Publisher Summary This chapter describes plecoptera (stoneflies). It comprises a hemimetabolous order of 16 families and more than 2000 species of aquatic insects distributed on all continents except Antarctica, and most major islands except notably Cuba, Fiji, Hawaii, and New Caledonia. They are primarily associated with running water, where nymphs inhabit mineral or organic substrates of streambeds, and the winged adults rest throughout their seasonal lives in streamside microhabitats such as rocks, moss, debris, leafpacks, and riparian vegetation. A few species occur in waveswept substrates of cold alpine and boreal lakes, or in intermittent streams. Stonefly adults are variable in size from about 5 to 50 mm, and in color from black to green or yellow, often marked with distinctive light or dark patterns. The aquatic adult of one species known from the depths of Lake Tahoe (Capnia lacustra) and a few other species are apterous (wingless), but most adults are winged. The wings of males and females of some species, or particular populations of a species, are shortened (brachypterous) and they do not fly, but the typical condition is of two pairs of wings as long as or longer than the abdomen (macropterous). As the ordinal name describes, the hind wings typically have an expanded posterior (anal) lobe that folds longitudinally under the main wing. Stoneflies are relatively slow, somewhat awkward fliers that typically fly short distances to disperse, to search for mates, or, for females to deposit eggs.