Wild Side: On the house fly

Surprisingly rare and possibly superfluous, but worthy of study.


“House fly.” I think we all get roughly the same image from that common name: a grayish fly, between a quarter- and a half-inch long, its wings spread as it perches, with an annoying habit of landing on whatever you’re about to eat.

That profile, though, could apply to dozens of species across three or four fly families. Often, the offending insect is a blow fly, in the family Calliphoridae; other times, it might be any of dozens of undistinguished gray jobs in the family Muscidae.

What it usually isn’t, at least on the Vineyard and at least as far as I can tell, is a true house fly, in the strict sense of the phrase. Inadvertently introduced here from Europe, Musca domestica — the scientific name literally means “house fly” — is really the only species we should refer to by that name.

I have to confess I’ve contributed to the confusion. In wanting to succinctly describe an insect, I occasionally say or write something like “about the size and shape of the familiar house fly.” But today, I’m here to come clean: Though I’m as familiar as any of us are with grayish flies in the house, it wasn’t until this past week that I was confident I had ever seen a house fly in the narrow sense. The species, deft at hitchhiking on human travel and commerce, has colonized much of the globe. But on the Vineyard, it has proven elusive.

Over the past few years, I’ve photographed many hundreds of flies, doing my best to identify them with as much precision as I could. For a good percentage of these flies, the ID proceeded only as far as the family level: I surely have scores of examples identified only as members of the house fly’s family, Muscidae, or, even worse, the super-family Muscoidea, which lumps Muscidae with several closely related families. Have I mentioned that flies are really hard to identify?

But one thing I was sure of was that none of those flies could possibly be Musca domestica. They all lacked either a distinctive bend in a prominent vein in the wing, or yellow coloration on the abdomen. Absence of either or both of those features rules out the true house fly.

Last summer, though, while I was manning a booth for my organization at the Agricultural Society Fair, I noticed that an unfamiliar fly became increasingly common as the fair progressed. A little more than a quarter-inch long, this species fit the profile I began with for a generic “house fly.” But it had the yellow patches on the abdomen characteristic of M. domestica, and so I nabbed a specimen in a plastic vial and put it in the office freezer when I had a chance.

I recently got around to pinning that specimen, so I could get it under a microscope and identify it using a scientific key. There are always some challenges to this process: A particular characteristic, for example, may be visible only if the specimen is illuminated from a certain angle, and unless you find that angle, a mistaken assumption may send you down the wrong path. But I took my time, and without much trouble established that my mystery fly was indeed a member of the genus Musca.

Unfortunately, the key I have only takes you to the genus level, and I couldn’t find another key that would take me to species. So I was left with the less reliable method of comparing my specimen to online photographs of flies that had been identified by experts.

Happily, only two members of the genus Musca are known in North America. And while these two — M. domestica and the so-called face fly, M. autumnalis — are similar to each other, males are readily distinguished by how widely spaced their eyes are. Fortuitously, I had caught a male, and compared with reference photos, his eye spacing revealed him to be M. domestica.

Musca autumnalis has also been documented here, in the form of a couple of flies posted in the community science platform iNaturalist. So I’m confident we have both North American members of the genus here. Both, though, seem to be scarce, or at least concentrated solely around livestock. So the “house fly” besmirching your salmon steak is likely not a house fly in the strictest sense.

Does it matter? Well, in one sense, no. The two Musca species have similar ecology (and similar ability to carry pathogens), and scores of other Vineyard fly species roughly share the same habits. So M. domestica is probably a redundant cog in our ecosystem — and, being non-native and a notorious disease vector, an undesirable cog at that.

And yet, as a naturalist, I basically want to know everything about everything in the natural world. Though no fan of Musca domestica, I’m glad I have learned a little about it. So it matters to me.