Order Plecoptera - Stoneflies

• Perla sp.

Soft-bodied insects with long thread-like antennae and cerci. Weak biting mouthparts. Wings membranous, hind pair usually larger than front pair, and folded flat over the body at rest. Simple metamorphosis, with over 30 nymphal moults before the adult stage in some species. The nymphs are aquatic, often with filamentous tufted gills on various parts of the body (usually near the bases of the legs and cerci), although they obtain much of their oxygen by simple diffusion through the whole body surface. Stoneflies are a relatively small Order with about 3,000 known species, of which only 34 occur in the British Isles.

All species of Plecoptera are intolerant of water pollution and their presence in a stream or still water is usually an indicator of good or excellent water quality. Nymphs of larger species are much prized by anglers as bait for trout.

Description. This genus includes the larger stoneflies, with adults and nymphs of some species up to 30 mm or more long. Adults generally dark brown in colour, similar to the species illustrated (P. marginata). Nymphs usually patterned with black and yellow, with lateral bunches of fine, white filamentous gills on each thoracic segment.

Biology. Mainly found near clear, fast-flowing, upland streams and rivers. Freshly-emerged insects are often seen resting on waterside stones near their empty nymphal skins. Adults probably need very little nourishment and may not feed at all, although they appear to drink plenty of water.

The aquatic nymphs live under stones and debris on the stream- or river-bed. They are predatory and feed chiefly on the larval stages of other aquatic insects. Development probably takes two to three years.

Distribution. Representatives of the genus occur in temperate regions worldwide, including Britain and other parts of Europe.

Photos: (adult) James Lindsey, CCA Share Alike License; (nymph) Bohringer Friedrich, CCA Share Alike License

Adult

Nymph