PLECOPTERA (Gr. plaited, and 7rrep6v, a wing) is that order of insects which comprises the stone-flies : by some authorities they are termed Perlaria from Perla, the principal genus. Stone-flies are dull coloured insects, either black, brown or grey, or in some species green. They are poor fliers and do not wander far from water where their early stages are passed. Their habitation is the margins of streams and lakes, especially in hilly or rocky districts and they are usually found resting on stones, palings or tree-trunks, or crawling about over stones or plants : the green forms frequent bushes or other herbage and are diffi cult to detect. Fewer than Soo species are known and these are classified into seven families: about 3o species inhabit the British Isles and a considerably larger number is found in the United States. The richest fauna is found in the southern hemisphere and the earliest forms are confined to the Australian region and Chile.
Stone-flies are soft-bodied insects with long thread-like anten nae : the wings are membranous, folded flat over the back in repose and the hind pair is usually the largest with a plicated posterior lobe. The mouth-parts are weak and of the biting type, the tarsi are three-jointed and the body is generally terminated by long, many-jointed tail-feelers. These insects derive their name from the fact that their nymphs are often common beneath stones in the beds of streams. The females discharge their eggs in masses
into the water : metamorphosis is incomplete and the nymphs only occur in clear, unfouled streams or lakes, not in stagnant water. Some are carnivorous, while others feed upon organic particles of various kinds. In form they resemble the perfect insects very closely except for the absence of wings : they are provided with lateral tufts of abdominal tracheal gills or with a terminal group of these organs around the anus. When fully grown the nymph crawls out of the water and the imago emerges. A few Plecoptera in the fossil condition are met with in the Upper Jurassic rocks of Europe, and in the Lower Permian of Kansas.
The nymphs serve as food for certain fishes, such as trout, and are in that respect valuable. The species Taeniopteryx pacifica, has been recorded as damaging the buds of fruit trees in Washing ton State.
Further information on these insects will be found in works on aquatic insects (see INSECTS) , while for the British species see papers by K. J. Morton in Trans. Entomological Soc. London (1894, 1896). The North American species are dealt with by J. G. Needham and C. P. Claassen, The Plecoptera or Stone-Flies of North America (1925), and for the general classification of the order see R. J.