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.
(Hydrophyllus piceus.) As the name indicates, this beetle is black, shining with a rich, purple lustre. (See illustration.) It is of larger size than the preceding, and strong in proportion.
The Black Water Beetle.
The beetle itself is a vegetarian, and as such, is not directly dangerous to the fish, its larva, however, is voracious without limit, destroying all that comes in its way.
The female of this species spins a white cocoon around the posterior portion of its body, with the aid of its hind legs, the cocoon, when completed, being the size of a hazel nut. In this it deposits its eggs, and after closing it carefully, fastens it to a floating leaf, adding to it a little projecting point on the top, which by the way resembles a small mast, retires to the water underneath and mounts guard. After a few days the young grubs make their appearance, at first resembling little whitish worms, but possessing six legs near the yellow head.
It is by the motion of these legs that the grub is propelled through the water, continually on the search for something to eat. When at rest on a water-plant, the head with its fearful apparatus, formed of a strong pincher with two pairs of adjuncts, which can be moved in any direction, is placed in such a deceiving position as to almost always lure an unsuspecting little fish, tad-pole or insect, within its reach.
As the grub gets larger, it turns darker in color, until having attained a size of about four inches in length, it has become nearly black on the back; the under part is then of a creamy white, and the sides have been fringed with hair. . In this state its appearance is extremely repulsive, being about as ugly as anything can be imagined. The earliest and best time to destroy them is when the cocoon has been finished, and the female is standing guard in the water beneath, both can then be captured and obliterated, in this way great damage is prevented before there has been an opportunity for development; very much on the principle of the old proverb : "A stitch in time saves nine."
The grubs breathe through the posterior part of the body, and have to come to the surface occasionally for that purpose, at which time they are easily caught with a dip-net.
In general appearance, the color excepted, the grub of the black water-beetle resembles that of the preceding.
The beetle, moreover, is very prolific, spinning several cocoons at two different periods, namely, in the spring and high summer.
Other varieties of this insect exist, the one under discussion being the most dangerous to the fish. So far as the others are concerned, it is sufficient to remember the injunction, allow nothing alive to remain in the company of the fish when newly hatched.