Colin S. Milkins, Discovering Pond Life, Hove East Sussex, Wayland, 1989, p. 24.
Some types of caddis-fly larvae resemble caterpillar both in appearance and in the way they feed. They chew up and swallow plant material. Caddis larvae that eat only plants are often called cased caddis. This is because they build a tube, or case, which they drag around with them. They begin making the case by spinning a tube of silk around the body. Then pieces of dead plant, tiny stones or even empty snail shells are stuck on to the silk to complete the tube. The shape of the tube and the choice of building materials varies from species to species. The case camouflages the larvae, so hiding it from predators. Also the case may simply make the caddis larvae too painful to swallow. When fully grown, the larva closes off the entrance to the tube, and over several weeks it changes its form to become a pupa.