Stonefly

Stoneflies belong to the order Plecoptera, an ancient lineage of insects closely tied to cold, clean, fast-flowing freshwater throughout their nymphal life. They are considered one of the most pollution-sensitive insect groups, and biologists routinely survey stonefly nymphs to gauge the health of streams and rivers.

Adults are relatively drab, flattened insects with two pairs of membranous wings that fold flat over the body, giving them a somewhat plain, unassuming look compared to showier aquatic insects like dragonflies. They are generally weak, clumsy fliers and stay close to the water where they developed.

Stonefly nymphs, sometimes called naiads, are flattened and streamlined for life clinging to rocks in strong currents, equipped with gills that in many species are visible as tufts on the body or legs, another useful identification clue tied directly to their need for highly oxygenated water.

• Adult body flattened and elongated, typically brown, black, or yellowish, with two pairs of membranous wings folded flat over the back at rest (not tent-like)
• Long antennae and two long, thread-like tail filaments (cerci) at the abdomen tip
• Weak, clumsy flier; often seen walking or resting on streamside rocks and vegetation rather than in flight
• Nymphs are flattened, gilled, and cling tightly to the undersides of submerged rocks in fast current
• Some species have visible gill tufts near the legs or thorax on the nymph
• Lookalikes: mayflies (nymphs have three tail filaments and gills along the abdomen rather than near the legs) and caddisfly larvae (soft-bodied, often cased, not flattened)

Stoneflies are found on every continent except Antarctica, with the greatest diversity in cold, clean mountain and northern streams. Nymphs require consistently cold, highly oxygenated, unpolluted water and cling to rocks in riffles and rapids, making them scarce or absent in warm, slow, or degraded waterways. Adults emerge along stream banks and remain close to the water, active mainly in cooler months or early morning and evening depending on species and region.

Adult stoneflies feed little; some species do not feed at all as adults, while others graze on algae or lichen on streamside rocks and vegetation. They are generally sluggish fliers and spend much of their short adult life resting on rocks, bridges, or plants near the water. Nymphs graze on algae and decaying plant matter or, in some larger species, prey on other aquatic invertebrates, and their sensitivity to oxygen levels and pollution makes them one of the most valuable biological indicators of stream water quality, in addition to being an important food source for trout and other stream fish.

Females typically drop or dip egg masses directly into the water while in flight. Nymphs develop underwater through numerous instars over a period ranging from about one year to as long as three years in larger, cold-water species, clinging to submerged rocks and gradually growing wing pads. When ready to emerge, nymphs crawl out of the water onto rocks or vegetation, where the final molt to a winged adult takes place directly, without a pupal stage. Adults live for only a few weeks, focused on mating and egg-laying before dying.