Sam S.Blanc, Modern science : Earth, Life and Man,  Holt, Rinehart and Winston, 1971, p. 228-229

Pour the water in a clean aquarium and observe the action of the caddis worm. How many different kinds cases did you find ? Of what help is the worm case to the worm ? In what way is the worm case not helpful ? The worm is just one stage in the life cycle of the caddis fly. It begins as an egg and hatches as a larva. As it matures it enters a resting stage by closing the doors of its « house » and spinning a silken material to keep it shut.