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Plecoptera are an order of insects, commonly known as stoneflies. There are some 3,500 described species worldwide (Fochetti & Tierno de Figueroa 2008), with new species still being discovered. Stoneflies are found worldwide, except Antarctica. Stoneflies are believed to be one of the most primitive groups of Neoptera, with close relatives identified from the Carboniferous and Lower Permian geological periods, while true stoneflies are known from fossils only a bit younger. The modern diversity however apparently is of Mesozoic origin (Zwick 2000).
Plecoptera are found in both the Southern and Northern hemispheres, and the populations are quite distinct although the evolutionary evidence suggests that species may have crossed the equator on a number of occasions before once again becoming geographically isolated (Hynes 1993, Zwick 2000).
The nymphs are aquatic and live in the benthic zone of well-oxygenated lakes and streams. A few species found in New Zealand and nearby islands have terrestrial nymphs, but even these inhabit only very moist environments. The nymphs physically resemble wingless adults, but often have external gills, which may be present on almost any part of the body. In addition, they can also respire through the general body surface, and some even lack gills altogether. Most species are herbivorous as nymphs, feeding on submerged leaves and benthic algae, but many are hunters of other aquatic arthropods (Hoell et al. 1998).
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.
The insects remain in the nymphal form for one to four years, depending on species, and undergo anything from 12 to 33 molts before emerging and becoming terrestrial as adults. The adults generally only survive for a few weeks, and emerge only during specific times of the year. Some do not feed at all, but those that do are herbivorous (Hoell et al. 1998).