Horst Janus, Nature as Architect, New York, Frederick Ungar Publishing, 1957, p. 6-7.

To protect their tender abdomen, they build shells, the so-called caddis cases, which they drag about with them, much as the hermit crab does his borrowed snail shell. In constructing these cases the larvae cement together the various building materials by  means of a sticky secretion from their silk glands. A given species will often use the same materials. In the present instance the animal empoyed the empty shells of a small water snail, but depending upon the resources of the stream bed, the same larvae might use quite different materials, such as pebbles, chewed off bits of wood, etc. The pupation of the larvae also takes place in these cases.