Besides birds, small larvæ of several insects will eat, or at any rate kill, the ova in considerable numbers. Caddis-worms are among these larvæ which eat ova. This seems to be one of the few cases in which nature is just, for caddis-worms are taken very readily by even small trout. Large trout will take them very greedily, cases and all. Therefore, I should advise the fish culturist to cultivate them as food for the fish he is rearing, but to be very careful that they do not get into the rearing boxes or hatching trays when he has ova in them. The caddis-worms kill the ova by making a small hole in them and sucking some of the contents out; from this hole some more of the contents escapes, and as it comes into contact with the water becomes opaque.
Caddis-worms are the larvæ of an order of four-winged flies commonly known as sedges, caddis-flies, or water-moths. The latter appellation is of course a misnomer, as these flies (Trichoptera) have nothing whatever to do with moths. They resemble moths, however, in that they have four wings which when at rest lie in much the same position as do those of moths, and as many of them have their wings thickly covered with hairs, this resemblance is sometimes very marked. The larvæ (caddis-worms), being eagerly sought as food by many fish, and having very soft bodies, make for themselves cases. Some of these cases are made from small sticks, some from little pieces of stone or sand, and some from a mixture of all of these substances. As these cases resemble such small pieces of rubbish as are frequently found in streams, care should be taken that they do not get into the hatching trays containing ova.
Many of the water beetles, and practically all of their larvæ, will attack the ova; they should therefore be carefully excluded from the hatching trays. As there are about 114 different species of beetles in the family of Dytiscidæ alone, my readers will appreciate my reason for not attempting to enumerate them. It will be a sufficient warning to state the fact that they are all carnivorous, and their relative sizes is the only thing which will decide whether the beetle will eat the fish, or the fish the beetle.
Very similar to beetles are some of the water-bugs. They may, however, easily be distinguished from beetles, as the outer or anterior wings of the bugs cross each other at their lower ends, while the elytra of beetles, which much resemble the horny, anterior wings of some of the water-bugs, meet exactly in the middle line. These water-bugs, though some of them are excellent food for even the small fish, will attack the ova, and therefore they should be kept out of the hatching trays. The fish culturist should, however, whenever it is possible, cultivate such of these water-bugs as are good food for the fry in separate ponds, as I have before recommended. The best of these water-bugs are Corixæ. Others, such as the water-boatman, water-scorpions and pond-skaters, are not of any value as food for the fish.
The larvæ of Ephemeridæ are very good food for the fish, and should be cultivated in separate ponds if possible, and some turned into the ponds containing the little fish occasionally. A fair proportion should, however, be kept in the ponds and protected, so that a good stock of the flies may be available next year.
The larvæ of Ephemeridæ may be obtained in many streams, and are best caught with a fine gauze net. Some of them swim, but most are generally captured with such a net at the bottom of the water among the débris. Eggs of Ephemeridæ may be obtained sometimes from another locality if they cannot be got on the spot. These should be carefully preserved for the first year at any rate, and a good fly may thus be introduced into a water where it was before unknown.
I have already spoken of the fresh-water shrimp and the water-flea (Daphnia pulex). These valuable articles of diet should be introduced whenever it is possible. Daphnia must be reared in a stagnant pond, the fresh-water shrimp (Gammarus pulex) in running water, with plenty of weeds.
Other useful creatures besides those snails and mussels which I described in a previous chapter, are the water-louse (Asellus aquaticus), Cypridæ, and Cyclops quadricornis. Asellus is very similar in size and shape to the common garden-louse, which is found in decaying wood. It will live either in stagnant or running water. Cypridæ are very much smaller, being generally only as large as a large pin's head. They have a bivalve shell which makes them look something like a small mussel. They are, however, very active, swimming by means of two pairs of legs. They also possess two pairs of antennæ and one eye. (The species belonging to the genus Candona of the family Cypridæ, do not swim.) Cyclops is another very small crustacean, shaped like a large-headed club. It swims very actively, and, like the Cypridæ, is an excellent article of diet for very young fish. Both these crustacea live in stagnant water, and must, therefore, be kept in a separate pond, whence they may be taken as required to be given to the fry.