Flightless snow flies in the US and Canada can amputate their legs to survive as they begin to freeze, researchers have discovered.
Lab experiments in which the flies were chilled gradually to sub-zero temperatures revealed they can detach one or more of their six legs, an apparent “last-ditch tactic” to protect their internal organs from the advancing cold.
“It is gruesome,” said John Tuthill at the University of Washington. “This wave of ice is going to crash into their bodies and kill them so they’re doing this amputation to prevent that happening.”
About 20% of snow flies collected in the wild by the researchers were already missing legs, he added.
Snow flies, being crane flies, are from the same family as daddy long legs. Unlike the well known British insect, North American snow flies habitually walk across snow and ice in the wild and have been observed doing so at temperatures as low as -10C.
The researchers used dozens of snow flies from four different but closely related species collected from icy, mountainous habitats in Washington state, Colorado and Vermont in the US, and British Columbia and Yukon in Canada.
In their experiments, the team placed individual flies on laboratory cold plates and observed their behaviour with a thermal imaging camera as the temperature slowly dropped.
The flies could still walk even when their bodies reached -7C, and the thermal imaging camera was able to capture the precise moment at which the flies’ legs began to freeze, prompting the amputation response. This happened in 31% of cases where freezing began in a leg. Some flies removed as many as five legs before succumbing to the cold.
The team has published a paper online detailing their findings, though it is yet to be peer-reviewed.
Other crane flies are known to self-amputate their legs – but for different reasons. They tend to do it when a predator grabs on to one of their limbs. According to Tuthill, there are muscles inside the flies’ legs that seemingly allow them to pinch off or detach a limb at will.
Certain geckos, crayfish and squid can also self-amputate their limbs or tails to escape predators. The African spiny mouse can even shed huge quantities of its own skin in a tight spot.
However, leg self-amputation as a response to potentially freezing to death was described as “unique” by the researchers.
“It’s an extreme adaptation,” said Erica McAlister, the curator of flies and fleas at the Natural History Museum, while praising the research.
In their paper, Tuthill and colleagues speculated that the flies may be able to detect a slight rise in temperature inside their legs, which is – somewhat counterintuitively – caused by a release of energy when ice crystals form. Researchers plan to investigate this hypothesis in future experiments, Tuthill said.
McAlister noted that the hardy flies have an evolutionary advantage in being able to survive, much of the time, in spite of the snow and ice. But that may evaporate as mountainous areas lose their snow cover due to global heating.
“When the snow disappears and this selective advantage of them being able to peg it out of the situation goes, you’re going to have a deleterious effect,” she said.