Dave WhitlockL.L. Bean Fly Fishing Handbook, Lyons Press, 1984, p. 71-72.

Many species of caddisfly larva construct a case which they live. These cases are made uniquely according to species from glue and a silklike filament in combination with aquatic plant pieces, sand grains, or terrestrial plant part. The cases are for protection and camouflage. Along the bottom, they look like short sticklike structures.