The Plecoptera are a relatively small order of insects. They are characterized by an aquatic immature (nymph) stage (for most species) and terrestrial adults. The nymphs are found mainly in streams and rivers, although some species can live in the wave-washed shores of lakes especially in cool northern or high altitude lakes. Adults are found near streams, either in vegetation, or on or under loose rocks along the edges. Worldwide, stoneflies can be found on all continents except Antarctica.

Stoneflies usually complete their development from egg to adult within a year, though some species have shorter life cycles and some have cycles as long as four years. Depending on species, eggs may be dropped on the water surface from flight or while walking over rocks near shore. In a few species, the females enter the water and walk on the stream bottom to deposit their eggs. The eggs have attachment structures or sticky coverings that allow them to attach to substrates so they are not washed away. In most species, the eggs develop quickly and are ready to hatch within 3-4 weeks, though some species in intermittent streams or streams showing wide temperature extremes may show arrested development that allows them to hatch when conditions are the most suitable. Others may suspend development for several years.

For most species, the majority of the stonefly life cycle is spent as an immature nymph in the water, and habitat conditions during development are critical to their survival. Stoneflies require well oxygenated and clean water for survival, and most species have very specific requirements for certain water temperatures, substrates, and food resources. These lead to species-specific distributional differences along the lengths of streams, as well as within microhabitats in stream sites. For example, some species prefer to live in packs of deposited leaves, whereas others may be on the surfaces or sides of rocks, or even deep within the stream gravelly substrates.

Nymphs of most temperate and northern species hatch from the eggs in summer, and grow through autumn and spring to emerge as adults sometime during the open water season in summer. A few species, known as “winter stoneflies” (some species in Capniidae and Taeniopterygidae) emerge as adults when ice is still present, and may mate on or under the ice, without leaving the stream. Most northern species are found in water bodies that do not freeze solid, and the nymphs are active under the ice. Species in small or intermittent streams, however, may spend the winter frozen in the ice, either as an egg or nymph that can stop their development in a process called diapause.

Stonefly nymphs show a variety of feeding types, depending on species, but fall mainly into two categories: herbivore-detritivores feeders and predators. The herbivore-detritivores include “shredders”, which tear and eat relatively large pieces of dead plant material, such as fallen leaves, in the water, and “collectors”, which gather and feed on fine detritus particles. The predators feed mainly on other aquatic insects. These feeding categories are not fixed, however, and many of the predaceous species may feed on detritus when they are small, and some of the detritus feeders may also feed by scraping algae off of rocks and submerged vegetation. Of those found in the NWT, members of the families Capniidae, Leuctridae, Nemouridae, Taeniopterygidae, and Pteronarcyidae feed on detritus, and members of the Chloroperlidae, Perlidae, and Perlodidae are considered primarily predaceous.

Adults may live for one to a few weeks, and generally stay close to the stream or lake from which they’ve emerged (‘hatched’). During emergence, nymphs of most species crawl out of the water onto streamside rocks or vegetation, then the adult form crawls out of a split that forms in their nymphal skin (exoskeleton) along the back. Newly emerged adults are soft and generally light in colour, and at this stage are called “teneral”. They harden quickly, though, and move to vegetation or crevices along shore to begin hunting for a mate. Some do not feed as adults, while others feed on algae, lichens, pollen, or nectar. Adults do not fly very well, and are most often seen clinging to vegetation or hiding under stones.

Stoneflies in the northern hemisphere have a unique method of locating their mates, known as “drumming”. They start by gathering near the stream or lake margin, then the males call by tapping their abdomens on a hard surface in species-specific patterns. Virgin females answer the male call, and then stay in one spot so the males can find them. Both call and answer until the male has located the female, and then they mate.

Stoneflies have long been known to be important as “fish food” in our streams and rivers, and are usually quite well known to fly-fishers. Due to their diverse feeding habits, they are important in all levels of aquatic food webs: some species contribute to the breakdown of detritus in streams, some are predators in their own right, and all provide a food source to other predatory animals in the streams or lakes in which they reside. Adults may also be important food for birds and other terrestrial insectivores.

The stoneflies’ requirements for clean and well oxygenated water have also led to their use as “biological indicators”. We can assess stream health by looking at the diversity and abundance of different species of stonefly and other stream invertebrates. Unfortunately, pollution in many industrialized areas has led to major declines in stoneflies throughout the world.

There are 37 species of stoneflies confirmed present in the NWT. One more species is expected to be present in the NWT.