Vinson Brown, How to make a Miniature Zoo, Boston, Little Brown and Company, 1956, p. 91.

All kinds of interesting experiments can be made with water insects. For example, caddis worms like to build cases around their bodies to protect them from their ennemies. These cases are made of all manner of tiny materials, from pieces of bark to bits of gravel. You can separate your caddis worm from their usual covering material and see what they select from teir usual covering material and see what they select from other kinds of materials you give them.