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.
"Knowledge is power."
It has been stated elsewhere in these pages that the raising of goldfish consists largely in the protection of them against their enemies, and to make the defense most effective it is essential that we know something about the transgressors, for by being acquainted with them and their habits, we can more intelligently combat them. For this end the author has described them in the following lines, adding to those that are less familiar to the general reader, an outline of their natural history and viewing them in the successive stages, during which they endanger the life of the fish.
Before beginning the description of insects, however, it is proper to say that the spawn of the goldfish immediately after its deposition, is sought for by other fish and devoured, the spawners themselves also engaging in this nefarious practice. Those eggs that have escaped the notice of the fish are consumed by various smaller enemies, foremost among which is which devours them.
This little creature, of which the accompanying illustration gives a good idea, is a crustacean, not more than one-half of an inch in length. It crawls about upon the bottom of the ponds and over the water plants, searching for food, part of which consists of fish eggs, to which they are very destructive, devouring them wherever found.
Another voracious enemy of the crustacean tribe is or Flea Crab, Buck Crab, etc. This lively little creature is closely related to the shrimp found in the ocean. It furrows through the water in any direction lying on its side, because its back is naturally bent. (See illustration.) In all stages of its growth it feeds upon the fish eggs, but in turn, furnishes an excellent food for young fish.
The Water Flea (enlarged).
The Water Asell (enlarged).
(Notonecta glauca,) or "Shoemaker," as it is commonly called, is a most voracious insect. The body is long, contracted posteriorly, convex above and flat below, having hair at the sides and extremities, which, when spread out, supports the insect upon the water. The head is large and presents a large eye upon each side, giving the possessor the power of vision in all directions. The color of the body is a greenish grey, the wings are white, of the legs, the four nearest the head are short, but the third pair are very long, different in shape from the others, very much resembling boat oars. When in the water, the insect swims upon its back, using the hind legs as oars for propulsion, while the front ones are instrumental in seizing its prey. Young fish, tad-poles, and other insects, all contribute to supply it with food, to the former, especially, it is a very dangerous enemy. The instrument or weapon with which the insect makes the attack upon the victim is a strong, conical beak.
It is believed that when making the attack, the boat-fly injects poison into the wound it makes, as seems to be proven by the fact that when once attacked, though subsequently escaping, the victim always dies in a short time. When upon land, this fly crawls along, in an upright position, dragging its oars behind it. In the evening, and at night, it likes to leave the water and make excursions to other ponds or creeks, from this habit the culturist may take warning. Its eggs are deposited against the stems of aquatic plants in the early spring, and again in mid-summer, so that one season produces two crops of them.
The Boat Fly.
Larva of Dragon Fly.
The young make their appearance soon after, immediately following the example of the parents by swimming upon the back and eating almost anything they happen to meet. The accompanying illustration shows the insect as seen from below when in the water.
There are two or more varieties of this fly that differ in coloring, and of smaller size than the one described, though all are extremely destructive to the young fish - the one just delineated, more especially.
This rather pretty beetle, lives entirely below the surface of the water, never leaving it, except during the night when the air is damp or in rainy weather, and then for the purpose of making excursions to other localities. The body is of a greenish black color, encircled with a brownish yellow band - this feature giving it its name. When taken from the water it exudes a milky fluid of a most offensive and disgusting odor. The hind legs are shaped very much like those of the boat-fly, and serve the same purpose. This beetle is very courageous, attacking fish of any size, as large ones have been caught, into whose flesh the beetle had eaten large holes, the beetle itself found in the hole hard at work eating up the fish. The larva, which is produced twice within the same season, lives and grows upon tadpoles and young fish.
Yellow-Banded Water Beetle and its Larva.
When of sufficient size, and the proper time has arrived, it changes into a pupa, which in turn, becomes the perfect beetle. (See illustration.)