes, sex, and time of day. A few species of Odonata that feed on mayflies, only forage at dawn or dusk (Merrit and Cummins 1996). During the breeding season, food intake is reduced as energy spent foraging is transferred to breeding. Females accumulate considerable mass before returning to reproductive sites. P. J. Mayhew found that differences between sexes were small in comparison with total variation in feeding activity, but female feeding activity did increase until reproductivily mature (Mayhew 1994). Mayhew also found that feeding in Calopteryx splendens, was primary between 10:00 - 16:00 h; times that probably present the optimum environmental conditions for flight. The time of day spent foraging is often associated with the activity of prey species.
 
Conclusions   Feeding behavior is widely varied between adult and larvae Odonata. The morphology in these insects plays a vital role in prey selection and foraging activities. The adults are specialized with chewing mouthparts that can catch and devour prey while the animal is in flight. The adults are also have modified legs that can form a basket to catch prey. The remarkable eyesight in Odonates is the primary sense used to track down their prey. The larval Odonates have evolved physical mechanisms that allow them to be superb predators. Their ability to thrust a modified labium to spear prey,  gives the larvae the ability to feed on any aquatic animal of appropriate size that it can catch.

     Prey selection in Odonata is varied as these insects are generalist predators. Adults tend to prey upon selected species that form swarms that the dragonfly can fly through eating as many as possible. Mosquitoes and Chironomididae sp. Appear to be favorites. The Odonata larvae show less active selection in prey, any organism they can capture and eat makes up their diet. There is evidence that the larvae might discriminate against (certain sized ) small prey. This fits into the optimality feeding model. There is also evidence that smaller prey might be taken more frequently, because of a cost in handling larger prey in terms of time spent capturing the prey and eating it.

     The adult Odonata are active hunters, Foraging over great distances to capture food. The foraging times of adults appears to correlated with peak times of prey swarming. The larvae can be described as sit and wait predators that ambush their prey or  in some species active hunters. The larvae use sight and touch to detect prey. There is an apparent relationship between prey selection and prey items isolated by habitat confines.

      The behavior of Odonata can be looked as how they locate food, prey selection, how they capture food, and how they  consume their prey. These “primitive” insects have evolved many morphological and behavioral traits that allow them to be top invertebrate predators in their ecological niche.
 
 

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