They fix to rocks

Charles Edward Walker, Old Flies in New Dresses : how to dress dry flies with the wings in the natural position and some new wet flies, Londres, Lawrence & Bullen, 1898, p. 45.

Every fisherman knows the Caddis-worm, which is the larval form of the Caddis-fly. As the number of different species of Trichoptera is very large, these are many different sorts of Caddis-worms. Some of these make cases which they fix to rocks ; most of them however have cases which they drag about with them, and retire into it when any danger approaches. These cases vary much in shape and the materials of which they are made.