Albro Tilton Gaul, The Pond Book, New York, Coward-MacCann, p. 86, 1965.
The tubes may be made of sand grains, bit of wood or leaves, or even snail shells. All these bits are carefully cemented together with a wonderful « underwater glue » which the larvae make in their salivary glands.
The larvae are the cadis worms in their cases. Each case is the home of a single caddis worm. As the worm grows, it adds to the front of its case making it longer and biger around. Then it crawls up to the new part where it fits more confortably. The case is a protection for it, since small fishes and little insects do not tend to bother a bunch of sticks or sand grains, while they might readily gobble the poor worm if it had no home.