On most spate rivers and upland streams stoneflies are a vital source of trout food, particularly in spring and autumn.
Flies of this order, which contains around thirty species in the British Isles, are also known as the hard-winged flies.
The stonefly has a life cycle similar to that of an upwinged fly except that there is only one winged adult stage.
Stonefly nymphs have two tails, and so they can be immediately distinguished from the three-tailed nymphs of the upwinged flies.
Stoneflies generally hatch by crawling out onto stones in shallow water or up emerging vegetation. This method of hatching is not an easy antic to mimic without the artificial fly getting snagged on the bottom. The adult stoneflies are therefore of greatest importance to flyfishers.
Most of the stoneflies of early and late season are dark brown or grey, often with a yellowish tinge to the underside of the body. Apart from the dark stoneflies, there are also two very common summer species with yellow wings and a few others with mottled brown wings.
After mating, the female stonefly lays her eggs, either by dropping them on to the water or by dipping down to the surface. Spent stoneflies often struggle and sink below the surface film; perhaps that is why some of the most effective imitations are tied in a spidery wet fly style.
Pryce, D., Macadam, C., & Brooks, S. (2007) Guide to the British Stonefly (Plecoptera) Families: adults and larvae. Taunton: Field Studies Council.
Hynes, H.B.N. (1977) Adults and Nymphs of British Stoneflies (Plecoptera). Ambleside: Freshwater Biological Association.