by Beth Fetterley, Director of Education, Urban Ecology Center
Sometimes called “mosquito hawks,” dragonflies are my favorite insects to observe. On a sunny afternoon, in a field not too far from water, these insects can be seen eating mosquitoes, moths, flies, grasshoppers, and even other dragonflies! In the insect world, they are the master hunters. Outstretched legs form a basket while four wings, beating independently, aim the “basket” and solid jaws toward its prey as the dragonfly emerges from the water. Strong flight muscles glisten in the sun, reflecting red, blue, yellow, or green. They dragonfly’s multifaceted eyes make escape unlikely. Evidence of the dragonfly’s presence on earth, determined by fossil records, dates back 300 million years. Dragonflies are in the order Odonata, meaning ‘toothed jaws.’ There are two suborders of Odonata, dragonflies (Anisoptera) and damselflies (Zygoptera). Damselflies are distinguished from dragonflies by the position of their wings at rest, wing shape, and location of gills in the nymph stage. Immature dragonflies, called nymphs, live in the water. Their gills are located inside the tip of the abdomen. They draw water in to the rear of the abdomen in order to breathe. To jet-propel their bodies forward, they then can quickly expel the water. Damselflies have external gills, which look like feathery tails, at the tip of the abdomen. Both the nymph and the adult are avid predators. The nymph has a strong, foldable bottom lip, or libellum. While keeping its body still, the dragonfly nymph can extend its bottom jaw to grasp its prey. Immature dragonflies eat aquatic insects, as well as tadpoles and even small fish. As adults, dragonflies catch and eat their prey in flight. Both odonates go through several instars, or stages between molts. Each time the insect molts, it looks more and more like the adult. This process is called incomplete metamorphosis. Complete metamorphosis occurs in butterflies, for example, where the caterpillar looks very different from the adult butterfly. When the dragonfly reaches its last nymphal stage, it climbs out of the water, up the stalk of a plant or onto a rock. It then loses its skin, taking on its adult form. If you look carefully on plants along the shore of a river or pond, you may find the skin, or exuvia. The first few hours of its adult life are tenuous. While its wings dry, the dragonfly is vulnerable to predators. After a few days, however, it is ready to hunt again. A dragonfly depends on its flying talent to catch prey and to mate. A dragonfly’s body is shaped for flight. The thorax, where the wings attach, contains the dragonflies’ thickest and heaviest muscles that are used for flight. The legs are tilted forward, making walking nearly impossible. The structure of the legs, however, is perfectly adapted for perching and for catching flying prey. Finally, the wings’ ability to flap independently allows the dragonfly to hover, flutter, zoom, and even fly backwards! Perhaps because of their strength and boldness, dragonflies have earned a bad reputation among some humans, though they do not sting or bite people. Next time you see one, watch carefully. Count the colors on its thorax, tail, wings and head. If you watch long enough, you may even see a bizarre mating behavior, which happens (of course) in flight. To find out more about the dragonflies in Wisconsin, check out the Guide to Common Dragonflies of Wisconsin by Karl Legler, Dorothy Legler and Dave Westover and the Urban Ecology Center. This book is filled with color photographs, identification tips, and life history information. You can contact the Urban Ecology Center at 964-8505 or e-mail uec@execpc.com. Riverwest Currents – Volume 1 – Issue 4 – May 2002