In the stomach

Francis E. Sell, Pratical Fresh water Fishing, New York, The Ronald Press Company, 1960, p. 31.

A caddis fly larva builds itself a case from bits of wood, gravel, and other material. These cases are often noticed in the backwaters and shallows of trout streams. Frequently in opening trout, bits of gravel and tiny pieces of wood are found in the stomach contents, evidence that they have been feeding on caddis fly larvae.