This section is from the book "The Goldfish And Its Systematic Culture With A View To Profit", by Hugo Mulertt. Also available from Amazon: The goldfish and its systematic culture.
The dragon flies (commonly known as snake-feeders) may be divided into three classes, all very destructive enemies of the fish.
1. The Libellula possesses a short, flat body, about two inches in length. (See illustration).
2. The Aeshma is longer than the above, its slender, round body sometimes measuring six inches in length.
3. The Agrion is not large, the body small and slender, varying in length from 1 1/2 to 2 1/2 inches.
The wings of the first two named, are, when the insect is at rest, always expanded horizontally, while those of the latter are folded together, pointing backward.
The Dragon Fly.
The hind part of the body in all of them is long, slender, and composed of ten rings. On the forepart of the body, they have three pairs of legs, and two pairs of transparent, webbed wings, the latter in some species glitter like gold, in others they are dotted with spots of different color; in the Agrion species they are of the same color as the body. The coloring of the bodies of all, especially the aeshma, is very brilliant, being of a bright green, blue or scarlet, and sometimes mottled and spotted with various colors.
The eyes are large and prominent, giving the insects a very large field of vision.
They all fly very rapidly, feed upon insects of every description that they catch flying about, and from this fact they may be made useful to destroy the mosquitoes in bedrooms and elsewhere. Although very voracious, they are perfectly harmless to man - they can not injure him in any way. The manner of their copulation is somewhat curious. The male fastens the extreme back part of its body to the neck of the female, and thus attached, both fly 'about for one or two hours, when, over some water, they separate. The female then deposits her small white eggs by immersing the posterior part of the body in the water, attaching them to the submerged surfaces of water-plants; there they remain until hatched.
The larvae or grubs of the dragon-flies live in the water; those of the libellula are short and thick, while those of the other genera are more slender, corresponding with the shape of the adult. The color of these grubs varies from blackish-brown to a brilliant green. They breathe through the posterior part of the body, which apparatus is also used to propel them forwards through the water, making them good swimmers.
They are extremely destructive to young fish and fish-eggs, upon which, together with tadpoles and snails, they manage to make a good living. Instead of hunting their victims, they lay concealed in the mud with the eyes only protruding from the surface. Whenever a victim comes within reach, they produce their concealed pincers by a rapid motion, rarely missing the mark they aim at. (See illustration.)
There are instances on record where one of the larvae of the libellulae, which was overlooked in the fish-tank, destroyed two thousand (2,000) young fish in a week's time.
After they have attained their full growth, the grubs leave the water, climb upon some object projecting from it, when the perfect fly makes its appearance through the back of the grub, rising upon its wings into the air as soon as they are unfolded and dry.
The eggs are also produced twice in a season, the grubs from the last deposit, living in the mud during the winter, and produce in the early spring the first dragon flies of the season.
Their natural enemies are the frog and the water-spider. The latter, small as it is, compared with their own size, is, nevertheless a powerful antagonist, attacking them when in the act of depositing their eggs. The attack is made upon the eye, the largest dragon-fly thus being easily overpowered by its small but intelligent enemy.
How strange it is that just those animals with which man has the least sympathy are among his best friends! Such are the toad and the spider!