Chionea valga is an unusual fly that, at first glance, looks like a little spider. But when you see it up close, there’s no doubt about it: a head, a thorax, an abdomen, six long and strong legs – it’s definitely an insect!
Flies, as we know, have just one pair of wings. But C. valga doesn’t have any. So what makes it a fly? The answer can be found on its thorax, where atrophied forewings are still quite visible and from where there emerge, on each side, little projections called halteres. These structures, acquired by diptera (flies and mosquitoes) in the course of their evolution, are vestiges of hind wings. In an “ordinary” fly, the two halteres maintain equilibrium during flight.
A species whose feet never freeze!
Of course, this insect doesn’t fly. But as its name indicates (Chionea valga means bow-legged snow fly), it walks and performs its activities on snow-covered ground. Moreover, it’s during these wintertime strolls that this Greek divinity (Chionea) of the snows meets a male and takes advantage of the opportunity, sometimes, to attempt to mate. This phenomenon, rare among insects, can be observed from late November to late March, in overcast windless weather, when the outdoor temperature reaches the freezing point.
The biology of C. valga is poorly documented. But we know that the insect, adapted to cold weather, normally lives between the snow cover and the frozen ground. This microhabitat, called subnivean space, provides it with a stable environment in which the average temperature is around -5° C. Confined to that space, C. valga will lay its eggs there somewhere between mid-December and late January. The insect will however be easy prey for shrews, which during the winter also occupy this restricted space.
Rare environment and fragile species
To date, 32 species belonging to the genus Chionea have been described, 18 of them in North America. In Québec, besides C. valga, C. scita, the Appalachian snow fly, can also be observed. It appears that these two species are closely linked to mature boreal forests characterized by among other things the accumulation on the ground of a significant amount of decaying dead wood. It’s this cover of woody debris, forming a specific habitat where species of Chionea can survive and reproduce, that would explain the abundance of these insects in the old forests of our belle province. What a shame that the shrinking of these old-growth forests, today threatened by human activity, at the same time affects the survival of special species of fly like these that surprise hikers and fascinate nature’s most avid students – even in winter!