The Plecoptera is a small order of aquatic insects with a world total of little more than 2000 species. They occur across all continents except Antarctica and from sea level to an altitude of about 5600 m (in the Himalayas). They are soft bodied, four-winged, rather uniform in appearance and vary in length from 4 to 50 mm. Adults of most species are fully winged (maximum wing span about 110 mm); some are brachypterous and apterous forms are known in several genera.
Plecoptera are superficially similar to and share some primitive character states with Orthoptera and Embioptera. They differ from Orthoptera in wing venation and in the more equal texture of fore and hind wings, by having only three tarsal segments, and by usually having aquatic larvae with tracheal gills. They can be distinguished from Embioptera by their unspecialised fore tarsi, distinctive wing venation, mouth-parts structure and by having ocelli. Plecoptera do not show close phylogenetic relationships with any other neopterous group and possibly they are the sister group of the Neoptera as a whole.
The literature on world Plecoptera up to 1972 is covered in three catalogues (Claassen 1940; Illies 1966; Zwick 1973). Comprehensive treatments of parts of the Australian stonefly fauna are those of Hynes (1978) on the larvae of Vic., Hynes (1981, 1982) and Hynes and Bunn (1984) on larvae and adults of Tas. and W.A. and Theischinger and Cardale (1987) on the adults of all Australian species. Michaelis and Yule (1988) catalogued the Australian species.
Mandibulate exopterygote Neoptera, with two, subequal pairs of membranous wings; larvae almost all aquatic.