Donald J. Borror & Dwight M. Delong, An Introduction to the Study of Insects, New York, Rinehart and Company, 1954, p. 437.
One of the most interesting characteristics of these insects is the habit of most species of constructing portable cases, in which they live and in which they pupate. These cases vary considerably in form ; some are straight and slender, some are oval, and some are coiled (Figure 25-6). The cases may be made of bits of leaves, twigs, sand, grains, pebble, or in some instance entirely of silk. The materials used in making the case are fastened together with silk, or they may be cemented together. Each species builds a very characteristic type of case, and in some species the young larvae build a case different form that made by older larvae.