Concept

Plecoptera

Summary
Plecoptera is an order of insects, commonly known as stoneflies. Some 3,500 species are described 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. Their 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 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. Stoneflies have a generalized anatomy, with few specialized features compared to other insects. They have simple mouthparts with chewing mandibles, long, multiple-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 their backs. 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) 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 lives, but can leave the water to travel.
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