As winter blankets the landscape in snow and ice, most insects disappear. However, a unique group, known as “snow flies,” emerges onto the snowy surface. These unique creatures offer a glimpse into unexpected activity that persists even in freezing temperatures.
What is a Snow Fly
The term “snow fly” refers to cold-adapted insects, primarily crane flies from the genus Chionea and specific winter stoneflies. Chionea species are small, flightless crane flies, typically around 7 mm (just over 1/4 inch) long. They have long, spindly legs and are often dark, appearing as small black specks crawling across snow, ice, or frozen ground. Winter stoneflies, such as Taeniopteryx or Capnella, are also small, dark-bodied insects, generally about a half-inch long, found near waterways. Both groups are found in montane, forested environments across the Northern Hemisphere, including North America, Europe, and Asia.
Surviving Winter
Snow flies have adaptations that allow them to remain active in freezing temperatures. They produce cryoprotectants, such as glycerol or trehalose, in their bodily fluids. These compounds act like antifreeze, preventing ice crystals from forming within their cells, which would cause fatal damage. While some insects can tolerate temperatures as low as -25°C, snow flies are active in temperatures ranging from 0°C to -6°C, with some observed at -10°C or even -17°C.
Their dark body coloration helps absorb solar radiation, maintaining a slightly higher body temperature in sunlight. Snow flies also have modified nerve and muscle functions, enabling activity even as their body temperature approaches freezing. If a limb begins to freeze, snow flies can self-amputate the affected leg. This unique adaptation stops ice crystals from spreading to their main body, providing a chance to escape lethal conditions. This self-amputation can occur within about 2.5 seconds of a leg freezing.
Snow flies emerge as adults in the fall or early winter and can live for up to two months, a considerably longer lifespan than many other crane flies. They often reside in the subnivean zone, the insulating space between the snowpack and the ground, or in rodent burrows, where temperatures are more stable and closer to freezing. Adult snow flies are not known to consume solid food, instead primarily drinking water from snowmelt, though laboratory observations indicate they will readily sip maple syrup. Their primary adult activity is finding a mate, which may be facilitated by the uniform surface of snow and the reduced presence of predators.
Role in the Ecosystem
Snow flies contribute to nutrient cycling within their cold habitats. The larvae of Chionea species develop in decaying organic matter found in forest leaf litter, under logs and stones, or within mammal burrows, where they may feed on plant debris or rodent feces. Similarly, winter stonefly nymphs consume algae or plant detritus in aquatic environments.
As adults, snow flies serve as a food source for other winter-active organisms. Predators like “ice crawlers” (Grylloblattidae), mice, rats, and various winter birds feed on them. Their presence during winter provides sustenance when other insect prey are scarce. These insects are not considered pests and do not threaten human health, agriculture, or homes. Winter stoneflies are also recognized as bioindicators, meaning their presence signifies clean, well-oxygenated water, as they are sensitive to pollution.