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Stoneflies or salmonflies are a group of insects with aquatic nymphal stages in the order Plecoptera. Stoneflies have a simple metamorphosis, with three life-history stages: egg, nymph or naiad, and adult.

Adult stoneflies have two pairs of membranous wings that are folded back over the abdomen when not in use. Stoneflies are rather weak fliers, and are not usually found very far from the aquatic habitat of their nymphs. Adult stoneflies do not feed, their short life being devoted to procreation.

The nymphs of stoneflies are soft-bodied, rather flattened, and usually occur in well-aerated, running waters of streams and rivers, or sometimes along rocky lake shores, where they hide in gravel and organic debris. Stonefly nymphs have an abdomen with ten segments, and with two extended appendages known as cerci sticking out from the back end. Stonefly nymphs are most easily differentiated from superficially-similar mayfly nymphs (order Ephemeroptera) on the basis of their two cerci (mayflies almost always have three abdominal "tails"). The nymphal stage of stoneflies is long-lived, commonly lasting for a year or more. There can be more than 20 nymphal stages, each separated by a molt.

Stoneflies are an abundant component of the bottom fauna of many aquatic communities, especially in streams. Although their biomass and productivity are not usually as large as that of mayflies, stoneflies are an important food of sportfish such as trout and salmon, and for this reason these insects are economically important. Some of the best designs for fish-catching "flies" are based on the shape and color of stoneflies, especially the adult insects.

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Science EncyclopediaScience & Philosophy: Stomium to Swifts