Winter, naturally, is a slow season for insect observation, with only a modest variety of particularly hardy or specially adapted species active. Some of these, though, can be plentiful, especially in mild winters like this one.
A good example, and a species that has seemed especially abundant this winter, is the black blowfly, more formally known as Phormia regina. As the common name suggests, these medium to large flies are dark in color, though not really black: Especially when seen in good light, Phormia is subtly iridescent, with bluish-green highlights. This species is easily distinguished by color alone from more familiar blowflies, such as the “bluebottle” flies of the genus Calliphora (less glossy than Phormia and marked with light patches on the thorax and abdomen) or the “greenbottle” flies in the genus Lucilia (iridescent green with gaudy, golden highlights).
When I first encountered Phormia, I thought I had found something unusual. My first one popped out of the leaf litter one winter day as I rummaged around at Menemsha Hills in hope of finding the odd beetle. The date and the remote location made me think the fly had to be some species specialized for oak woodland.
Subsequently, though, I’ve realized that black blowflies can turn up anywhere from pristine natural environments to the exteriors of human dwellings. Moreover, it turns out that this species occurs throughout the Northern Hemisphere. Nobody seems to know where it originated; perhaps it had succeeded in colonizing much of the globe on its own even before human commerce began transporting wildlife from continent to continent. Or perhaps Phormia began hitching rides so early on in human history that it was already established most places by the time the first biologists turned up and began looking at flies. In any event, Phormia is a common insect across a vast portion of the globe.
Oddly, I’ve never found Phormia during the warmer months, although records from other New England observers suggest that these flies are active year-round in our region. The pattern of records suggests to me that Phormia cycles through multiple generations in the course of a year, reproducing irregularly as conditions allow and larval food supplies are available. Perhaps I’ve simply overlooked them during the warmer months, among the abundant blowflies of other species. I’ll make a specific effort to look for Phormia this coming summer.
While it would be misleading to say that Phormia is truly optimized for life in winter, these are certainly very hardy insects. The breeding biology of these flies has been very well studied, and it’s clear that temperatures at least in the reasonable range — from the low 50s on up in our archaic Fahrenheit scale — are needed for optimal adult activity and for good growth by larvae.
But as with some other blowfly species, adult Phormia routinely survive the winter, becoming active when conditions allow, and presumably mating and reproducing in early spring, when temperatures get back into their comfort zone. Larvae, too, are quite hardy, simply slowing their growth rates when the mercury dips. (Feeding actively in swarms, larvae can generate some warmth on their own.) Adults can be active in air temperatures in the 30s, if the flies can find a sheltered, sunny spot. I’ve been finding them all winter on the shingle siding of the office building I work in in Vineyard Haven.
In terms of its biological classification, Phormia resides in a subfamily that is, if possible, even less popular than blowflies in general. The subfamily Chrysomyinae is dominated by a group of species known as screwflies. The larvae of screwflies have the disconcerting habit of feeding on wounds in living flesh, often or even preferentially mammalian flesh. Screwflies can be serious agricultural pests, zeroing in on minor wounds and laying their eggs there. The feeding larvae can be numerous enough to weaken or even kill their host as they feed on the flesh exposed by the wound.
Phormia, happily, engages in no such shenanigans. For whatever evolutionary reason, this genus either didn’t develop the harmful habits of screwworms, or else it evolved back into the more usual blowfly habit of using dead flesh as larval food. Like other blowflies, Phormia has a preternatural skill at sniffing out corpses, attracted by chemicals produced by even the earliest stages of decomposition, and laying eggs on the dead flesh. Adults, again as is the case with other typical blowflies, are equipped with spongy mouthparts that are suitable for sucking up nutrient-rich liquids of nearly any kind; the juices found in decaying flesh or vegetation, or even feces, attract and sustain the adults.
Phormia seems less strongly associated with human activity than some other blowflies are (I can’t, for example, recall ever seeing Phormia inside a building). This habitat probably makes them less of a health threat than some of their relatives, which can transport germs from their food onto yours. So while its habits may repel humans, Phormia regina is a benign presence, helping recycle nutrients from dead animals while doing little harm.