Frederick Vincent Theobald, Insect Life: A Short Account of the Classification and Habits of Insects, Londres, Methuen & Co., (1896), 1905, p.204.

The Caddis-worms are very curious creatures. Their body is soft, but the head and thorax are horny. To protect their soft bodies theses Caddis-worms construct tubes or « houses » of various foreign materials. Some build them of pieces of sticks, others of leaves, others of stones, and one of small fresh-water shells. Inside these curious cases the larvae spend their lives, with only their head and thorax with the six legs protruding ; these are immediately withdrawn into the tube on the approach of danger…/…

../.. So abundant were these Trichoptera in olden times that we find a deposit, of what is know as indusial limestone, two to three metres thick and of wide extent, in Auvergne, formed of their cases or indusiae.