|A snowy creek bank teeming with insects? Not exactly what those of us in colder climates would expect to see on a winter day. Because insects are cold-blooded and need warmth in order to function, they typically avoid cold conditions by hibernating (referred to as diapause in insects) or finding shelter. A few, such as green darner dragonflies and monarch butterflies, migrate. Winter stoneflies do the unexpected: they brave the cold. |
Winter stoneflies (or snowflies) comprise a large number of species mostly belonging to two families, Taeniopterygidae (winter stoneflies) and Capniidae (small winter stoneflies), in the order Plecoptera. Similar to dragonflies, damselflies, mayflies, and caddisflies, stoneflies spend the bulk of their life as aquatic nymphs. While most aquatic insects develop fastest in warm weather, winter stoneflies do the opposite. As summer approaches and water temperatures rise, the nymphs burrow into the stream bed and become inactive. When conditions cool in late fall and winter, the nymphs return to the water, resume feeding (on detritus), and grow rapidly. Nymphs mature in winter or early spring and exit the frozen river via holes or cracks in the ice. Once out of the water, the stonefly undergoes its final molt into a winter-loving adult.
And what's not to love? Outlandish though it may seem, emerging during the coldest months of the year has its advantages. According to Jonathon Neal, an entomology professor at Purdue University, a huge advantage for winter stoneflies is the reduced number and relative inactivity of predators in mid-winter. With fewer predators to worry about, the conspicuous stoneflies are free to move about and mate with low mortality. Still, winter is not entirely devoid of risk, and a small number of birds, including blue jays, cedar waxwings, and robins, have been documented feeding on winter stoneflies.
The real question is why do these insects not freeze to death? The secret lies in the production of antifreeze compounds such as glycols, sugars, and proteins that disrupt the formation of ice crystals in the insect haemolymph (blood), allowing the body fluids to remain liquid at temperatures several degrees below their freezing point.
Once out of the water, winter stoneflies mill about the snow and ice in search of mates. Male stoneflies attract females by drumming their rear end against the snow and ice. The vibrations are transmitted through the substrate, and females feel, rather than hear, the male calling. Virgin females will drum in reply, and the two insects will continue to drum until they meet and mate. Shortly after, the female releases her fertilized eggs into the water where they quickly hatch into young nymphs.
Winter stoneflies have very particular water quality requirements, and are among the first animals to disappear from polluted or degraded streams and lakes. As such, they are frequently used as sentinel organisms in biomonitoring. Finding them indicates that a water body is on the clean and healthy side of things, while not finding them, particularly in places where they were once common, can be a cause for concern.
In light of this, the Xerces Society has recently completed status reviews for three seriously imperiled winter stoneflies. The straight stonefly (Capnia lineata) and the Idaho stonefly (C. zukeli) are known solely from a small network of streams and creeks in northwest Idaho, and both species are threatened by water quality impairment due to large scale logging, agriculture, and cattle grazing in the watershed, as well as thermal and chemical pollution from waste water treatment plant discharges. The Arapahoe snowfly (C. arapahoe) lives only in two small streams in northern Colorado. This species is on the brink of extinction due to habitat damage from intensive recreation, livestock grazing, logging, stream de-watering, insecticide applications, runoff from roads and trails, and pollution from residential and resort septic systems. Responding to a petition from the Xerces Society, the U.S. Fish and Wildlife Service recently verified that protection of the Arapahoe snowfly may be warranted and began its own status review, a necessary step for lising under the Endangered Species Act. Listing would result in the protection and restoration of any remaining habitat for this winter stonefly.
So keep your eyes open when you are on a snowy walk and you may just find winter stoneflies on the banks of a partially-frozen stream. And while you're at it, look out for other unusual insects on the snow, including snow fleas, winter craneflies, and winter-active caddisflies. As the University of Minnesota's Jeffrey Hahn so aptly put it, "there's snow place like home for these insects".
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Winter stonefly by Ashley Bradford.
Female winter stonefly by Wayne Mumford, http://willfishforwork.com/.
Male winter stonefly by Wayne Mumford, http://willfishforwork.com/.