caddis fly, any of various insects of the order Trichoptera, with four hairy wings usually held back rooflike over the abdomen, long antennae, and chewing mouthparts. The aquatic larvae, or caddis worms, which somewhat resemble caterpillars, are food for many freshwater fishes; they are called creepers when used as bait. The larvae build and inhabit underwater cases or nets made from a silken threadlike material they produce, or from materials such as twigs, sand, and leaves. Most larvae feed on plants and debris caught in the cases; among the net-building species some are predacious. Many seal their cases, and spin cocoons and pupate within. Caddis flies are classified in the phylum Arthropoda, class Insecta, order Trichoptera.

Any member of about 7,000 species of mothlike aquatic insects (order Trichoptera) found worldwide, usually in freshwater habitats but sometimes in brackish and tidal waters. Generally dull brownish, caddis flies have long antennae and hairy wings that fold rooflike over the abdomen. They feed primarily on plant juices and flower nectar, though a few are predaceous. Many caddis-fly larvae construct a portable case from grains of sand, bits of shells, and plant debris glued together by a sticky substance they secrete. This case surrounds the larva's abdomen while it matures. Caddis flies are important to freshwater ecosystems because they clean the water by consuming plant and animal debris and serve, as larvae and adults, as an important food for fish, particularly trout.

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