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Damselflies, dragonflies prey on insects



Published: Sat, July 10, 2010 @ 12:00 a.m.

Violet dancer, fragile forktail, unicorn clubtail, dragonhunter, ruby meadowhawk — though these descriptive names might conjure up images of characters in a fantasy novel, they are actually the common names of some North American damselflies and dragonflies. These insects, members of the insect order Odonata, are common wherever there’s water.

Though ferocious in appearance, odonates are harmless to people. They are, however, voracious predators of flying insects of all sizes. The aforementioned dragonhunter, which measures almost 3.5 inches, eats other dragonflies and even large swallowtail butterflies. Most odonates eat smaller insects, including deerflies, horseflies and mosquitoes.

Several differences between damselflies and dragonflies are readily apparent. Damsels are slim and dainty; at rest they fold their wings and hold them above their back. Dragons are bigger and heavier; at rest, they hold their wings outstretched and flat like a moth.

Though adult odonates are “good” insects because they eat lots of flying in-sects, their primary purpose is to reproduce. Most species live as adults for only a few weeks or months. They spend most of their lives, sometimes years, as aquatic larvae called nymphs.

Midsummer is a great time to observe odonate mating behavior. Thanks to the male’s unique genitalia, mating is a multistep process. First, the male transfers sperm from the tip of his abdomen to an “accessory” organ on the second or third abdominal segment. Then he grabs a receptive female by the head with claspers at the tip of his abdomen.

At this point, the pair is in “tandem,” and the male tows the female in flight. To complete the act, the female bends her abdomen beneath the male and wraps her legs around the male’s abdomen. She then bends her abdomen upward until the tip touches the male’s accessory organ. This is called the “copulation wheel,” and sperm is transferred to the female for fertilization of her eggs.

Next time you visit a body of water, watch for dragonflies in the tandem and wheel positions. What may have once seemed to be bizarre behavior will now make perfect sense.

Shortly after mating, females lay their eggs directly in the water or in another moist habitat. Some species drill holes in plant stems and deposit their eggs inside the moist plant tissue. Most, though, lay their eggs in water or saturated soils. Upon hatching, nymphs embark on a completely predatory lifestyle.

I first learned about the carnivorous nature of dragonfly nymphs when I was a boy. I collected an assortment of aquatic invertebrates at a nearby farm pond and placed them all in a large jar. The next day only one dragonfly nymph remained. It had eaten everything else in the jar. After consulting an encyclopedia (remember those?), I learned about the feeding habits of larval dragonflies.

Dragonfly nymphs are well-armed predators. Their lower jaw (the labium) is more like an extra arm; it is hinged at rest, folds under the head, and is tipped with two sharp pincers. When a food item is detected, the nymph explosively extends the labium, and the pincers impale and grab the prey. It then returns the labium to its resting position, and the food is consumed.

Odonate nymphs are opportunistic predators — they eat whatever is available. Larger species, which can reach lengths up to two inches, take small fish and tadpoles.

Depending on the species, nymphs can be active or passive predators. Active species prowl aquatic plants in search of prey. Passive species ambush their prey. They hide in vegetation or bottom sediments until an unsuspecting prey item passes by.

As adults, odonates are equally predatory. Any flying insect is fair game. Thanks to a pair of enormous compound eyes and incredibly aerodynamic wings, they are masters of aerial predation.

Dragonflies use their keen vision to detect flying prey, and their four wings operate independently to give them amazing agility. They can fly forward, backward, hover, and make quick zigzag turns. Nothing flies like a dragonfly.

To actually capture prey, dragonflies can take them in flight, a behavior called hawking. Or they can surprise and grab a perched prey, a behavior called gleaning.

Send questions and comments to Dr. Scott Shalaway, RD 5, Cameron, WV 26033 or by e-mail via my Web site, http://scottshalaway.googlepages.com.


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