Dragonflies

Dragonflies are large flying insects in the order Odonata. Dragonflies can be as large as 3 in (7.5 cm) in length, with a wing span of up to 8 in (20 cm). The fossilized remains of a huge dragonfly-like insect that had a wingspread of more than 2 ft (70 cm) is known from the Carboniferous period, some 300 million years ago.

Dragonflies are very distinctive insects, with large eyes that almost cover the entire head, a short thorax, a long slender abdomen, and glassy membranous wings. Dragonflies are classified in the suborder Anisoptera since their hindwings are larger than their forewings, and the wings are habitually held straight out when at rest. They feed on other insects, which they catch in flight.

Dragonflies are usually found around streams and ponds, where they feed, mate, and lay their eggs. The mating habits of dragonflies are conspicuous and unusual. The male generally sets up a territory over a part of a stream or pond which he patrols for most of the day. When a newly emerged female flies into the territory, the male flies above her and lands on her back, bends his abdomen far forward and deposits sperm on the underside of his second abdominal segment, which is the site of his penis. Then, grasping the female behind the head with a pair of forceps-like structures at the end of his abdomen, he flies off with her in tandem. When she is ready to mate, she curls her abdomen down and forward to place its end under the male's second abdominal segment, which has structures to hold it in place while the sperm are transferred to her reproductive tract. The pair may fly around in this unusual "wheel" configuration for several A spotted skimmer dragonfly taking a drink. Photograph by J.H. Robinson. The National Audubon Society Collection/Photo Researchers. Reproduced by permission.


minutes. Egg-laying begins within a short time, with the male either continuing to hold the female while she dips her abdomen into the water to lay the eggs, or waiting above her and then regrasping her after each egg-laying session. The eggs hatch into aquatic larval form (naiad) after a few days.

Like the adults, the wingless naiads feed on insects and other small aquatic animals. The lower lip (labium) of the larvae is retractable with jaws that can be thrust out in front of the head to catch and pull the prey back to the chewing mandibles. The naiads have gills in the last segments of the abdomen and ventilate the gills by pumping water in and out. The contraction of the pumping muscles also allows the larvae to "jet" forward rapidly out of harm's way. During the winter, the larvae live in the water, where they grow, shedding the external skeleton (molting) several times. In the spring, the larvae climb out of the water, molt again, and the newly-transformed adult dragonflies emerge and unfurl their wings.

Some 5,000 species of dragonflies are known, living in every continent except Antarctica, and on most islands as well. The principal families of dragonflies are the high-flying darners, the Aeshnidae, and the skimmers, the Libellulidae.