Stoneflies are members of a small insect order known as the Plecoptera. There are about two thousand species worldwide, living in or around streams, in most temperate and tropical ecosystems. The people who know them best are fishermen, because stonefly larvae are an important food for many species of freshwater fish, including trout. The young stoneflies, or nymphs, live a flightless aquatic existence while adult stoneflies are typical insects with soft greenish or brownish bodies, two pairs of membranous wings and long antennae. Plecoptera means folded wing and refers to the fact that the second pair of wings are pleated and folded under the front wings. Plecopterans are an old order and fossils exist from the Permian period 250 million years ago. Today’s stoneflies are little changed from their ancestors.
Like many insects, the stonefly spends most of its life as a nymph and a very short period as a sexually active adult. Larval stoneflies have to pass through a large number of molts before becoming adults and this takes at least a year and sometimes as many as three or four years. Larval stoneflies flourish in clean, cool, well-aerated waters, preferably with a gravel bottom. They live under the water and either breathe by diffusion or use external tufted gills on the mouthparts, thorax, legs, abdomen or even extruded from the anus. Many are vegetarian but some are carnivorous. A few species have terrestrial larvae that live in moist soils.
The adults have medium-sized flattened bodies from one to five centimeters long. Their four membranous wings are held flat across the body and the hind wings tend to be broader than the front pair. They have relatively long, thread-like antennae usually at least half the length of the body, and a pair of thread-like cerci at the back of the abdomen. Their tarsi have three segments each.
Stoneflies are usually poor fliers and spend much of their time sitting on waterside vegetation. They often depend on camouflage and blend in well with the substrate on which they rest. Some will hide under loose bark or in logs. Some species are wingless or have reduced wings. They are usually yellow-green or brownish in colour and live for 2 to four weeks, during which time they mate and lay eggs. Many do not eat during this period, Those that do are vegetarian, scraping algae from rocks or eating pollen from nearby trees. Mating occurs on vegetation and then the female lays up to a thousand eggs just under the surface of the water. The eggs are sticky and can adhere to rocks or gravel in the stream.
The strangest stonefly is a species of the family Nemouridae which lives its entire life, nymph and adult, at a depth of sixty meters or more in North American lakes. It has no external gills and so must absorb oxygen by diffusion. Obviously the adults of this species are wingless.
Stoneflies depend on clean, well-oxygenated water for their nymphs to survive. As such, they are a good indicator of the health of a stream or river. Only a small amount of pollution will eliminate these animals and remove an important part of the food chain for the fish of those systems.
References: C. O’Toole 1986. The Encyclopedia of Insects. George Allen and Unwin. CSIRO 1979. The Insects of Australia. Melbourne University Press. http://tolweb.org/Plecoptera
