Phylum Arthropoda
Subphylum Hexapoda
Class Insecta
Order Plecoptera
Common name: stoneflies
Overview
The Plecoptera (stoneflies) is a small order of insects with terrestrial adults and aquatic nymphs. Adult stoneflies are soft-bodied, elongate and flattened and up to 50 mm long; they have long, many-segmented antennae and mandibulate mouthparts. They have two pairs of membranous wings that are folded around the body at rest and can have a wingspan of 100 mm. A few species are wingless (apterous) or have reduced wings (brachypterous). Plecoptera are also characterised by relatively long legs and two long cerci at the end of the abdomen. Adult stoneflies are always found near freshwater and, being weak fliers, have poor dispersal capabilities. They rarely move far from the water bodies from which they emerge. The aquatic nymphs look like wingless adults and typically have external filamentous gills along the edge and on the end of the abdomen.
Distribution and diversity
Stoneflies are found throughout Australia, except for the Northern Territory. Their distribution is considerably restricted in the more arid environments of central Australia. Most species inhabit the cooler regions of the continent, particularly in the south-east and Tasmania. Worldwide over 2,000 species of stoneflies are recognised. For Australia 192 described species are recorded, in 26 genera assigned to four families, but these are known largely from adult specimens; the nymphs of many Australian species still unknown.
Life cycle
Typically adult stoneflies mate during the day and sperm transfer is direct. Up to 100�1000 eggs may be laid singly or in masses. In some species, the females fly slowly over the surface of the water and eggs are washed from the tip of their abdomen. Others crawl under water and lay their eggs on the undersides of stones. Some species with long ovipositors place their eggs deep into wet rock crevices. Many Australian species seem to oviposit at night. Eggs hatch within a few weeks or may take up to a year. The aquatic nymphs resemble the adults but lack wings. Depending on the species, stonefly nymphs can moult up to 15 times before reaching maturity, which can take several months to 2�3 years. Mature nymphs crawl out onto plants, rocks or logs at the edge of the water where the adult emerges.
Feeding
Adult stoneflies are herbivores and detritivores. They use their mandibles to break apart and feed upon algae, lichens, rotting wood or other detritus. Nymphs are also mostly herbivorous and detritivorous and feed on a wide variety of small plants, algae or dead plant matter; a few species are predatory.
Ecology
Typically stonefly nymphs prefer cool, fast-flowing, well-aerated streams and lakes, and live among the gravel and sand on the bottom. Adults can be found nearby on tree trunks, vegetation and rocks in the riparian zone. Because stoneflies depend upon cool, well-oxygenated water for their nymphal development, they are very susceptible to any pollution that decreases oxygen content or riparian habitat clearing that reduces shade and thus leads to an increase in water temperature. As such, stoneflies are often used as ecological indicators for monitoring changes to water quality.
Stonefly from Tasmania
Image Credit: Photographer: Mark Harvey � Western Australian Musuem
