TRICHOPTERA, the term used in zoological classification for that order of insects whose members are commonly known as caddis-flies. They are sombrely coloured insects, being generally some shade of brown often with darker markings, and are of small or moderate size. Caddis-flies are weakly flying insects of moth like appearance and are closely related to Lepidoptera : they are not often seen on the wing unless disturbed, while many are noc turnal and are attracted to a light. They are found in the vicinity of ponds and streams in which the early stages are passed.
Caddis-flies have four membranous hair-covered wings with a predominantly longitudinal venation : the hind wings are usu ally the broader and have a plicated posterior lobe. The mouth parts are of the biting type but are weakly formed and man dibles are either wanting or reduced to vestiges. Metamor phosis is complete, the larvae being more or less eruciform or campodeiform and mostly live in cases formed of various foreign materials: the pupae have the appendages free and strong mandi bles are present. More than I,000 species are known and they are classified into thirteen families: about 400 species inhabit North America and over i7o are found in the British Isles.
The eggs are laid in masses covered with a mucilage and they are deposited in or near water. The larvae are aquatic and are the familiar objects known as caddis worms. They have a well developed head, biting mouth-parts and rather long legs : respira tion takes place either by means of lateral filamentous tracheal gills or through the skin only. The last segment of the body car ries a pair of appendages terminated by strong hooks. Most of the larvae construct cases or shelters in which they live : these struc tures are commonly tubular, and are formed of a warp of silk binding together fragments of leaves, straw, sticks, gravel, sand or even of the shells of small molluscs—the material utilized often being constant for particular species or genera.
Since these cases are open at either end a current of water, in duced by the undulatory move ments of the larva, flows through the case and out at the hind ex tremity. Some larvae (Hydro psyche, Philopotamus, etc.) con struct silken snares, instead of cases, which serve to collect food particles wafted through them by the water. Pupation takes place either in the larval case or, in species without the latter, in specially constructed shelters of minute stones, sand, etc. The pupa bites its way out by means of its jaws and either swims or wriggles to the surface of the water to allow of the emergence of the perfect insect.
Caddis-flies are found on all continents but relatively little is known concerning the tropical species. They are to be regarded as beneficial insects, since their larvae are eagerly devoured by trout and other fishes. In the fossil state they first appear in the Lias of Europe and later in Oligocene Baltic amber.
BunmoRAPHY.—Further information on caddis-flies will be found in works on aquatic insects (see INSECTS) : see also R. MacLachlan, Trichoptera of the European Fauna (1874-8o), and for the British species see the same author in Trans. Entomological Soc. London (1865, 1882). Life-histories of North American species are given in J. T. Lloyd, Bull. 21 Lloyd Library (Cincinnati, 1921). (A. D. I.)