Wild Side: The flies of winter

Among them, ‘arguably the most amazing insect nobody has ever heard of.’

Among the easiest flies to find in winter is the blowfly Calliphora vicina. — Matt Pelikan

Insects, of course, are primarily warm-weather friends. And with flies in particular — the order Diptera — summer is rife with familiar examples. Mosquitoes (yes, they’re flies), black flies (related to mosquitoes), deer flies, greenhead flies perforating beachgoers, blowflies applying their spongy mouthparts to your picnic: All prominent features of the warmer months. For many humans, this list sums what the word “fly” calls to mind.

I’ve been mildly surprised, then, to find that flies are the most prevalent order of insects on the wing in midwinter. “Prevalent,” of course, is a relative term. Springtails, or snow fleas, six-legged arthropods that aren’t technically insects, can be locally abundant, and of course the winter moth and one or two close relatives can be plentiful earlier in the season. But no insect groups are truly diverse outdoors in the winter months.

If you’re not paying attention, you notice no insects at all in late January. But of the insects I do manage to find at this point in the season, the lion’s share are in the order Diptera. I can think of five fly families that I’ve encountered on mild days in January or February. (If the sun is out; anything above freezing is warm enough to get some flies aloft on their single pair of wings.)

One of these families, amazingly, is optimized for winter, normally active as adults then, and highly adapted to reproducing in cold weather. Trichoceridae, the winter crane flies, appear to be represented on the Vineyard by just one species. But it can be an abundant one.

Our winter crane flies are small, spindly insects, about a half-inch long, with thin legs, transparent wings, and slender bodies. Though there isn’t much substance to them, they’re capable of flight in temperatures around freezing, and can survive nighttime lows down into at least the lower teens. Adults commonly turn up at porch lights, and these insects can be present by the thousands in natural habitats around the Island.

They lay their eggs in winter after mating. Hatching in spring, the larval flies spend the warmer months dining on detritus in the leaf litter in moist areas. It’s not until early November that the first adults appear here, and while numbers probably peak sometimes in early December, I continue to see them in decreasing numbers into early February or so. Their resistance to cold, based on unique proteins that remain functional at low temperatures, makes them arguably the most amazing insect nobody has ever heard of.

Flies in the family Heliomyzidae (the family has no common name) show somewhat less specialization for life in cold conditions, but some species are most active in winter. One that I think is in the genus Orbellia has been fairly common around our house in Oak Bluffs this winter, sometimes turning up in sunny spots in the yard and sometimes perching on the inside or outside of window screens (heat leaking through the glass may attract them).

Tiny and not at all distinctive, Orbellia appear to have been little studied. They likely follow the typical pattern of their family, living among and feeding on decaying organic matter both as larvae and as adults. The nature of their adaptations to the cold and the evolutionary reasons for the odd timing of their life cycle are beyond me; some years I find them and some years I don’t, and that’s about all I know.

Taking a different approach to life are the seaweed flies, in the genus Fucellia (we may have only one species). Small flies, perhaps a quarter-inch long, these represent a family with the unfortunate common name “root maggot flies” (larvae of many species feed on plant roots). They are active year-round on the wrack line on our beaches, where the tidal cycle constantly replenishes the supply of organic material.

Adult Fucellia rarely stray from the upper beach, and the larvae of this genus invariably live in sea wrack and feed on decaying weed. Indeed, the process of decay generates heat, making the wrack line a centrally heated refuge for the flies. In winter, you won’t see swarms of these flies, as you may during summer; but if the sun is out and the temperature is above freezing, a patient observer can always scare a few up off the heaps of moldering seaweed. With their physiologies specially tweaked to make them tolerant of salt, seaweed flies are another group that I find fascinating in their complete weirdness.

Also in winter you can often find some of the same species of blowflies that you recall from summer. This family both eats and lays its eggs on a range of (mostly unpleasant) organic substrates: rotting vegetables, rotting flesh, even feces. I presume that blowflies overwinter mainly as pupae or nearly mature larvae. But a few adults clearly survive deep into the winter, perhaps right through to spring, and can be found basking or zipping about on mild days.

It’s hard to love a blowfly, but still, I enjoy spotting them in winter — they remind me of warmer days, and of the remarkable diversity and resourcefulness of flies as a group.