STONEFLIES: Plecoptera

PHYSICAL CHARACTERISTICS

Adult stoneflies are brown, black, green, or yellow and are usually marked with distinctive light or dark patterns. Their bodies are somewhat flattened with legs outstretched to the sides. They range in length from 0.19 to 1.97 inches (5 to 50 millimeters). The broad head is equipped with compound eyes that have more than one lens each, with simple eyes with only one lens, and with chewing mouthparts that are directed forward. The antennae (an-TEH-nee), or sense organs, are long and threadlike. Nearly all species have four fully developed wings that are held flat over the back. At rest the wings are usually as long or longer than the abdomen. A few species of stoneflies are completely wingless or have short wings that are not capable of flight. The hind wings are folded lengthwise when held at rest under the forewings. The abdomen is ten-segmented and is tipped with a pair of short to long, threadlike projections.

The larvae (LAR-vee), or young form of the animal, may or may not closely resemble the adults. Like the adults, their bodies are flattened, with short, sometimes pointed wing pads and outstretched legs. Plant feeders and scavengers (SKAE-vihnjers), or animals that eat decaying matter, have specialized mouthparts that allow then to scrape algae off rocks, collect fine bits of plant food, shred living and dead leaves into smaller pieces, or chew chunks from leaves. Predatory larvae have sharpened mouthparts that help them to grasp and hold their prey, or animals hunted for food. The larvae may have simple or feathery gills located on their head, thorax or midsection, and abdomen. They always have a pair of segmented projections on the tip of their abdomen.

GEOGRAPHIC RANGE

Stoneflies live on all continents, except Antarctica. They are also found on most larger islands except Cuba, Fiji, Hawaii, and New Caledonia. There are approximately two thousand species of stoneflies worldwide, with about six hundred in the United States and Canada.

HABITAT

Both adults and larvae live primarily in cold, running streams and rivers. The larvae live on the bottom of streams and rivers. A few species prefer the gravelly shores of mountain lakes where there is a lot of wave action. Each species of stonefly lives in its own special habitat, including rocky bottoms, in spaces among loose gravel, piles of waterlogged leaves and debris, or on submerged logs. Adults are found during the day resting on vegetation, rocks, and debris along running waters. They are sometimes attracted to lights at night.

DIET

The adults of some species feed on algae (AL-jee), lichen (LIE-kuhn), pollen, or nectar, but the food preferences of most species are still unknown. Some adults apparently do not feed at all. The larvae feed on living and dead plant or animal materials. Some species shift their food preferences from one food group to another as they mature.

BEHAVIOR AND REPRODUCTION

In North America and Europe male and female stoneflies usually find one another by joining in mating swarms near streams and rivers. Males and females locate one another through a complex series of vibration signals known as drumming. These signals are attractive only to individuals of the same species. Stoneflies drum by tapping, rubbing, or scraping their abdomens on a rock or log. Other species produce signals by doing pushups or rocking back and forth. Males typically drum as they search for females. If interested, a female perched nearby will drum back. Both the male and female continue to communicate this way until they locate one another and mate. Males will attempt to mate many times, but females will mate only once. Mated females will not answer male drumming calls.

Males transfer sperm directly to the female's reproductive organs during mating. The eggs are laid in pellets or masses containing many eggs. The female then flies over the water, either dipping her abdomen in the water to deposit the eggs, or simply dropping them from the air. In some species the female runs along the shore to lay her eggs directly in shallow water. Other females submerge themselves completely to place their eggs directly on the stream bottom. The eggs either hatch within three to four weeks or enter diapause (DIE-uh-pawz), a period of rest that lasts three months to one or more years.

Stonefly larvae somewhat or closely resemble the adults and develop gradually. They molt, or shed their exoskeletons or hard outer coverings, ten to twenty-five times before reaching adulthood. Their life cycle may take months or years depending on species and local conditions, such as water temperature. When mature the larvae crawl out of the water and molt for the last time. Their shed exoskeletons are commonly found on rocks and vegetation near streams and rivers in spring and summer. The adults live only briefly and do not provide any care for the larvae.

STONEFLIES AND PEOPLE

Stoneflies are a very important part of stream food webs. Since nearly all species require clean water to reproduce, their presence in a stream or river is used as an indication of good water quality. Both the adults and larvae are an important food source for fish. Fishermen make lures called flies that imitate the forms of both adult and larval stoneflies and use them instead of living bait to catch fish.

HOW CLEAN IS YOUR STREAM?

Aquatic insects are used to measure the quality of freshwater habitats. One method of measuring water quality is to count the total number of larvae of three pollution-sensitive insect groups: mayflies, stoneflies, and caddisflies. This number is compared with the total number of pollution-tolerant fly larvae known as midges. Streams with more mayflies, stoneflies, and caddisflies and fewer midges are less likely to be polluted than other streams in the same area with relatively more midges.

CONSERVATION STATUS

Four species of stoneflies are listed by the World Conservation Union (IUCN). One is listed as Extinct, or no longer living; two are listed as Vulnerable, or facing a high risk of extinction in the wild. The fourth species lacks sufficient information to determine the chances of it becoming extinct. Habitat destruction caused by development, logging, and other man-made or natural events that change water quality are the greatest threat to stonefly populations.

Stonefly species are sometimes found only in a particular stream system or are considered rare and restricted to a small geographic region. This has resulted in the development of local, regional, and state lists that identify these species and their need for special consideration and further study. Stonefly larvae have similar space and clean water requirements to small trout and other fish of similar size. Techniques used to maintain healthy native fish populations might work equally well to manage populations of stonefly larvae. However, these practices are rarely used to protect stonefly habitats.

GIANT SALMONFLY (Pteronarcys californica): SPECIES ACCOUNT

Physical characteristics: The giant salmonfly is a large stonefly measuring 1.18 to 1.97 inches (30 to 50 millimeters) in length, with a wingspan of 2.25 to 3.25 inches (58 to 84 millimeters). They are dark brown, with a reddish line down the middle of their midsection. The larvae have branched gills under the first two abdominal segments. These gills are reduced to small stubs in the adults.

Geographic range: The giant salmonfly is widespread in western North America.

Habitat: Adults and larvae are associated with fast-moving mountain streams.

Diet: The larvae feed on algae and shred plant materials. The adults do not feed.

Behavior and reproduction: Adult males and females emerge in spring and gather along streams and rivers on vegetation for drumming and mating. Males produce heavy six-beat signals, and females answer with similar signals. The larvae require two to three years to reach adulthood.

Giant salmonflies and people: The giant salmonfly is an important food source for trout. Both the larvae and adults are used as models by fly fishermen to create artificial lures.

Conservation status: This species is not endangered or threatened. ∎

FOR MORE INFORMATION

Books:

Resh, V. H., and D. M. Rosenberg. The Ecology of Aquatic Insects. New York: Praeger Publishers, 1984.

Stark, B. P., S. W. Szczytko, and C. R. Nelson. American Stoneflies: A Photographic Guide to the Plecoptera. Columbus, OH: Caddis Press, 1998.

Stewart, K. W., and B. P. Stark. Nymphs of North American Stonefly Genera (Plecoptera), 2nd ed. Columbus, OH: Caddis Press, 2002.

Periodicals:

Amos, W. H. "Unseen Life of a Mountain Stream." National Geographic 151, No. 4 (April, 1977): 562–580.

Web sites:

Gordon's Plecoptera (Stonefly) Page.http://www.earthlife.net/insects/plecopt.html (accessed on September 29, 2004).

"Plecoptera. Stoneflies," Ecowatch.http://www.ento.csiro.au/Ecowatch/Insects_Invertebrates/plecoptera.htm (accessed on September 29, 2004).