That's No Flea - It's a Snow Fly

Feb 19, 2017

When I’m out in the woods in winter, I tend to keep my eyes on the ground. I’m looking for tracks, scat - signs of warm-blooded life. About the last thing I’d expect to see is an insect. But a few weeks ago, on a ski up at Lolo Pass, that was exactly what I found – and not just one insect; dozens.

Most temperate invertebrates live out their adult lives in the summer or spring, surviving through winter in a state of dormancy. However, I had seen invertebrates out in the winter before – tiny snow fleas, or Collembola, insect relatives that come out en masse on warm winter days, peppering the snow with their dark bodies. But snow fleas, while numerous, are tiny – barely the size of a freckle – and typically seem to show up on days warm enough to melt snow. The strange insects I was seeing seemed happy with the temperature below 20 Fahrenheit, and were much larger. And they were scattered all over, never occurring near each other, but widely spaced across the snow. And perhaps most notably, they all seemed to be on the move. They were going somewhere. And with long spiderlike legs, they were actually pretty quick about it.

I later learned that these strange, leggy insects were snow flies. Snow flies are related to summer’s crane flies, and resemble them quite closely, except for one notable feature – snow flies are wingless. Rather than flying through the cold winter air, they traverse the snow on foot. This may be a way of conserving energy – both flying and keeping warm are pretty energetically expensive, so it may be that the most “cost effective” way for these insects to get around is simply to walk. It’s not known if they even eat as adults, so cutting energetic costs may be especially important.

There’s actually very little that is known about these strange creatures – probably owing to the fact that they show up only occasionally on the surface of the snow. They actually spend most of their adult lives beneath the snow, in what is known as the subnivean environment, between the snowpack and the ground below. Snow flies spend most of their time in rodent tunnels, and cavities in the snow created by vegetation. Completely opposite to most insects, snow fly larvae grow through summer and don’t pupate until fall, with adults mating in winter.

So if they spend most of their time beneath the snow, why the mid-winter excursions? The best-supported explanation seems to be that snow flies make their winter treks to avoid inbreeding. Groups of snow flies live in isolated chambers under the snow. This may be beneficial in some ways, but doesn’t do much for genetic diversity. A good solution may be to get out and find a new place to hunker down, perhaps with a member of the opposite sex, if a fly is lucky.

The more I learned about these strange flies, the more I came to appreciate them in all their weirdness. Now, each time I’m out in the snow, I keep my eyes peeled not just for tracks and scat, but for insects as well. I’d count myself lucky to see them again, but even when I don’t, I still know they’re down there, beneath my feet, living their strange subnivean lives – wingless flies, finding each other beneath the snow.

'Field Notes' is produced by the Montana Natural History Center

Broadcast: "Fieldnotes," 2/19/17 & 2/24/17. You can hear the program on the radio Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., and Fridays at 4:54 p.m., or listen via podcast.