A caddis fly is a small, moth-like insect that spends most of its life underwater as a larva before transforming into a winged adult. The larva, called a caddisfly larva or caddis worm, is famous for building itself a protective case out of whatever materials it finds on the stream bottom: tiny pebbles, bits of wood, sand grains, or even small shells. Each species has its own architectural style, gluing materials together with silk it produces from special glands.

If you flip over rocks in a clean, cold stream, you might spot these cases slowly crawling along, with the larva's head and legs poking out the front. The cases look like miniature log cabins or stone towers, and they protect the soft-bodied larva from predators like fish and other insects.

After spending months or even years building and rebuilding its case as it grows, the caddis fly larva eventually seals itself inside and transforms into an adult. The adult flies away from the water, looking somewhat like a drab brown moth. Adult caddis flies live only a few weeks, just long enough to mate and lay eggs back in the water.

Fishers pay close attention to caddis flies because trout and other fish love eating both the larvae and the adults, making caddis fly patterns some of the most important artificial flies for fly fishing.