Stoneflies

If you think the best thing about a PEI winter is the absence of insects, I have bad news for you: this is peak season for some of our invertebrates! Carla McKie recently sent me this photo of one the creepie-crawlies on the snow in Rock Barra, wondering what they might be. They are Winter Stoneflies, also known as Willowflies (Taeniopterygidae Family).

At first glance, you might think this is an Earwig out for a mid-winter stroll. Like Earwigs, Winter Stoneflies have six legs, two antennae at the front, and a pair of structures called cerci at the rear. However, Winter Stoneflies are smaller than Earwigs, and those cerci are thin and straight rather than pincer-like. The cerci’s form reflects their function: in Stoneflies, they detect air and water vibrations; Earwigs use theirs for defense.

Stoneflies live among rocks and stones on the bottom of fast-flowing, freshwater streams. There are hundreds of species in North America, usually grouped by when they emerge from their aquatic habitat: Winter Stoneflies appear from January to April, Spring Stoneflies (Nemouridae Family) appear from April to June, and Common Stoneflies (Perlidae Family) are around from April to September.

Adult Stoneflies take flight and drop their eggs into clean, oxygen-rich streams where they settle to the bottom and eventually hatch into nymphs. Depending on the species, nymphs may be predators or shredders (feeding on fallen leaves) and may take one year or as many as four years to mature. Once they emerge as adults, Stoneflies have only a few weeks to mate, lay eggs, and start the cycle again. Not all adults feed, but Winter Stoneflies do eat lichens and algae to get the energy they need to survive the cold.

Stoneflies are prey for fish, predators of other invertebrates, and recyclers of nutrients (they break vegetation down into finer bits that can be used by other organisms). They are also very sensitive to sediment and pollution, making them excellent indicators of water quality and stream health. Seeing them is a good sign, and no surprise in the area of Eastern Kings County where these were found: streams there are ranked very good to excellent in PEI’s watershed water quality report cards.

Stoneflies – and other aquatic invertebrates – are important parts of PEI Untamed!