Caddis-fly (Phryganea, &c.), a name common to the members of a very distinct family of Neuropterous insects. The family (Phryganeidae) is co-extensive with a special division of the order known as Trichoptera, and regarded by some as a foregrounding of Lepidoptera. The adult insects are somewhat moth-like, and the larvæ often not unlike caterpillars. The two pairs of wings are different; both bear hairs or scales, but the posterior pair are usually broader and can be folded. The mouth organs are not adapted for mastication, the mandibles are degenerate and membranous, and the first pair of maxillæ are fused to the second. Besides these more distinctive features, the caddis-flies have the following general characters: the head is small, the antennæ long and bristle-like, the compound eyes hemispherical, the eye-spots three in number, the wings with very few transverse veins or none, the feet with attaching processes. The adults are very active, especially in the evening and at night.
The eggs are laid in gelatinous masses on plants or stones near or in the water. Into this the larvæ speedily pass, and surround themselves with a sheath usually composed of minute fragments of wood, grass, moss, leaves, stone, shell, and the like, bound together by the silken secretion of a spinning gland which opens on the second pair of maxillæ. Within these tubes the larvæ are both masked and protected. Sometimes they are fixed, sometimes carried about. The tubes which are open at both ends vary greatly, but seem to be constant for the species. Two British species (Setodes) have a primitive type of tube, which is silken and nothing more. Sometimes a few strips of grass stem are simply glued on, but in most cases the sheath is complex, definite, and composed of a multitude of little fragments. In Helicopsyche the sheath is spirally twisted like a minute snail-shell. The larvæ feed principally on aquatic plants, and are in turn much eaten by fishes and other animals. The abdomen of the larva discharges aquatic respiration, and bears external respiratory filaments. These are absent in Enocycla pusilla, which lives out of the water amongst moss at the roots of trees. After a while the larva water into aerial life. Some of the smaller species rise to the surface in their cases, and take wing thence, as from little boats.
The caddis-flies form the largest family of Neuroptera; some 800 species are known; Mr M'Lachlan in his monograph enumerates 474 European forms, of which about 200 occur in Britain. Besides the typical genus Phryganea, with the very common species Phryganea striata, Limnophilus, Brachycentrus, Apatania, Molanna, Setodes, &c. are represented by British species. The larvæ are very common in ponds and streams in spring, are well known as caddis-worms to anglers, and form deadly bait.