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.