A tapering case

Preston J. JenningsA Book of Trout Flies, New York, Crown Publishers, 1970, p. 73.

The newly hatched Caddis worm or creeper immediately starts the construction of a suitable case or house, employing such materials as best suit its needs. The type of case is typical of a specific group of these flies, and every member of that group wild build a case of approximately the same material and will pattern it a after the same general pattern, so that Caddis worms of the same species may be identified by the type of case in which they live. Some species employ gravel, or vegetable matter, still others a combination of the two materials, binding and cementing them together with a silken substance which the worm itself excretes. The interior of the case is also lined with the same material. The case are usually cylindrical, although some species, such as the Grannom, build a tapering case that is perfectly square at the larger end. The case are open at both ends but the hind end is smaller than the front end.

The newly hatched worms occupy these houses and as they grow and expand they have to enlarge the case by adding more material at the larger end. Some species of Caddis worms fasten their cases to stones and brush submerged in the water, while other species drag their cases around with them as they crawl over the bottom of the stream.
The Caddis case or house is designed for protection from ennemies and a very good job it does as it blends with other material of the stream-bed. If the Caddis worms did not have the benefit of this protective covering they would soon be completely eliminated from  the scheme of things, as fish seem to relish them. There are also carnivorous insects, such as the Dragon fly Nymphs and Stone fly Nymphs, with are always ready and waiting to take advantage of any carelessness on the part of a Caddis worm.

The Caddis case, except from the point of view of camouflage, does not furnish much protection against large fish as they do not hesitate to swallow worm, case and all. Stomach examinations of large trout taken this season, 1935, disclosed Caddis cases two inches in length. The worm is apparently dissolved out of the case by the action of the juices in the trout’s stomach and the case itself taken care of in the regular elimination.