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.