A long cylindrical tube

Robert HegnerParade of the Animal Kingdom, New York, the MacMillan Company, 1971, p. 148-149.

These Caterpillar-like larvae of the Caddis Flies build a long cylindrical tube, adding material in front as they grow larger, and dragging it with them as they crawl about on the bottom of a fresh-water stream or pond. Each species uses some particular kind of building material, such as grains of sand, small leaves, or minute sticks; these are glued together with saliva and lined with silk.