Edith M. Patch & Harrison E. Howe, Science at Home, New York, The MacMillan Company, 1934, p. 63-64.
Bird and Insect Masons
Long, long ago, there were certain kinds of caddis worms (larvae or young, of caddis flies) that made shelters for themselves with pebble, as they do still. They found the pebbles in the water where they lived. Each caddis worm made its own little tube-shaped rock house. It fastened the tiny stones together with silk that oozed from its silk glands. One end of the tube was open and the larva could push its head and legs out or pull them Inside. It could drag its little stone house from place to place easily as a nail moves its shell.