Georges F. Mason, Animal Homes, New York, William Morrow, 1947, pp. 87-88.

The caddis fly larva is another underwater architect whose case-like home is often seen on the bottom of streams when the water is clear.The larva is an ingenious little fellow, capable of constructing his home out of whatever material happens to be on the stream bottom. There are many kinds of caddis flies, and it appears that each one has a preference for certain kinds of materials used in making the cases, although this may not hold true in many places where the desired materials are not available. Where the stream flows through a hemlock forest, you will find that many cases are made with the tiny needles shed by the overhanging boughs ; or where the river bed is sandy and there are not many trees and bushes, the cases will be made of sand. Other materials that may be used are leaves, pebbles, sticks, and even the shells of small snails.
The arrangement of whatever material the larva uses to build his home is always neat and symmetrical, for he is a skillful and painstaking craftsman.The interior of the case, which is lined with silk, is tube-like and open at each end, except for a silken partition at one end. The partition is pierred with small holes to allow water to flow through the case. During its early stages of development when it needs protection, the larva may close both ends of the case with porous screens.