The Stonefly or Plecoptera are an order of aquatic insects. There are some 3,500 described species worldwide, and new ones are still being discovered. Although stoneflies are found worldwide, they are absent from Antarctica. Stoneflies are believed to be one of the most primitive groups of Neoptera. Their close relatives are identified from the Carboniferous and Lower Permian geological periods. True stoneflies, known from fossils, are only a little younger. The larva have claws on their legs. This allows them to better hang on to things in their environment and withstand stronger current. They need fast moving water in streams and rivers to survive. They cannot tolerate the low oxygen saturation found in warm water. They usually live in cold clean streams with a lot of turbulence (such as rapids). The adult stoneflies are easily identified by their wings that fold over their abdomen. The "pteronarcys" Stoneflies, found only in North American waters, live for several years.