Stoneflies need cool, well oxygenated water, just like the trout, so their presence can be an indication of high quality water. There are perhaps 2000 species worldwide with around 35 in the UK. Not particularly great fliers they spend a lot of time on the ground, some species don't fly at all. They also crawl from the water, rather than swim and so are known as creepers at the nymphal stage.
After mating the female lays her eggs on the water.
After the eggs hatch, the nymph stage lasts from one to four years during which the nymph will go through many skin changes. The nymphs feed on algae and aquatic vegetation, larger ones will feed on smaller nymphs, small worms and even fish-fry. As already noted, they are not great swimmers and so spend their nymphal stage creeping about on the bed. This can provide an interesting challenge for the angler trying to imitate a creeper.
The mature nymph crawls from the water and sheds its skin to transform into the adult stonefly. The adult has three body sections and four wings which fold flat along its back. The name plecoptera describes the wings, "braided wings", if you can stretch your imagination, they actually have a similar appearance to a stained glass window where all the glass is the same colour. The adult stonefly will survive for up to about 4 weeks. It is the imitation of a female laying eggs which is most commonly used by the angler.