Alexander B. Klots & Elsie B. Klots, Living Insects of the World, Londres, Hamish Hamilton, 1959, p. 105,

There are twenty or more families of true case-bearers, which start their case by constructing a basic silken tube. In this are then embedded pieces of sticks, leaves, stones, shell or sand with skillful precision and specific uniformity. The Phryganeidae, a family that includes some of the largest species use bits of leaves.Some species cut these into rectangular pieces and fit them together around the tube ; other arrange thin strips side by side to form a narrow ribbon coiled spirally around he case. The Molannidae make a tube of small stones that has a broad, flat flange along each side. Some Leptoceridae and Odontoceridae use fine grains of sand to make curved, cylindrical tubes, which may increase so regularly in diameter from the rear to the front as to look like horns or trumpets. The Limnophilidae build as though with imagination, sometimes using relatively large sticks that may be placed lengthwise or crosswise in log-cabin style or sometimes pebbles, large or small, or even snail shells. But we like best of all the cases of two of the Sericostomatidae : the spiral case of Helicopsyche, made of fine grains of sand in an almost perfect duplication of the form of a snail shell ; and the tapered tube of Brachycentrus nigrisoma, which in an exact square in cross section and is made from bits of twigs and roots cut neatly and accurately to the proper size. Every available material is used by one or another of thsee builders to form the distinctive case of its species. Stones and sand may seem like too heavy material for a case that must be carried everywhere ; but a bubble or two of air inside the case counteracts this an actually makes it weightless or lighter than water.