A grisly trick helps snow flies survive freezing: self-amputation

The insects cast off freezing limbs, preventing ice from spreading through their bodies

A photo of a snow fly standing on ice.

In the wild, snow flies are often missing legs, like the insect shown here. The flightless flies my self-amputate limbs as a way to survive the cold.

Trasroid/Flickr (CC BY-NC 2.0)

A few years ago, John Tuthill was trail running in the Cascade mountains in Washington state when he spotted something dark skittering across the snow.

It was about the size of a wild blueberry, with an elongated body and six legs that moved in a blur.

Tuthill was surprised to see an insect out and about on that cold October day. “I was kind of blown away that there was this animal out running around,” says Tuthill, a neuroscientist at the University of Washington in Seattle. It was a Chionea fly, he later learned. Also known as a snow fly, it could somehow walk around at temperatures well below what most other insects can tolerate.

Now, Tuthill and colleagues have shown that a grisly trick helps snow flies survive sub-zero conditions. When a leg begins to freeze, the insects can rapidly self-amputate it, preventing ice crystals from creeping up into their bodies, the team reports in a paper posted online May 30 at bioRxiv.org.

Snow flies may drop one or more legs to prevent themselves from freezing. Even missing multiple limbs, one fly (shown here) can navigate over snow in the wild.

Many animal species, including spiders, lizards and crabs, can drop a limb or tail to escape a predator (SN: 2/17/22), but the new work is the first to show an animal using this life-saving measure in response to the cold, says Christine Miller, an evolutionary biologist at the University of Florida in Gainesville. The study describes “a new phenomenon that hadn’t been documented before and a very interesting species that we know very little about.”

Snow flies are a type of flightless crane fly, relatives of the “big, spindly, goofy-looking flies that you see bumbling around in your house,” Tuthill says. The insects, which can live up to two months, aren’t easy to study: They can’t be bred in the lab, and they’re tough to collect from the wild. Snow flies can live in alpine areas that are difficult for people to reach, where the threat of avalanche looms.

The best way to find them, Tuthill says, is to spend a lot of time wandering around, looking at snow. Backcountry skiing fit the bill. In an eight-hour trip, he says, “probably seven hours of that is walking slowly uphill.” From 2020 to 2022, he, his wife, and some friends and volunteers collected hundreds of snow flies, scooping them into plastic tubes they brought back or mailed to Tuthill’s lab. Most of the snow flies that arrived alive came from excursions in Washington.

Tuthill’s team used a thermal camera to record 77 snow flies as they walked on cold plates. The insects kept trekking even when their body temperature fell to an average of –7° Celsius, the researchers found. But more than half of the snow flies tested dropped at least one leg during the experiments.

A sharp-eyed research tech, Dominic Golding, noticed a temperature spike in the flies’ legs just before they fell off. That spike is a sign of ice formation, Tuthill says. Liquid water releases heat as it crystallizes into ice. Neurons in the leg may sense this temperature shift and trigger amputation, preventing the icy crystals from spreading, the team suggests. The flies “have about half a second to get rid of their leg before that wave of ice crashes into their body and then freezes all of their internal organs,” he says.

Snow flies didn’t lose their limbs when the researchers tugged on them — only in response to freezing.

A thermal camera can pick up the moment ice begins to form in a snow fly’s leg and track when the insect self-amputates it. Colors represent temperatures, shown from warmer (yellow) to colder (purple).

Other animals have devised different strategies to fend off frost, says Katie Marshall, an entomologist at the University of British Columbia in Canada. Some insects pump out antifreeze proteins; certain snails and frogs simply withstand ice forming in their bodies.

“The cool thing about snow flies is that they don’t actually follow either strategy,” says Marshall, who provided feedback on the team’s manuscript, which has not yet been published in a peer-reviewed journal. They let ice form in their legs, she says, and “self-amputate to get rid of it, which is just completely strange to me.”

Snow flies that amputated their gelid limbs survived more than a minute longer than flies that didn’t, the team found. That extra time may not seem like much. But in the wild, when night is falling and the temperature is dropping and the insects are “looking furiously for a place to hunker down,” Tuthill says, it could be the difference between life and death.

Meghan Rosen is a staff writer who reports on the life sciences for Science News. She earned a Ph.D. in biochemistry and molecular biology with an emphasis in biotechnology from the University of California, Davis, and later graduated from the science communication program at UC Santa Cruz.

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