These Animals Shouldn’t Be Alive, Much Less Sprinting

In October 2016, while hiking through a treacherous mountain pass, John Tuthill saw something move. He was almost 8,000 feet above sea level in Washington’s Alpine Lakes Wilderness, braving high winds and temperatures well below freezing. Although he and his hiking companion initially thought that they had the trail to themselves, they soon noticed small, brown specks zipping over the virgin snow. When Tuthill looked closely at them, he was astonished to see that they were insects, existing in an environment where insects shouldn’t be alive, let alone active.

As a neuroscientist who works with flies, Tuthill knew that insect nervous systems shut down when the creatures get too cold, so even species that can tolerate subzero temperatures tend to do so by entering a dormant state. And yet here were insects, running about in apparent defiance of both biology and physics. “It immediately blew my mind,” Tuthill told me. That’s how he came to study snow flies.

Snow flies are a group of insects so obscure that very few scientists in history have ever studied them. They don’t have wings, and so get about by sprinting in a spiderlike way. And they do so in conditions so cold that most insects would struggle to move at all. Tuthill, who is a neuroscientist, tells me that he often anesthetizes fruit flies by chilling them to 2 degrees Celsius (36 degrees Fahrenheit), which paralyzes them. But snow flies can keep running even when their bodies hit –7 degrees Celsius (19 degrees Fahrenheit). They actually prefer temperatures close to freezing: Hold them in your hand, and they’ll become agitated, but put them in a fridge, and they’re … well … chill.

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Very little is known about snow flies, in part because even collecting them requires being a good skier or mountaineer. In 2019, Tuthill tried encouraging members of the public to capture and send him snow flies, but few people tried, and most of those who did sent spiders by mistake. He and his colleagues had to collect most of the individuals they studied—a task he didn’t mind one bit. “They coincidentally have the same idea of what a good day is as a backcountry skier,” and are most active when the sun is shining on a few feet of fresh powder, Tuthill told me. “People in my lab have tried to raise them, but I personally don’t want that to work, because every time there’s a perfect day, I cancel my meetings and go out collecting snow flies.”

By studying the captured insects, Tuthill’s colleague Dominic Golding discovered how they cope with one of the biggest threats of subzero living—ice. Once ice starts to form in an insect’s bodily fluids, the spreading crystals will quickly kill them. But snow flies can sense when that fatal process begins, and stop it through self-amputation. Using thermal cameras, Golding filmed many moments in which a wave of ice would race up a snow fly’s leg only for that leg to detach from the body within seconds.

Their close relatives—spindly, slow-flying crane flies—use a similar trick if their legs are caught by predators, which they sense using neurons that detect pulls and tugs. Tuthill thinks that snow flies have placed the same defensive reflex under the control of neurons that sense temperature instead. These detect the small bursts of heat that occur when ice first forms, and trigger a muscular contraction that jettisons the freezing limb. This strategy is unorthodox, to say the least. Katie Marshall, a zoologist at the University of British Columbia, told me that cold-adapted animals cope with ice by either loading their blood with antifreeze that stops crystals from forming, or letting crystals form and withstanding them. Snow flies do neither; their unique solution—self-amputation—might be dramatic, but it is successful. In the wild, Tuthill has seen many three-legged snow flies, still moving at a pretty good clip, he said.

And their speed is all the more impressive given that their nervous systems shouldn’t work at all. To fire, neurons must pump electrically charged molecules out through their membranes and then allow those molecules to reenter by opening small gates. But as the temperature drops, the pumps slow down, the gates stop opening, and neurons stop firing. The snow flies must have adaptations that allow their neurons to defy these thermal constraints, but Tuthill doesn’t yet know what they are. Compared with other insects, he says, snow flies aren’t actually that cold-tolerant. Their superpower is the ability to stay active right until the moment they freeze.

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Adult snow flies don’t eat, so when they skitter over snow fields, they’re specifically searching for mates. When two find each other, they have sex immediately and audaciously: Despite being completely exposed to predators, and highly visible against a white backdrop, they’ll spend 30 minutes or more “in full view on the surface of the snow,” Tuthill writes. But hardly any predators are around to see them, which may be why they adapted to run over glaciers at all. And although animals that live in extreme environments are often treated as champions that endure barely bearable conditions, Tuthill wonders if snow flies actually have it pretty good. “If you go through metamorphosis on the right day, and it’s bluebird with two feet of powder, and you meet your lover … it’s a good life if it works out,” he told me.

Or while it lasts. Within the next two decades, Washington is set to end the winter with about half as much snow as it did last century. By the 2080s, it will have 70 percent less. Snow flies will likely be pushed to higher altitudes, but “at some point, they’ll run out of mountain,” Marshall told me. Most people won’t notice their absence; hardly anyone noticed they were there to begin with. But losing snow flies would still mean losing an incredible example of life’s tenacity—its ability to persist in the unlikeliest of environments using adaptations we barely understand, running against the odds until they eventually run out of time.