This Tiny Insect Should Freeze to Death but Somehow Doesn’t

Snow flies beat the cold by acting like a mashup of Arctic fish and polar bears.

Snow flies may seem like ordinary insects, but their survival strategy is anything but typical.

In a new study, researchers at Northwestern University examined how these tiny, wingless insects, which crawl across snowy surfaces to mate and lay eggs, manage to stay alive in freezing conditions. They found that snow flies rely on an unusual combination of traits. Like mammals, they can generate their own body heat, and like Arctic fish, they produce antifreeze proteins.

Most insects cannot survive in subzero environments. Snow flies, however, remain active even at temperatures as low as -6 degrees Celsius (or 21.2 degrees Fahrenheit).

These discoveries offer new insight into how living organisms adapt to extreme environments. They could also help scientists develop better ways to protect cells, tissues, and materials from damage caused by freezing.

The study will be published on Tuesday (March 24) in the journal Current Biology.

“Insects are cold-blooded, so they are at the mercy of external temperatures,” said Northwestern’s Marco Gallio, who led the study. “But they have a mind-boggling ability to adapt to extremes. When it gets cold, a common strategy is to find shelter and become dormant until conditions get better. But instead of slowing down, snow flies actually prefer freezing cold, snowy conditions and hide away when the snow melts and it gets warm. They really push the limit of what’s possible. Now we’ve found snow flies aren’t just tolerating the cold, they have multiple ways to counteract it.”

Gallio studies how temperature influences biology and serves as the Soretta and Henry Shapiro Research Professor in Molecular Biology as well as a professor of neurobiology at Northwestern’s Weinberg College of Arts and Sciences. He co-led the work with Marcus Stensmyr, a biology professor at Lund University in Sweden. Additional contributors from Northwestern include William Kath of the McCormick School of Engineering and Alessia Para from Weinberg. Gallio and Kath are also affiliated with the NSF-Simons National Institute for Theory and Mathematics in Biology (NITMB).

Strange Genes and Antifreeze Proteins

To understand how snow flies survive, researchers first needed to examine their genetic makeup. Gallio’s team became the first to sequence the snow fly genome and compare it with related insects that lack cold tolerance. They also analyzed RNA to determine which genes are actively used in cold survival. These detailed comparisons were carried out by Richard Suhendra, a Ph.D. student working with Kath.

What they found was unexpected.

“We couldn’t find many of the genes within any database,” Gallio said. “Initially, I thought we must have sequenced some alien species. It’s very rare for an active gene, which makes a protein, to not have a match.”

Further analysis revealed that these unusual genes produce antifreeze proteins. Similar to those found in Arctic fish, these proteins attach to ice crystals and prevent them from growing. This process protects cells from damage caused by freezing.

“Remarkably, some of the antifreeze proteins we found are actually structurally related to those of Arctic fish,” Gallio said. “That suggests evolution came to the same solution for a common problem.”

Heat Generation in an Insect

The researchers also identified genes linked to energy use and cellular processes involved in producing heat. This pointed to another surprising ability. Snow flies not only resist freezing but also generate their own warmth.

“We found genes that in larger animals are associated with mitochondrial thermogenesis in brown adipose tissue,” Gallio said. “Many animals like marmots and polar bears have brown fat, which is there to produce heat. When they go into hibernation, they burn this stored fat to produce heat rather than to produce chemical energy. So, in some ways snow flies use a combination of the strategies used by polar bears and by Arctic fish.”

Blocking Ice and Producing Warmth

To test the antifreeze proteins, Matthew Capek, a Ph.D. student in the Gallio Lab, modified fruit flies to produce one of the snow fly proteins. He then exposed them to freezing conditions in a lab freezer. The engineered flies survived at much higher rates than normal flies, confirming that the proteins act as microscopic barriers that stop ice from spreading.

In a separate experiment, the team investigated whether snow flies truly generate heat. They measured the insects’ internal temperatures while gradually lowering the surrounding temperature below freezing. Even as conditions cooled, snow flies stayed slightly warmer than expected by a few degrees Celsius compared to other insects.

“Other insects, like bees and moths, shiver to increase their heat,” Stensmyr said. “But we found no evidence of shivering. Snow flies instead likely produce heat at the cellular level, more similar to how mammals and even some plants generate heat.”

For an organism living on the edge of freezing, even a small increase in temperature can be critical. This slight warmth may give snow flies enough time to find shelter and avoid sudden freezing.

Reduced Sensitivity to Cold Pain

Snow flies have yet another adaptation that helps them endure harsh conditions. The researchers found that these insects are less sensitive to the painful effects of extreme cold.

Most people are familiar with the sharp sting that comes from touching ice or cold metal. This sensation is triggered by reactive molecules in cells, which signal the body to avoid harmful conditions. In snow flies, that response appears to be altered.

Gallio’s team discovered that a key sensory protein involved in detecting harmful stimuli is much less responsive in snow flies than in other insects. This reduced sensitivity allows them to tolerate higher levels of cold-related stress and continue functioning in environments that would overwhelm most species.

“It turns out that a specific irritant receptor is 30 times less sensitive in snow flies than in mosquitoes and fruit flies,” Gallio said. “So, they can cope with higher levels of noxious irritants produced by cold exposure.”

What Comes Next

The researchers now plan to further investigate how snow flies generate heat at the cellular level and to map out the full range of antifreeze proteins they produce. These future studies could reveal whether similar survival strategies exist in other species that live in extreme cold.

Reference: “Coordinated molecular and physiological adaptations enable activity at subfreezing temperature in the snow fly Chionea alexandriana” 24 March 2026, Current Biology.
DOI: 10.1016/j.cub.2026.02.060

The study, “Coordinated molecular and physiological adaptations enable activity at subfreezing temperature in the snow fly Chionea alexandriana,” will appear in the April 6 volume of the journal and feature on the cover. The work in the various labs was partially supported by the National Institutes of Health, the Pew Scholars Program, the McKnight Foundation, the Paula M. Trienens Institute for Sustainability and Energy, the Crafoord Foundation, the National Science Foundation, the Simons Foundation, and NITMB. External collaborators included the DNAzoo project and Olga Dudchenko and Erez Lieberman Aiden, who are both faculty members at Rice University and at the Baylor College of Medicine.

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