Adults are rarely noticed as they are relatively inactive, reluctant fliers, and blend into the background. They are largely nocturnal. 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, and the wings beat out of sync. Some males may be wingless, or have short, non-functional wings.
An adult stonefly can be recognised by the "double ladder" down its wings, this can be seen in the photograph above and below. This is formed by 3 almost parallel veins with a large number of cross-veins.
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.
The stoneflies above and below are in the Perlodidae family. The double ladder can be seen, but the antennae are blurred as they were in constant movement.
The male signals his desire to mate to the female by beating the ground with the tip of his abdomen. The pattern of beats is different for each species. If the female is receptive she replies with a series of taps on the ground with her abdomen. The pattern of taps is different in males and females, and between different species.
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.
The eggs have a gelatinous coating that swells up and absorbs moisture. This enables some eggs to survive in streams that dry up periodically.
In the U. K. most species have one generation per year. The eggs stage lasts about a month. Cold can halt the development of the nymph, and then a generation may take more than one year to complete.
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. They look similar to the adult (see below), 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, and detritus 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 and below is Leuctra sp. This genus is also known as needle flies because of the way the wings are folded around the body when at rest. All stoneflies in this family are relatively small, and the adults have short cerci.
Above is an adult in the Nemouridae family. All in this family are fairly small - less than 12 mm as adults - and their cerci are either short or inconspicuous.
Family Perlidae. This family have the largest nymphs of all U. K. stoneflies. They are relatively large and fat, and up to 35 mm long. They have a pair of white feathery gills at each of the 3 segments of the thorax.
Above is the nymph of Perla maxima, the largest species. It is most commonly found in hill streams.