Stonefly

Water quality indicator

Stonefly larvae are among the most pollution-intolerant of all aquatic invertebrates. They require water with very high dissolved oxygen content — cold, fast-flowing, and free of agricultural runoff, sewage, and acidification. Their presence in a river assessment sample is a reliable indicator of excellent water quality; their absence from historically suitable habitat signals degradation. River conservation organisations use stonefly survey data as one of the primary metrics for assessing and tracking river health.

Ancient lineage

The Plecoptera are one of the most ancient insect orders, with fossil stoneflies from the Carboniferous period over 300 million years ago that are recognisably similar to modern species. This evolutionary conservatism — the basic body plan preserved for hundreds of millions of years — suggests that the stonefly design is highly successful and that the ecological niche they occupy has been stable across deep time. They pre-date many of the fish species that now prey on them.

Larval life

Stonefly larvae (nymphs) live under stones, in gravel, and in the leaf litter of streambeds for one to three years depending on species and temperature. They moult repeatedly, sometimes up to 30 times, growing slowly. The larger predatory species are important controls on other invertebrate populations; the smaller herbivorous species process fallen leaves and algae, contributing to nutrient cycling. When ready to emerge, larvae crawl out of the water and climb onto stones or vegetation.

Importance to fly fishing

Stoneflies are among the most important invertebrates in fly fishing. The large Perlidae species — the stone flies of chalk stream tradition — are imitated by fly tyers and hatch in significant numbers on productive rivers. The emergence of large stonefly species in spring and early summer is one of the events that triggers selective feeding by trout, and accurate imitation of the adult stonefly is a significant part of the dry fly fishing tradition.