What Are Stoneflies?
Stoneflies are a relatively small but ecologically significant order of insects known scientifically as Plecoptera, meaning “braided wings.” With around 3,500 species worldwide and approximately 34 species in the UK, they are among the most important aquatic insects in freshwater ecosystems. Stonefly nymphs require clean, well-oxygenated water, making them one of the most reliable bioindicators of water quality. For anglers, ecologists, and freshwater biologists, stoneflies are a group of exceptional interest.
Why “Stoneflies”?
The common name derives from the behaviour of the aquatic nymphs, which are most often found clinging to stones on the beds of fast-flowing streams and rivers. Adults are frequently seen resting on riverside stones and boulders after emergence.
Identification
Adult Stoneflies
Adult stoneflies are soft-bodied insects with two pairs of membranous wings that are folded flat over the abdomen at rest, giving them a characteristically elongated, narrow appearance. Key identification features include:
- Two long, thread-like cerci (tails) at the tip of the abdomen
- Long, slender antennae
- Flattened body held close to the surface when at rest
- Wings folded flat over the body (not tent-like as in caddisflies)
- Generally dull colours: browns, yellows, greys, or black
- Body length ranges from 5 mm in small species to over 30 mm in the larger Perlidae
Nymphs
Stonefly nymphs are aquatic, flattened, and possess:
- Two prominent cerci (unlike mayfly nymphs, which may have three)
- Two claws on each foot
- Tufted or filamentous gills, usually located on the thorax or at the base of the legs
- Wing buds that develop externally as the nymph matures
| Feature | Stonefly Nymph | Mayfly Nymph | Caddisfly Larva |
|---|---|---|---|
| Cerci (tails) | 2 | 2 or 3 | 0 (or very short) |
| Gills | Thoracic tufts | Abdominal plates/filaments | Abdominal filaments (if present) |
| Claws per foot | 2 | 1 | 1 |
| Case | No | No | Yes (usually) |
| Body shape | Flattened | Varied | Cylindrical |
Lifecycle
Stoneflies undergo incomplete (hemimetabolous) metamorphosis, passing through three life stages: egg, nymph, and adult. There is no pupal stage.
1. Eggs – Females deposit egg masses into flowing water, often by dipping the tip of their abdomen into the surface while flying or by crawling to the water's edge. Some species drop egg clusters from the air. A single female may produce several hundred to several thousand eggs.
2. Nymphs – The nymphal stage is the longest part of the lifecycle, lasting from one to four years depending on the species. Nymphs live in clean, fast-flowing streams and rivers, typically under stones and gravel. They moult numerous times (up to 30 instars in some species) as they grow.
3. Adults – When fully developed, nymphs crawl out of the water onto stones or vegetation to undergo their final moult. Adults live for a few days to a few weeks. Many species are poor fliers and stay close to their natal stream. Some winter-emerging species are brachypterous (short-winged) and cannot fly at all.
Did you know? Some stonefly species emerge in the middle of winter, even walking across snow to find mates. These “winter stoneflies” (notably members of the family Nemouridae) are among the very few insects active during the coldest months of the year.
Stoneflies as Water Quality Indicators
Stonefly nymphs are exceptionally sensitive to pollution, particularly low dissolved oxygen levels and organic contamination. Their presence in a waterway is a strong indicator of good to excellent water quality. Environmental agencies across the world use stoneflies as part of biological monitoring programmes:
- In the UK, the Biological Monitoring Working Party (BMWP) scoring system assigns stonefly families the highest sensitivity scores.
- The absence of stoneflies from a previously occupied site may indicate deteriorating water quality.
- Species of the family Perlidae are particularly intolerant of pollution and are among the first to disappear from degraded rivers.
Ecological Importance
Stoneflies play vital roles in freshwater ecosystems:
- Nutrient cycling – Many nymphs are shredders, breaking down leaf litter that falls into streams. This process releases nutrients and makes organic matter available to smaller organisms.
- Food web – Stonefly nymphs are a crucial food source for trout, salmon, and other freshwater fish. Adult stoneflies are eaten by birds, bats, and spiders.
- Predation – Nymphs of larger species (Perlidae, Perlodidae) are active predators of other aquatic invertebrates, including mayfly nymphs and chironomid larvae.
Stoneflies and Fly Fishing
Stoneflies have a long history in the sport of fly fishing. Anglers have tied artificial flies to mimic stonefly nymphs and adults for centuries. Classic fly patterns such as the “Stonefly Nymph” and the “Yellow Sally” are staples of the trout angler's fly box, and hatches of stoneflies on upland rivers can trigger spectacular rises of fish.
Key Takeaway
Stoneflies are vital freshwater insects whose presence signals clean, well-oxygenated water. Their long aquatic nymphal stage, sensitivity to pollution, and roles in nutrient cycling and food webs make them both ecologically important and scientifically valuable. Monitoring stonefly populations is a key component of assessing river health across the UK and beyond.