Lenox Dick , The Art and Science of Fly Fishing, Ill. Alan Pratt, New York, Winchester Press, (1966) 1972, p. 56-57.
The caddis larva is best described as a round grub or creeper in a case. The type of case is typical of a specific group of these flies, and every member of that group will build a case of approximately the same material and pattern ; thus caddis worms of the same species are easily identified. Some employ gravel, some vegetable matter, and some a combination of both, binding them and cementing them together with a silken substance that the worm excretes. The cases are of various shapes, most often cylindrical or tapered.