Temporal range: 299–0Ma Permian–Recent
Plecoptera are an order of insects, commonly known as stoneflies. There are some 3,500 described species worldwide, 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.
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.
Stonefly Nymphs live in trees.
Description and ecology 
Stoneflies have a generalised anatomy, with few specialised features. They have simple mouthparts with chewing mandibles, long, multi-segmented antennae, large compound eyes and two or three ocelli. The legs are robust, with each ending in two claws. The abdomen is relatively soft, and may include remnants of the nymphal gills even in the adult. Both nymphs and adults have long paired cerci projecting from the tip of their abdomens.
The name "Plecoptera" literally means "braided-wings", from the Ancient Greek plekein (πλέκειν, "to braid") and pteryx (πτέρυξ, "wing"). This refers to the complex venation of their two pairs of wings, which are membranous and fold flat over the back. Stoneflies are generally not strong fliers, and some species are entirely wingless.
A few wingless species such as the Lake Tahoe benthic stonefly ("Capnia" lacustra[Note 1]) or Baikaloperla are the only known insects, perhaps with the exception of Halobates, 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 females lay hundreds or even thousands of eggs in a ball which they initially carry about on their abdomen, and later deposit into the water. The eggs typically take two to three weeks to hatch, but some species undergo diapause, with the eggs remaining dormant throughout a dry season, and hatching only when conditions are suitable. Stonelflys like to live in areas with running water.
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.
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.
Traditionally, the stoneflies were divided into two suborders, the "Antarctoperlaria" (or "Archiperlaria") and the Arctoperlaria. However, the former simply consists of the two basalmost superfamilies of stoneflies, which do not seem to be each other's closest relatives. Thus, the "Antarctoperlaria" are not considered a natural group (despite some claims to the contrary).
The Arctoperlaria, meanwhile, have been divided into two infraorders, the Euholognatha (or Filipalpia) and the Systellognatha (also 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 (meaning approximately "advanced Holognatha"). But the Scopuridae do not appear significantly closer to the Euholognatha than to the Systellognatha.
In addition, not adopting the clades Antarctoperlaria and Holognatha allows for a systematic layout of the Plecoptera that adequately reproduces phylogeny, while retaining the traditional ranked taxa.
Basal lineages ("Antarctoperlaria")
- Superfamily Eusthenioidea
- Superfamily Leptoperloidea
- Basal family Scopuridae
- Infraorder Euholognatha
- Infraorder Systellognatha
- Family Chloroperlidae (100+ species) - green stoneflies
- Family Perlidae (ca. 400 species) - common stoneflies
- Family Perlodidae (250+ species)
- Family Peltoperlidae (ca. 68 species) - roachlike stoneflies
- Family Styloperlidae (ca. 10 species)
- Family Pteronarcyidae (ca. 12 species) - salmonflies, giant stoneflies
- Romolo Fochetti & José Manuel Tierno de Figueroa (2008). Global diversity of stoneflies (Plecoptera; Insecta) in freshwater. In E. V. Balian, C. Lévêque, H. Segers & K. Martens. "Freshwater Animal Diversity Assessment". Hydrobiologia 595: 265–377. doi:10.1007/978-1-4020-8259-7_39.
- Brittain, 1987
- Peter Zwick (2000). "Phylogenetic system and zoogeography of the Plecoptera". Annual Review of Entomology 45: 709–746. doi:10.1146/annurev.ento.45.1.709.
- H. B. N. Hynes (1993). Adults and Nymphs of British Stoneflies. Freshwater Biological Association. ISBN 0-900386-28-2.
- Nelson, Riley. "Clean water has bugs in it, says BYU Biology Professor Riley Nelson". Retrieved 16 May 2013.
- Hoell, H.V., Doyen, J.T. & Purcell, A.H. (1998). Introduction to Insect Biology and Diversity, 2nd ed. Oxford University Press. pp. 383–386. ISBN 0-19-510033-6.
- S. C. Woodhouse (1910). English-Greek Dictionary - a Vocabulary of the Attic Language. London: George Routledge & Sons.
- C. Riley Nelson (January 1, 1996). "Capniidae. Winter Stoneflies". Tree of Life Web Project. Retrieved July 31, 2008.
- E. M. Holst (2000). "Lake Tahoe benthic stonefly (Capnia lacustra)". In D. D. Murhy & C. M. Knopp. Lake Tahoe Watershed Assessment (PDF). United States Department of Agriculture. pp. O–118 – O–120.
- C. Riley Nelson (January 1, 1996). "Plecoptera. Stoneflies". Tree of Life Web Project. Retrieved July 31, 2008.
- Nelson (1996b)