Invertebrates
Stonefly
Plecoptera · also called stoneflies
A stonefly is a plain, unassuming insect that biologists treat as one of the most reliable witnesses to a stream’s health. Its nymphs cannot tolerate warm, polluted, or sediment-choked water, so wherever stoneflies are thriving, the water almost certainly is too. Some species take this to an extreme, emerging as adults in the dead of winter and walking across snow.
Overview
Stoneflies are the order Plecoptera, ancient, flat-winged aquatic insects of cold, clean, fast-flowing streams worldwide. Depending on species, the nymph grazes algae or hunts other small invertebrates, while the short-lived adult, found resting on streamside rocks and vegetation, eats little or nothing.
Moulting on dry land
Unlike mayflies, which often moult near the surface, stonefly nymphs crawl fully out of the water onto a rock or log before shedding their final skin, leaving behind telltale empty nymphal cases along a healthy streambank. A number of species emerge in the depths of winter, walking across snow when almost nothing else is active.
A pillar of water testing
Together with mayflies and caddisflies, stoneflies make up the “EPT” group biologists survey to judge stream health — because stonefly nymphs are so intolerant of pollution, low oxygen, and sediment, their presence or absence is one of the most trusted signals of water quality anywhere.
Status
Least Concern as a group and often abundant in clean water, but declining across many industrialised regions from pollution, warming, sedimentation, and river damming — often one of the earliest measurable signs that a stream is in trouble.
Stonefly: common questions
Why do stoneflies crawl out of the water to moult?
Stonefly nymphs complete their final moult into a winged adult on land rather than in the water, climbing out onto a rock, log, bridge support, or streamside plant before splitting the old nymphal skin and emerging. This behaviour, different from mayflies which often moult at or just under the surface, may reduce the risk of the vulnerable, soft-bodied emerging adult being swept away or eaten by aquatic predators during the brief, delicate transition, and it commonly leaves behind numerous empty, intact, pale nymphal skins clinging to rocks and vegetation along a healthy stream, a useful visible sign that stoneflies are present even if the adults themselves are hard to spot.
What does it mean that stoneflies are part of the EPT group?
EPT stands for Ephemeroptera, Trichoptera, and Plecoptera, the scientific order names for mayflies, caddisflies, and stoneflies respectively, three insect groups that freshwater biologists commonly survey together because all three include species that are notably sensitive to pollution, low oxygen, and habitat degradation. Counting the number of different EPT species, or the proportion of all invertebrates in a stream sample that belong to these three orders, gives a simple, widely used, and reasonably reliable indicator of water quality, since a stream with abundant, diverse EPT insects is generally clean and healthy, while a decline or absence of these groups often signals pollution or other problems.
Can stoneflies really be active in the middle of winter?
Yes, a number of stonefly species, sometimes called winter stoneflies, are specifically adapted to emerge as adults during the coldest months, crawling out of streams and onto snow-covered banks, rocks, and even the surface of the snow itself on relatively mild winter days, when few if any other insects are active. This unusual timing may let them avoid competition and predation that would be more intense during the warmer months when most other aquatic insects emerge, and finding small, dark stoneflies walking across snow near a stream in winter is one of the more surprising sights for anyone exploring cold-climate waterways at that time of year.
Why are stoneflies considered such a good sign of water quality?
Stonefly nymphs generally have a low tolerance for warm water, low dissolved oxygen, fine sediment, and organic pollution, so they tend to disappear from a stream relatively early as water quality declines, well before many other, hardier invertebrates are affected. Because of this sensitivity, and because different stonefly species have somewhat different tolerance thresholds, the presence, absence, and diversity of stoneflies in a stream sample gives ecologists a fairly detailed and reliable picture of recent water conditions, making stoneflies, alongside mayflies and caddisflies, one of the standard groups used in biological water quality monitoring around the world.