The Dragon-Flies and Damsel-Flies
POND without dragon-flies darting above it, or without the exquisitely iridescent damsel-flies clinging to the leaves of its border would be a lonely place indeed. As one watches these beautiful insects, one wonders at the absurd errors which have crept into popular credence about them. Who could be so silly as to believe that they could sew up ears or that they could bring dead snakes to life! The queer names of these insects illustrate the prejudices of the ignorant—devil's darning needles, snake doctors, snake feeders, etc. Despite all this slander, the dragon-flies remain not only entirely harmless to man, but in reality are his friends and allies in waging war against flies and mosquitoes; they are especially valuable in battling mosquitoes since the nymphs, or young, of the dragon-fly, take the wrigglers in the water, and the adults, on swiftest wings, take the mosquitoes while hovering over ponds laying their eggs.
The poets have been lavish in their attention to these interesting insects and have paid them delightful tributes. Riley says:
While Tennyson drew inspiration for one of his most beautiful poems from the two stages of dragon-fly life. But perhaps Lowell in that exquisite poem, "The Fountain of Youth," gives us the perfect description of these insects:
It is while we, ourselves, are dreaming in the sun by the margin of some pond, that these swift children of the air seem but a natural part of the dream. Yet if we waken to note them more closely, we find many things very real to interest us. First, they are truly children of the sun, and if some cloud throws its shadow on the waters for some moments, the dragon-flies disappear as if they wore the invisible cloak of the fairy tale. Only a few of the common species fly alike in shade and sunshine, and early and late. The best known of these is the big, green skimmer, which does not care so much for ponds, but darts over fields and even dashes into our houses, now and then. Probably it is this species which has started all of the dragon-fly slander, for it is full of curiosity, and will hold itself on wings whirring too rapidly to even make a blur, while it examines our faces or inspects the pictures or furniture or other objects which attract it.
Another thing we may note when dreaming by the pond is that the larger species of dragon-flies keep to the higher regions above the water, while the smaller species and the damsel-flies flit near its surface. Well may the smaller species keep below their fierce kindred, otherwise they would surely be utilized to sate their hunger, for these insects are well named dragons, and dragons do not stop to inquire whether their victims are relatives or not. It is when they are resting, that the dragon and damsel-flies reveal their most noticeable differences. The dragon-fly extends both wings as if in flight while it basks in the sun or rests in the shadow. There is a big, white-bodied species called the whitetail which slants its wings forward and down when it rests; but the damsel-flies fold their wings together over the back when resting. The damsel-flies have more brilliantly colored bodies than do the dragon-flies, many of them being iridescent green or coppery; they are more slender and delicate in form. The damsel-fly has eyes which are so placed on the sides of the head as to make it look like a cross on the front of the body fastened to the slender neck, and with an eye at the tip of each arm. There are very many species of dragon and damsel-flies, but they all have the same general habits.
The dragon-fly nymphs are the ogres of the pond or stream. To anyone unused to them and their ways in the aquarium, there is a surprise in store, so ferocious are they in their attacks upon creatures twice their size. The dragon-fly's eggs are laid in the water; in some instances they are simply dropped and sink to the bottom; but in the case of damsel-flies, the mother punctures the stems of aquatic plants and places the eggs within them. The nymph in no wise resembles the parent dragon-fly. It is a dingy little creature, with six queer, spider-like legs and no wings; although there are four little wing-pads extending down its back, which encase the growing wings. It may remain hidden in the rubbish at the bottom of the pond or may cling to water weeds at the sides, for different species have different habits. But in them all we find a most amazing lower lip. This is so large that it covers the lower part of the face like a mask, and when folded back reaches down between the front legs. It is in reality a grappling organ with hooks and spines for holding prey; it is hinged in such a manner that it can be thrust out far beyond the head to seize some insect, unsuspecting of danger. These nymphs move so slowly and look so much like their background, that they are always practically in ambush awaiting their victims.
The breathing of the dragon-fly nymphs is peculiar; there is an enlargement of the rear end of the alimentary canal, in the walls of which tracheae or breathing tubes extend in all directions. The nymph draws water into this cavity and then expels it, thus bathing the tracheae with the air mixed with water and purifying the air within them. Expelling the water so forcibly, propels the nymph ahead, so this act serves as a method of swimming as well as of breathing. Damsel-fly nymphs, on the other hand, have at the rear end of the body, three long, plate-like gills, each ramified with tracheæ.
Nymphs grow by shedding the skin as fast as it becomes too small; and when finally ready to emerge, they crawl up on some object out of the water, and molt for the last time, and are thereafter swift creatures of the air.
References—American Insects, Kellogg. Comstock's Manual.
The Dragon-Flies and Damsel-Flies
Leading thought—The dragon-flies are among the swiftest of all winged creatures and their rapid, darting flight enables them to hawk their prey, which consists of other flying insects. Their first stages are passed in the bottoms of ponds where they feed voraciously on aquatic creatures. The dragon-flies are beneficial to us because, when very young and when full grown, they feed largely upon mosquitoes.
Method—The work of observing the habits of adult dragon-flies should be largely done in the field during late summer and early autumn. The points for observation should be given the pupils for summer vacation use, and the results placed in the field note-book.
The nymphs may be studied in the spring, when getting material for the aquarium. April and May are the best months for securing them. They are collected by using a dip-net, and are found in the bottoms of reedy ponds or along the edges of slow-flowing streams. These nymphs are so voracious that they cannot be trusted in the aquarium with other insects; each must be kept by itself. They may be fed by placing other water insects in the aquarium with them or by giving them pieces of fresh meat. In the latter case, tie the meat to a thread so that it may be removed after a few hours, if not eaten, since it soon renders the water foul.
The dragon-fly aquarium should have sand at the bottom and some water weeds planted in it, and there should be some object in it which extends above the surface of the water which the nymphs, when ready to change to adults, can climb upon while they are shedding the last nymphal skin, and spreading their new wings.
Observations on the young of dragon-flies—
1. Where did you find these insects? Were they at the bottom of the pond or along the edges among the water weeds?
2. Are there any plume-like gills at the end of the body? If so, how many? Are these plate-like gills used for swimming? If there are three of these, which is the longer? Do you know whether the nymphs with these long gills develop into dragon or into damsel-flies?
3. If there are no plume-like gills at the end of the body, how do the insects move? Can they swim? What is the general color of the body? Explain how this color protects them from observation. What enemies does it protect them from?
4. Are the eyes large? Can you see the little wing-pads on the back in which the wings are developing? Are the antennæ long?
5. Observe how the nymphs of both dragon and damsel-flies seize their prey. Describe the great lower lip when extended for prey. How does it look when folded up?
6. Can you see how a nymph without the plume-like gills breathes? Notice if the water is drawn into the rear end of the body and then expelled. Does this process help the insect in swimming?
7. When the dragon or damsel-fly nymph has reached its full growth, where does it go to change to the winged form? How does this change take place? Look on the rushes and reeds along the pond margin, and see if you can find the empty nymph skins from which the adults emerged. Where is the opening in them?
Observations on the adult dragon-flies—
1. Catch a dragon-fly, place it under a tumbler and see how it is fitted for life in the air. Which is the widest part of its body? Note the size of the eyes compared with the remainder of the head. Do they almost meet at the top of the head? How far do they extend down the sides of the head? Why does the dragon-fly need such large eyes? Why does a creature with such eyes not need long antennæ? Can you see the dragon-fly's antennæ? Look with a lens at the little, swollen triangle between the place where the two eyes join and the forehead; can you see the little, simple eyes? Can you see the mouth-parts?
2. Next to the head, which is the widest and strongest part of the body? Why does the thorax need to be so big and strong? Study the wings. How do the hind wings differ in shape from the front wings? How is the thin membrane of the wings made strong? Are the wings spotted or colored? If so, how? Can you see if the wings are folded along the front edges? Does this give strength to the part of the wing which cuts the air? Take a piece of writing paper and see how easily it bends; fold it two or three times like a fan and note how much stiffer it is. Is it this principle which strengthens the dragon-fly's wings? Why do these wings need to be strong?
3. Is the dragon-fly's abdomen as wide as the front part of the body? What help is it to the insect when flying to have such a long abdomen?
Outline for field notes—Go to a pond or sluggish stream when the sun is shining, preferably at midday, and note as far as possible the following things:
1. Do you see dragon-flies darting over the pond? Describe their flight. They are hunting flies and mosquitoes and other insects on the wing; note how they do it. If the sky becomes cloudy, can you see the dragon-flies hunting? In looking over a pond where there are many dragon-flies darting about, do the larger species fly higher than the smaller ones?
2. Note the way the dragon-flies hold their wings when they are resting. Do they rest with their wings folded together over the abdomen or are they extended out at an angle to the abdomen? Do you know how this difference in attitude of resting determines one difference between the damsel-flies and the dragon-flies?
3. The damsel-flies are those which hold their wings folded above the back when resting. Are these as large and strong-bodied as the dragon-flies? Are their bodies more brilliantly colored? How does the shape of the head and eyes differ from those of the dragon-flies? How many different colored damsel-flies can you find?
4. Do you see some dragon-flies dipping down in the water as they fly? If so, they are laying their eggs. Note if you find others clinging to reeds or other plants with the abdomen thrust below the surface of the water. If so, these are inserting their eggs into the stem of the plant.
Supplementary reading—Outdoor Studies, Needham, p. 54; "The Dragon of Lagunita" in Insect Stories, Kellogg.