Snow flies might seem like ordinary insects, but their survival strategy is anything but typical.
In a new study, scientists at Northwestern University investigated how these small, wingless insects, which move across snowy surfaces to find mates and lay eggs, stay alive in freezing conditions. They discovered that snow flies rely on a surprising mix of biological tools. The insects can generate their own body heat like mammals and produce antifreeze proteins similar to those found in Arctic fish.
While most insects cannot survive below freezing, snow flies remain active at temperatures as low as -6 degrees Celsius (or 21.2 degrees Fahrenheit).
These findings provide new insight into how life adapts to extreme environments. They may also help researchers develop new ways to protect cells, tissues and materials from damage caused by cold.
The study was published on 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 shapes biology and is 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 study with Marcus Stensmyr, a biology professor at Lund University in Sweden. Other Northwestern contributors 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).
Unusual Genes and Antifreeze Proteins
To understand how snow flies survive such harsh conditions, researchers first examined their genetic makeup. Gallio and his team were the first to sequence the snow fly genome and compare it with related insects that are not adapted to cold environments. They also analyzed RNA to identify which genes are actively used for survival in freezing temperatures. These complex comparisons were carried out by Richard Suhendra, a Ph.D. student working with Kath.
The results were 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 investigation showed that these unusual genes produce antifreeze proteins. Like those found in Arctic fish, these proteins attach to ice crystals and prevent them from growing. This process protects cells from damage during 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 Production Helps Snow Flies Stay Active
The team also identified genes linked to energy use and cellular processes involved in producing heat. This suggested another unexpected ability. Snow flies do not just resist freezing, they also generate their own heat.
“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 Creating Warmth
To test how the antifreeze proteins work, 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 temperatures in a lab freezer. The modified flies survived at much higher rates than normal fruit flies, confirming that the proteins act as barriers that stop ice from spreading.
In another experiment, researchers tested whether snow flies actually generate heat. They measured the insects’ internal temperature while gradually lowering the surrounding temperature below freezing. During this process, snow flies consistently remained slightly warmer than expected by a couple of 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.”
Even a small increase in temperature can be critical for survival in such extreme conditions. This brief warmth may give snow flies enough time to find shelter and avoid freezing when temperatures suddenly drop.
Reduced Sensitivity to Cold Pain
Snow flies also appear to be less sensitive to the painful effects of extreme cold. Most people recognize the sharp sting of touching ice or cold metal. This sensation is triggered by reactive molecules in cells that signal the body to avoid harm. In snow flies, this response is significantly reduced.
Gallio and his team found that a key sensory protein involved in detecting harmful stimuli is much less responsive in snow flies than in other insects. As a result, these insects can tolerate higher levels of cold-related stress and continue functioning in conditions 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.”
Future Research on Extreme Cold Survival
Next, the researchers plan to explore in greater detail how snow flies generate heat at the cellular level and to identify the full range of antifreeze proteins they produce. This work could reveal whether other organisms use similar strategies to survive in extreme cold environments.
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 Current Biology 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.