Egg shells

Robert W. Pennak, Fresh-water invertebrates of the United States, New York, Ronald Press, 1953, p . 570

Under normal conditions the same species select the same materials and build cases in the same way, but when the usual materials are not available, substitutes may be utilized. Experimental work has shown that a variety of unusual objects, such as paper and egg shells, may be used. Exposed larvae are often able to distinguish their own case from those of others.