John Goddard, Trout-Fishing Techniques, Practical Fly-Fishinf Solutions from an International Master, Ill. Charles Jardine, Guilford (CT), The Lyon Press, 2003, p. 45.

The majority of species live on or near the bottoms, on or among stones or weed, and make cases from various local materials. In stillwater they tend to use light materials such as vegetable matter or sections of leaves, while in running water they use heavy materials such as sand, gravel, small stones, or sections of discarded shells. These cases are often works of art, although they do restrict movement.