Dave Hughes, Handbook of Hatches, a basic guide to identifying trout foods ans selecting flies to match them, Harrisburg (PA), Stackpole Books, 1987, p. 82-83

Case-building larvae are the most common and most often observed. Their cases are held together with silk and secretions. If they are fast-water dwellers, they build their cases of sand or pebbles. This gives them the ballast they need to keep themselves firmly on the bottom. Slow-water larvae select lighter material. Some use pine meedles, others cut sections from leaves. Another makes its case by hollowing the center of a twig and crawling inside. These materials are not only convenient, but they also provide the insects with their best camouflage.