Insect external structure
Insect internal structure
Development of insects
Relationships of insects
Damsel fly picture
Insect identification > Odonata > Dragonfly
There are two groups of dragonflies. In one the insect is slender, the two pairs of wings are of about equal size and when not in use are held almost vertically above the body. These insects are often called damsel flies. In the other group the body is stouter and proportionally shorter, and the wings when at rest extend out horizontally at the sides of the body.
The bodies of dragonflies are often brilliantly colored and in some cases covered with a "bloom," giving them a whitish appearance. The adults feed on almost any flying insects smaller than themselves which they may capture during their flight. Flies and mosquitoes form a favorite food, and the attempt has been made to "tame" dragonflies and keep them in houses on this account, but without success. They are very voracious, one specimen having been known to consume 40 house flies in less than 2 hr.
Many dragonflies fly very swiftly either in direct lines or making sudden changes of direction while hunting their prey and are perhaps unequalled in this regard by any other insects. They also mate in the air. The eggs are laid either in the water, attached to water plants, or in the stems of plants under water. In the latter case they are laid singly but otherwise they are usually in clusters containing either a small or a large number of eggs. The eggs may hatch after a few days or, if laid in the fall, may not produce nymphs until the following spring. The young nymphs stay at the bottom of the water and are carnivorous, feeding on larger and larger animals as they grow, individuals of the largest species attacking small fish in some cases, though the bulk of their food is undoubtedly the aquatic larvae of insects. They lie on the bottom waiting for their prey to come within reach, and when it is near enough they thrust out the underlip (labium) and seize it (Fig. 42). This labium has been remarkably developed from its usual form, being drawn out into two long pieces with a pair of jaws or claws at the end. When not extended the piece connected at one end with the head is bent backward under the body, while the second piece, hinged to the other end of the first, extends forward so that its front end with the jaws lies near the front of the head, which it somewhat conceals, and this has led to calling the structure a "mask." When this is extended forward it reaches out more than twice the length of the head, thus enabling the nymph to capture animals which are not very close to it.
Breathing in the nymphs of the damsel flies appears to be, in part at least, by means of long and rather large, tracheal gills at the end of the abdomen, which are also used for swimming. In the other section of the order, the gills are found in the rectum, into which water is drawn, bathing the gills there, after which it is expelled, and if this is done quickly the recoil carries the nymph forward, thus providing one means of locomotion.
Molts are frequent, and when full-grown the nymph crawls out of the water and molts for the last time, whereupon the wings grow to full size and the adult insect is produced. Some dragonflies have two generations a year or possibly even more, while in other cases more than a year is necessary to a generation, but one each season is the usual condition.
Despite tradition and their bad reputation, dragonflies are in no way injurious to man, not stinging - they have nothing to sting with nor biting to such an extent as to cause the slightest pain, their jaws being too weak even to break the skin. They are beneficial insects as both young and adults, because so much of their food consists of injurious insects such as flies and mosquitoes, while the injury they cause by feeding on fish is usually so slight as to be negligible.
Dragonflies are sun-loving animals, concealing themselves during dark, cloudy weather. Over five thousand kinds are known, and the greatest number of these occur in the warmer regions. Fossil dragonflies or insects resembling them are numerous, and some of them were very large, one measuring more than 2 ft. from wing-tip to wing-tip.