Mitchell Beazley, Joy of Knowledge, the comprehensive home reference library, vol. 1, (1976), 1987, p.9

Caddisfly larvae build a total of five house (4) through their larval development, one for each of five moults and each bigger than the previous one. Cross-sections show the series of houses (1-7) and how they are anlarged by stones (8-12). In between, during moults, the larva blocks off the opening. The larva filters out small stones by raking (B) ; it rejects lare stones (C) by touch. The stones are fitted together (D) by silk spinning.