This issue of Wings is devoted to the order Odonata, dragonflies and damselflies, consummate predators and spectacular aerial acrobats.
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Our cover photograph shows a twelve-spotted skimmer (Libellula pulchella), easily identifiable by the marking on its wings. Photographed in Illinois by Robert A. Behrstock. |
Dragonflies and Damselflies: Bugs of Prey The damselflies and dragonflies that form the order Odonata vary greatly in size, color, habitat, and behavior, yet they have one thing in common-they are all consummate predators. Every species of odonate eats other animals: there's not a vegetarian among them. This is true for both the adults and the larvae. In this they differ dramatically from the Lepidoptera (butterflies and moths), almost all of which are leaf-eaters as larvae and liquid-feeders as adults. Dragonflies and damselflies are supremely adapted to be predators, with huge compound eyes; the largest species has up to 12,000 simple eyes (ommatidia) in each compound one. Odonates eat just about any arthropod their size and smaller, and they take their prey from just about everywhere. I should remark at the outset that we know nothing at all about the
foraging behavior and diet for the great majority of species, and this
includes whole families of tropical damselflies. It is a shame that
odonatologists have not made a concerted effort to study thoroughly
at least one species from each family, but with the publication of Philip
Corbet's book, Dragonflies: Behavior and Ecology of Odonata, it is now
much easier to see where further study might be most revealing. The order Odonata includes 5,600 named species, and is divided into two suborders. For this discussion, "odonates" refers to all members of the order, "dragonflies" to the suborder Anisoptera, and "damselflies" to their smaller cousins from the suborder Zygoptera. scroll down for complete article |
Odonates exhibit a lot of behavioral dichotomies; often, there are two distinctly different ways of doing things. For example, males search for females from a perch or in flight, and females oviposit either endophytically (the eggs inserted into plant tissues) or exophytically (the eggs laid free in the water). Some of this variation is related to the existence of two rather different suborders, but many of the dichotomies cross damselfly/dragonfly lines. These primary differences can be seen clearly in their foraging behavior. The basic divisions involve where prey is captured, either in the air or on the substrate, and whether foraging bouts start in flight or from a perch. When you combine these two divisions, you actually end up with a trichotomy: three different foraging styles.
The first foraging mode involves both predator and prey being in flight (the
parallel in the avian world is the "swallow" mode). It is typical
of larger, long-bodied dragonflies and is easily seen in darners (Aeshnidae)
and emeralds (Corduliidae). Many species of these families participate in
feeding swarms. They are attracted to areas where small insects such as midges
are themselves swarming. A "swarm" is probably nothing more than
a local concentration of dragonflies taking advantage of abundant prey. Most
of the species that forage this way take prey too small for us to see, but
you can often tell when a prey item has been captured, as the dragonfly slightly
interrupts its smooth flight
| The second foraging mode involves the predator perched and watching for flying prey, then leaving the perch to capture it (the "flycatcher" mode). This is used by some, perhaps many, damselflies, and is characteristic of most skimmer (Libellulidae) and clubtail (Gomphidae) dragonflies. This type of predation is very easy to see. Just watch a dragonfly perched in the open on a nice sunny day, and you will see it turning its head, presumably scanning for potential prey moving past. Of course if it's a male in its territory, it might be scanning for passing females instead, but most individuals are probably doing both; dragonflies are efficient in that way. Recent research has shown that dragonflies that fly after passing insects actually aim for where the prey is going to be when their flight paths intersect, a rather sophisticated sensory setup. |
Damselflies use similar hunting strategies as their larger cousins, the dragonflies. When perched, damselflies, unlike dragonflies, hold their wings above their bodies, as shown by this bluet (Enallagma boreale). Photographed in Washington by Netta Smith. |
Dragonfly eyes are so sharp, and their flight so competent, that it's easy to understand their ability to capture fast-moving aerial prey. But some of them instead choose a third foraging mode, flying slowly through vegetation and snatching insects from the substrate (this could be called the "kinglet" mode, named after birds that take prey from the vegetation while in flight). This is practiced primarily by damselflies (although not by all of them), and only occasionally by dragonflies. It is well known in the pond damsels (Coenagrionidae) and is doubtless the primary foraging strategy for many species in that family. The kinglet mode is not only interesting but also amusing to watch. Sometimes, a bluet or forktail flies slowly along and darts at a tiny dark object on a leaf, only to bounce off a spot of fungus. The contrast between potential prey and substrate isn't always a reliable cue, but it's the best they've got. One would think depth perception was an important component of this foraging mode, but, although damselflies have widely separated eyes-perhaps enhancing their depth perception-they aren't always successful in the hunt.
A specialized example of this mode of predation is shown by the "helicopter damselflies" (family Pseudostigmatidae). These are among the largest odonates, with very long wings and bodies, and they fly slowly through Neotropical forests apparently searching for the webs of orb-weaving and other spiders. When they find one, they are likely to orient toward the center of the web and pluck the spider from it. They often hover for several seconds, perhaps judging the size of and distance to the spider. Or are they mesmerizing the spider? Years ago, it was reported that spiders that made no move when approached by a "helicopter" would immediately retreat if approached by a human!
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Dragonflies capture insects in flight using a "basket" formed by their legs. After a successful flight, this female clubtail (Ictinogomphus clavatus) is eating a cicada. Photographed in Kyoto, Japan by Dennis Paulson. |
How about prey size? Most odonates take prey much smaller than themselves. Tiny flies, beetles, aphids, and leafoppers figure prominently in their diets, even among quite large species. Prey of this size is consumed in flight by the species that remain in flight to feed, but larger prey is quickly taken to a perch. A relatively small number of odonates specialize in large prey. This may be true in general of clubtails, which have relatively large mandibles and powerful jaws; they are among the dragonflies that call for a little extra care in extricating one from a net, as the larger species can give a painful nip. The largest clubtail in North America, the dragonhunter (Hagenius brevistylus), specializes in other dragonflies, and one of its common prey species is the ebony jewelwing (Calopteryx maculata), a lovely black-winged, iridescent green damselfly. Jewelwings are conspicuous and slow flying, and the swift-winged dragonhunter is able to capture them with ease. Pondhawks (Erythemis), a New World genus of brightly colored skimmers, are specialized for taking large prey, including dragonflies up to their own size. If you look closely at one, you can see that the spines on the basal leg segments are especially large, and these are presumably used in prey capture. |
Of course, we must not forget that the larvae are also accomplished predators. While adults capture prey in their spiny legs and immediately transfer it to their mandibles, the larvae have neither wings to chase down prey nor long legs with which to capture it.
Instead, they have a specialized mouthpart, the labium, that is large and hinged. Hydrostatic pressure shoots this "killer lip" forward from under the head; at the tip are a pair of labial palpi (small, finger-like projections on the labium) that open and then close on the prey and capture it. Being aquatic, the larvae prey on just about anything in the water smaller than themselves, including many kinds of small crustaceans and insect larvae.
The labia of odonate larvae are of two sorts, again contributing
to a dichotomy in feeding modes. Darners, clubtails, and damselflies, among
others, have a flat labium with narrow labial palps, typically with a hook
on the end with which prey items are pierced. Labium length varies greatly,
even within a genus. For example, in mosaic darners (Aeshna), the labium of
the longest-lipped species can be three times the length of that of the shortest-lipped.
Right now we don't have a clue about the adaptive significance of these differences,
other than that they may indicate the average distance at which the larva
strikes at prey. It is an amazing thing to see a darner larva stalking its
prey, which may be as large and elusive as a tadpole or small fish. The dragonfly
advances slowly, step by halting step, just like a cat stalking a mouse. When
it is within range, the labium shoots out, almost too rapidly to see, and
the prey is suddenly back at the mouth being chewed up. Movement is an important
stimulus, but it has been shown that these larvae will stalk absolutely motionless
prey, apparently recognizing them by shape.
The other type of labium is spoon-shaped, with the basal part concave and
the palpi much expanded and furnished with long bristles. Dragonflies with
this labium-which include skimmers, emeralds, and spiketails (Cordulegastridae)-engulf
their prey rather than grabbing it with precision; they can afford to be a
bit sloppy in their aim, but are equally effective predators as their more
accurate relatives. Many members of this group are sit-and-wait predators,
well camouflaged among the bottom detritus and alert for any passing critter.
All animals must eat to live, and dragonflies do it very well indeed. They
are among the most effective predators natural selection has designed, and
there would be no better way to understand the diversity of insect life than
to spend a day watching both butterflies and dragonflies, larvae and adults,
as they search for their very different forms of sustenance.
About the author
Dennis Paulson is Director of the Slater Museum of Natural History
at the University of Puget Sound, where his general interests focus on biodiversity
and biology of dragonflies and birds. He has written many academic papers,
field guides, and books on general natural history.
Other articles in this issue include:
-"Spiketails," by Sidney W. Dunkle
-"Hawaiin Damselflies: Insular Diversity at Risk," by Dan A. Polhemus
-"The Endangered Hine's Emerald Butterfly," by Dan M. Johnson, Daniel
A. Soluk, and Diane Debinski
-"Discovering More About Odonata," by Michael Bean
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