The stone files are thought to have first appeared in the Permian.

Nymphs (see the drawing below) are usually found on stones on stream beds, and are an important food item in the diets of many fish caught by fly fishermen. They are poor swimmers though. Most species can live only in very clean running water, so their presence/absence is often used as an indicator of water quality. They look similar to the adult (see right), but are wingless. They get most of their oxygen by diffusion over the body surface, but some species also have gills. They are mainly vegetarian, eating moss and algae, but some larger nymphs will eat the nymphs and larvae of other aquatic insects. Development from egg to adult can take 1 - 3 years, and during this time the nymph can moult as many as 33 times. In winter the nymphs hide in the mud.

Above is a Leuctra sp. nymph.

On the left is the nymph of Perla maxima, in the Family Perlidae. This family have the largest nymphs of all U.K. stoneflies, and Perla maxima is the largest species. It is most commonly found in hill streams.