Caddis Flies — Part 1

(Order Trichoptera.)

Almost everyone who likes to watch curious living creatures knows the caddis worms--the strange little larvae which construct cases of bits of leaves, twigs, small stones, or sand, and which live under the water of permanent streams or ponds. They are very good objects for the aquarium, and are structurally of great interest. While the case-bearing larvae are so well known, it is quite different with the adult insects, which not only are seldom seen in collections, but have been studied to a comparatively slight extent in this country. They have four wings, which are more or less clothed with hairs, so that the caddis flies look very much like moths. The mouth-parts are rudimentary, and the antennae are thread-like, frequently very long and many-jointed. The pupae resemble the perfect insects in general appearance, and become active before the last molt. The hind wings of the flies are often broader than the forewings, and may be folded in repose.

The caddis flies are often seen about the margins of streams and frequent shady places. They seldom fly during the day, but are often attracted to light at night.

The eggs are laid in a double mass, which is gelatinous, and usually green in color. This mass is usually attached to the surface of some water-plant, but it is supposed that certain species creep down the stems of aquatic plants under water for the purpose of laying their eggs. The larvae are all aquatic, with the exception of one form, which lives in damp moss on land, and they are nearly all protected by a case of some form or another. The cases, as just stated, may be composed of leaves, either attached by their edges or placed longitudinally, or they may be bits of stick arranged in many different ways, or stones, or grains of sand, or even water-snail shells may be used for the purpose, attached to the outside of the case, in such cases sand being the main material. All of these different substances are fastened together by means of silk, spun by the larvae, and they serve to protect the caddis worms from the predatory insects so often found in streams, and also from fish as well.

In the majority of instances the cases are cylindrical, but sometimes they are curved in a horn-like manner, and in other cases they are very strangely involuted, like a snail-shell. In fact, the case of a caddis worm was once described by a conchologist as a new species of snail-shell. Then there is another form in which the case resembles a bottle with the bottom cut away and the lower part compressed until only a slit is visible. In the majority of instances the cases are free, and the larva crawls or swims about with only its head and thorax protruding from the orifice, but in other instances they are firmly attached to rocks or submerged logs.

The larva itself is elongate and usually cylindrical, and while the head and thorax are tough and horny, the abdomen is thin-skinned, delicate, and of a pale color. It breathes by means of tracheal gills, which issue from the sides of the abdomen. It is not known how many times any of them shed the skin and a molt has never been observed to my knowledge. They live several months in the larval stage, and most of them pass the winter in that condition. When ready to transform to pupa, both ends of the case or tube are protected by a silk netting, spun by the larva, which transforms in security, well drawn back from either orifice. When ready to transform to the adult stage the pupa works its way through the guarded orifice, swims to the surface of the water, and crawls out, where possible upon a twig or other vegetation, or upon the bank. It may cast its pupal skin while still in the water, the wings remaining unexpanded, but as soon as it leaves the water the wings expand instantly to their full size, and the fly departs through the air. This statement is based upon an interesting observation by Comstock, who states that the instant expansion of the wings is necessitated by the fact that the insects studied by him normally emerge from rapidly flowing streams, which dash over rocks, and if much time were required in this wing-expansion, the water would destroy the wings for purposes of flight, and dash the insect down. The larval food of the caddis flies is mainly vegetable, but there is one group which is carnivorous. The order is not a very large one, but more than one hundred and fifty North American species have been described.