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

Nymphs 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.

Adults are rarely noticed as they are relatively inactive, and blend into the background. They live for just 2 or 3 weeks, and they spend most of this time crawling around on stream side stones or nearby vegetation. Many do not feed as adults, but those that do eat algae or pollen. They tend to fly only when disturbed. The hind wings tend to be slightly broader than the fore wings. Some males may be wingless, or have short, non-functional wings. The cerci may be long and filamentous. The antennae can have up to 80 segments. Their compound eyes give them good vision, and they also have 3 ocelli. Mating usually happens on the ground or on vegetation. The female then flies over the water dipping her abdomen in to wash off the eggs, or she swims on the surface while laying eggs. Each female can lay hundreds of eggs.

On the right is Leuctia sp. This genus is also known as needle flies because of the way the wings are folded around the body when at rest.