Insect guide > Dragonflies

Dragonflies


Dragonflies(Order Odonata.)
Excepting the butterflies, there are few more attractive and graceful insects than the "dragonfly", as the members of this order are generally termed. They are insects which have always attracted attention, and which are known by a variety of vernacular names, of which dragonfly is the commonest English term.

They are known in some parts of the country as "devil's darning needles" ; elsewhere as "snake feeders" or "snake doctors" ; in Scotland as "flying adders", and in some parts of England as "horse stingers". Although the insects are perfectly harmless, these names well indicate the existence of numerous popular superstitions. Some believe that they will sew up the ears of bad boys; others that they sting horses; still others that they act as feeders and physicians to snakes, especially to water snakes.

The Odonata are slender insects with a very large head which moves most easily upon its slender neck, even rotating to a
considerable extent. The eyes are very large, but the antennae are small and short. The wings are elongate, nearly equal in size, and have many veins, both longitudinal and transverse, so that the entire surface of the wing is cut up into many small cells. The legs are placed near the front of the thorax, and all curve forward and are used for grasping the prey of the dragonfly, and never for walking. In fact, the legs are unfitted for walking, although they are used to grasp the twig or other object upon which the dragonfly may rest.

All of the dragonflies are aquatic in their early stages. The metamorphosis is complete in so far that the larva differ radically in appearance from the adults, but the pupa is not quiescent at any time. It is very active, and feeds up to the moment when the final metamorphosis begins. The jaws in all stages are strong, and both larvae and adults are extremely active and are among the strongest and most graceful flyers of all insects. Their flight is so perfect that it has been seriously suggested that flying machines should be modeled after the flight mechanism of these insects.

A very peculiar feature of the adult is the curious separation in the male of the intromittent organs from the opening of the ejaculatory duct. The former are placed on the under side of the second abdominal segment, while the latter are on the next to the last ventral plate. Therefore, before copulation, the male curves his abdomen around beneath, so that the ninth segment of the abdomen is brought into contact with the second, thus transferring the fertilizing fluid to the intromittent organ. The tip of the abdomen of the female is bent around and joins with the under side of the second segment of the male's abdomen, the male frequently grasping the female around the neck with certain appendages at the extremity of his abdomen. He retains this hold after fertilization, and frequently during the entire process of egg-laying. Even with such species as descend under the surface of the water to lay their eggs the male has been observed to still retain his grasp of the female's neck, and to be carried down under the water with her.

Dragonflies capture their prey on the wing, and feed upon almost all flying insects, especially the small ones - that is, the gnats and midges. They alter their direction with perfect facility, and dart here and there, unerringly capturing their prey. Possibly some of the smallest are seized with the jaws, but the larger ones are undoubtedly captured by the legs, and are consumed during flight, and so rapidly is all this done that it is practically impossible to see the operation. The only way, in fact, that one can know that an insect has been captured is, as Dr. Needham expresses it, to see that the place that once knew them knows them no more.

Flies seem to be their commonest food, but large dragonflies will eat small ones. Leafhoppers and even small butterflies and moths are captured by them. Some forms will occasionally pick up a moth from a weed or a grass stem on which it is resting, and even one of the large swallow-tailed butterflies has been seen captured by a dragonfly, while Williamson states that he once saw one holding a large wasp in its jaws. The voracity of a large dragonfly may easily be tested by capturing one and holding it by its wings folded together over its back, and then feeding it live house flies. I should hesitate to say how many it will accept and devour, as I never tried one to the limit of its capacity. Beutenmueller found that one of the large ones would eat forty house flies inside of two hours, while a smaller one ate twenty­five in the same time. It is an odd fact that a dragonfly will eat its own body when offered to him. Even when insufficiently chloroformed and pinned, if one revives, it will cease all efforts to escape if fed with house flies, the satisfying of its appetite making it apparently oblivious to the discomfort or possible pain of a big pin through its thorax. There is one record to the effect that a dragonfly has been observed feeding upon the flesh of a dead reptile.

Although dragonflies are frequently very abundant in swampy regions and about ponds, there are times when they swarm in enormous numbers. Koppen, a German entomologist, has published a chronological account of the records of dragonfly migrations, from 1494 to 1868. Such migrating swarms seem to have been more frequently noticed in Europe than in this country, but several have been noticed in the United States. For example, Mr. A. H. Mundt, of Fairbury, Illinois, says that between the hours of 5 and 7 P. M., August 13, 1881, "the air for miles around seemed literally alive with these dragonflies (Aeschna heros) from a foot above ground to as far as the eye could reach, all flying in the same direction, a southwesterly course, and the few that would occasionally cross the track of the majority could all the more easily be noticed from the very regular and swift course they generally pursued; but even these few stray ones would soon fall in with the rest again. Very few were seen alighting, and all carefully avoided any movable obstacles." This migration was probably caused by the very dry season which had resulted in the drying up of ponds and swamps, and it is probable that other similar recorded migrations have arisen from the same cause.

Among the insects killed by dragonflies there must be, of course, some mosquitoes, although the beneficial work of these insects in this direction is greater in the larval stage than in the adult. Dragonflies are day flyers, but in cloudy weather and toward evening many mosquitoes are undoubtedly killed by them. Dr. E. A. Mearns, U. S. A., (quoted by Beutenmueller) states that at Fort Snelling, Minn., mosquitoes appeared in vast swarms, and were soon followed by large numbers of dragonflies after which the mosquitoes were considerably reduced in numbers. Dr. Robert H. Lamborn, noticing in the Lake Superior region the activity of dragonflies in this regard, years later offered a prize for the best essay on the artificial multiplication of dragonflies for the destruction of mosquitoes and house flies.

The eggs are laid either in the water or are inserted in the stem of some aquatic plant. In the dragonflies of two families there is no apparatus for the insertion of eggs into plant stems, and they are therefore either dropped loosely in the water or attached to submerged objects by means of a mucilaginous substance which surrounds them. With others, however, there is a curious modification of the end of the body. The sides of the vulva are pointed or roughened, and cut into plant tissue so that the eggs may be pushed into the cuts. Here the female gradually crawls down the stem of a water plant until she is often completely submerged. She is always incased with an air film so that she can continue to breathe under water to a certain extent. The number of eggs is variable, but is usually large. Some very curious Hymenopterous parasites live in the eggs of dragonflies.

When the eggs hatch the young immediately begin an active, predatory life under the water, feeding upon other aquatic insects. This food habit is continued throughout their larval or nymphal existence, and as they grow larger they are able to overpower larger and larger insects and even small fish and other aquatic animals. They will kill others of their own kind, and nymphal dragonflies have been seen to catch and destroy adults in which the wings were not yet expanded. They molt probably a number of times, but the exact number has not been recorded for any species, so far as I know. The most peculiar feature of the larva or nymph is the strange modification of the mouth. There has been a backward growth of the lower lip and this has become hinged so as to form a long, hinged apparatus with sharp teeth at its extremity. It can be folded to cover the lower face like a mask. The structure has been called a mask, and when it is folded the head of a dragonfly larva seen from the front looks like that of a bulldog. It is more innocent looking than that of a bulldog simply because we know what a bulldog can do, but the moment that the larva approaches near enough to its prey the innocent looking mask is unfolded and darted out, and the probably unsuspecting aquatic insect or small fish is seized by the teeth at the extremity and drawn back into the mouth.

Dragonfly larva breathe in a peculiar way modified to some extent with the members of the different families. The rectum is furnished with very many tracheal branches, forming numerous loops and even penetrating the walls of the intestine. Water is sucked into the rectum, and these "rectal gills", as they are termed, derive their oxygen from this water. This same feature affords with some species a means of locomotion, for this water which is sucked in for breathing purposes may be ejected violently, the effect of which is to send the larva ahead. This same principle has been used in certain mechanical toys, and applications have even been made for patents on a similar method of propulsion for vessels. With some there are external abdominal gills, both lateral and caudal, while when the nymph becomes full grown and is ready to leave the water breathing is taken up by certain obscure spiracles. There is still some doubt as to the exact method by which they begin to breathe air after leaving the water, and it is a subject which will bear much further investigation.

When the nymph becomes full grown, it has changed its form from a rather slender creature to a broad and flattened one, not resembling the slender bodied adults in the least. It crawls out of the water on the bank upon the stems of water plants or upon the rocks, and later its skin splits down the back, and the adult dragonfly emerges. Empty skins of these nymphs are very common objects about watercourses.

Rather more than two thousand species of dragonflies have been described, and of these something less than three hundred inhabit the United States, of which about two hundred and twenty-five species are peculiar to this country. According to Kellicott, about one hundred species are found in the State of Ohio, and Williamson thinks that even more are to be found in Indiana. In many places dragonflies are disappearing, owing to the drainage of their breeding places. On account of the beauty of the adults and the interest attaching to their habits, they are becoming favorite subjects for collections, and there are now a number of earnest students of the Odonata in this country. The death of the great master of dragonfly science, Baron de Selys-Longchamps, of Belgium, has called renewed attention to this fascinating group.

The nymph dragonflies are well adapted to aquarium study. They are easily collected and easily kept. The debris at the bottom of ponds can be brought up with a rake, and the nymphs thus collected placed in a bucket and carried home to the aquarium, which should be furnished with sand and aquatic plants. The best time for collecting them is in the spring and early summer.