The common name for insects in the order Plecoptera, stoneflies, is derived from the preference of the nymphs for the rocky substrates of streams, rivers, or lake margins. Stoneflies are important predators and shredders in aquatic ecosystems and are an excellent source of food for many fish. As a family, they are especially sensitive to human disturbance in watersheds and thus can be excellent indicators of water quality. According to The Nature Conservancy and NatureServe (2000), stoneflies are the most at risk group of all insects in North America, with 43% of all stonefiles vulnerable to extinction, imperiled or extinct. Unfortunately, the conservation needs of this family are often overlooked. The Xerces Society has developed species profiles for three at-risk stoneflies of the intermountain west in the red list of aquatic invertebrates.
Stoneflies make up one of the most ancient insect orders. They undergo incomplete metamorphosis, so the aquatic nymphs look similar to the terrestrial adults. Some adults even retain gill remnants on their thorax or neck. Stonefly adults are generally weak fliers and stay close to stream, river, or lake margins where the nymphs are likely to be found. The nymphs occur mostly in flowing water, but can also be found under the stones of rocky lake margins or (in the case of one species in the family Capniidae) deep in Lake Tahoe, California.
Plecoptera typically have flattened, somewhat roach-like bodies that are well-adapted for living under and between rocky substrates. Their shape also helps them move aerodynamically in fast-flowing water. There are two tails, or cerci, at the tip of the abdomen, each with many small segments. They have two claws at the end of each leg and usually have finger-like gills on the head, thorax, or between the tails.