<\/a>A snow fly wanders on fresh snowfall near the Mt. Baker region in Washington state. (University of Washington photo)<\/p><\/div>\n\t\t\t\t
Then a friend called my attention to a recently published article about a strange little crane fly. They are not the usual sort of crane fly, but have been classified in a separate subfamily and, indeed, are sometimes assigned to a different but closely related taxonomic family. There are over 30 species of these, known as snow flies (genus Chionea), distributed in the northern hemisphere. They are wingless, unlike most crane flies, and adults are typically seen walking on the snow in winter. That seems to be the mating season, when they wander over the snow looking for mates. Why would they use that season? They are not feeding then, but predators are probably fewer than in warmer times. The females are presumed to lay their eggs down under the snow in the debris and leaf litter, but that part of the life cycle has not been reported, apparently.<\/p>\n\t\t\t\t
But winter can be dangerous in other ways. Most insects (including typical crane flies) become paralyzed when temperatures drop below freezing, and there is also the risk of their body fluids freezing, which is typically lethal. But snow flies are able to remain active at temperatures down to minus seven or more degrees centigrade, well below the temperatures at which other crane flies become paralyzed. But below that limit, they freeze to death due to internal ice formation. Before reaching that limit, they are “supercooled,” their small volumes of fluids remaining liquid because they contain no particles that serve as nuclei for ice crystals to form. That’s a feature found in many small insects, most of which are inactivated at lower temperatures, but somehow snow flies can maintain normal locomotor activity right down to the lethal freezing point. That may allow them to seek shelter in snow burrows at times and continue mate-searching for longer periods.<\/p>\n\t\t\t\t
They have another unusual feature too. If a leg starts to freeze, they can self-amputate it before the freezing spreads fatally to the rest of the body. Detaching a leg takes only two or three seconds, sometimes less; when successful, detachment happens before the fast-moving wave of ice reaches the site of amputation. In experimental situations, snow flies that self-amputated a leg survived at lower temperatures and for longer times.<\/p>\n\t\t\t\t