Joseph Leconte, A compend of Geology, New York, American Book Company, 1898, p. 372.
In Auvergne, France, there is a Miocene fresh-water deposit, one layer of which two to three feet thick, is almost wholly composed of the cast-off cases (indusia) of caddis-worms, and is therefore called indusial limestone. The caddis-worms (larva of the caddis-fly) of to-day is a wingless creatures , living wholly in the water. It has the curious habit of gathering bits of wood, small dead shells, or even grains of sand, and webbing them together to form a cylindrical hollow case in which it lives. When it wishes to walk about, it puts out the head and legs for that purpose, as seen in the figure. These cases are left when the worm changes into the caddis-fly. We may imagine, then, that in Auvergne, in Miocene times, there was a lake in which live countless generations of caddis-worms, and their cast-of cases accumulated until a depositt, two ro three feet thick, was produced.