Snow flies self-amputate frozen limbs to survive in sub-zero temperatures

Researchers from the University of Washington, Seattle, have made a fascinating discovery about the survival tactics of the snow fly (Chionea spp.), a flightless crane fly found in cold northern environments.

Their study, titled “Snow flies self-amputate freezing limbs to endure sub-zero temperatures,” published in Current Biology, delves into the remarkable behavior of snow flies, which can remain active in extremely cold conditions, even down to –10°C. This sets them apart from most insects, which can’t function in freezing temperatures.

To gather data, citizen scientists, including skiers and mountaineers, helped collect 256 adult snow flies from remote alpine areas in the Pacific Northwest. Surprisingly, 20% of these flies were already missing one or more legs.

The researchers used thermal imaging to observe these insects, which could still walk with an average body temperature of −7°C. At this point, ice crystals began to form within their bodily fluids, starting from the extremities.

Examples of snow fly locomotion in the wild. Credit: Current Biology (2023). DOI: 10.1016/j.cub.2023.09.002

Snow flies have a unique survival strategy – they amputate their own legs when ice crystallization threatens to reach their vital organs. This process must be incredibly fast, as ice that starts in the leg can reach vital organs in just half a second.

The self-amputation consistently occurs at the joint between the femur and the trochanter. While leg amputation is common in crane flies as a predator escape mechanism, snow flies don’t respond to mechanical stimuli but may rely on thermosensory neurons to detect temperature changes during ice crystallization.

Living in such a harsh environment has its advantages for snow flies. Their habitat is mostly devoid of other animals and predators, allowing them to lay undisturbed eggs beneath the snow. They’ve even been observed mating on the snowy surface for extended periods.

Unfortunately, their unique habitat is under threat due to human-induced climate change. High-elevation ecosystems, including Washington, are losing their snowpack rapidly. This loss will increase temperature fluctuations and harm the plant life that snow fly larvae feed on. Ultimately, these environmental changes may lead to the extinction of this extraordinary species.

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