A sac

Frederick George Shaw, The Complete Science of fly Fishing and Spinning, Londres, MacMillan, 1915, pp. 65-66.

The larva of one, after leaving its egg, spins a cylindrical sheath around itself (see Plate VII., Fig. 2), which forms its future home, and to which it attaches small stones, sand, wood, etc.; these cover and mask this case, and at the same time act as ballast. The case thus forms an armour-like protection against ennemies….The larva of the other family of the Caddis-fly forms its home by spinning a sac, like a bag attaching it to some sheltered spot, and covering it with stones, etc.