Plecoptera are an order of insects, commonly known as stoneflies. There are some 1,700 recorded species worldwide, and new ones continue to evolve. 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.
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.
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 nymphs are aquatic and live in the benthic zone of lakes and streams. Nymphs of this order are hunters of other aquatic arthropods or shredders of large organic particles, such as leaves. Some also graze on benthic algae. They undergo many molts as aquatic nymphs (10 instars?) before emerging and becoming terrestrial as adults.
A few wingless species such as the Lake Tahoe Benthic Stonefly ("Capnia" lacustra) or Baikaloperla are the only known insects that are exclusively aquatic from birth to death. Some true water bugs (Nepomorpha) may also be fully aquatic for their entire life, but can leave the water to travel.
The Arctoperlaria, meanwhile, have been divided into two infraorders, the Euholognatha (or Filipalpia) and the Systellognatha (aso called Setipalpia or Subulipalpia). This corresponds to the phylogeny, with one exception: the Scopuridae must be considered a basal family in the Arctoperlaria, not assignable to any of the infraorders. Alternatively, the Scopuridae were placed in an unranked clade "Holognatha" together with the Euholognatha (hence the name of the latter, meaning approximately "advanced Holognatha"). But the Scopuridae do not appear significantly closer to the Euholognatha than to the Systellognatha, and therefore the "Holognatha" are also not used here either. In addition, not adopting these two dubious "clades" allows for a systematic layout of the Plecoptera that adequately reproduces phylogeny, while retaining the familiar ranked taxa.
Basal lineages ("Antarctoperlaria")