Royal Alberta Museum: Invertebrate Zoology - Bug Facts

[Caterpillars and Other Immature Insects]

Fact Sheets: Commonly Observed and Asked About Insects and Spiders Found in Alberta

Giant stoneflies (Pteronarcys spp.)

Giant Stonefly
Credit: Terry Thormin

INTRODUCTION

This large, impressive insect can often be found crawling around the shorelines of streams and rivers, especially in rocky areas. Occasionally individuals will fly some distance from the water and turn up in people's yards, being attracted to lights at night.

IDENTIFICATION

Giant stoneflies can be up to 40 mm long, including the wings. Some small species of stoneflies are only about 10 mm long. The long lacy wings that overlap one another make this a rather distinctive-looking insect.

DISTRIBUTION

Giant stoneflies are found throughout the province near streams and rivers. Some smaller stoneflies are found near lakes. All stonefly larvae are aquatic.

TIME OF YEAR

Adults of the Giant stoneflies are generally out from late May through early July, but some smaller species may emerge by early May and other species are primarily out in the fall. The larvae of some species live for up to three years and may remain active all year long.

HABITAT AND HABITS

All larvae are aquatic and are found primarily in streams and rivers. Many species are sensitive to a lack of dissolved oxygen in the water, and are good indicators of organic pollution. They feed on plants, decaying leaves and detritus. They are also a favorite food for trout, as most fishermen will know. Adult giant stoneflies do not feed and thus usually live only for a week. Other species may live for more than a month as adults. Males of many species attract a mate by drumming their abdomens on the substrate, the females feeling the vibrations rather than hearing the sound. .

SIMILAR SPECIES

There are over 200 species of stoneflies found in Alberta that range in size from about 10 mm to 40 mm. Most are grey and black, some having orange areas between the thoracic plates. Some of the smaller species are greenish.

COMMENTS

As large as they are, these insects are incapable of inflicting any harm; in fact the largest ones, the giant stoneflies, have no mouthparts as adults. They are an important part of stream and river fauna, providing food for fish and birds like the Dipper, and their absence is often a good indicator that a stream is being polluted.

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Last update: August 29 2008