Wild Side: Thick-headed flies

A native species that deserves respect.

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Physocephala tibialis uses its wasplike appearance and habits to ambush the true wasps it parasitizes. —Matt Pelikan

The fly family Conopidae suffers from a ludicrous common name: They are generally known as “thick-headed flies.” The name presumably comes from the blocky, large heads of some members of this family, rather than from any assessment of their intellectual abilities! But even so, the name describes only a few members of the family.

Conopidae is not an especially diverse family, at least by fly standards. Of about 800 species known worldwide, fewer than 70 occur in North America. On the Vineyard, I’ve found only four conopids so far, representing three genera, though the odds are very good that I’ve missed at least a few.

Members of Conopidae are all internal parasites of other insects; female conopids lay their eggs one at a time on suitable hosts, and when each egg hatches, it burrows into its host and then begins consuming it. As you’d imagine, this does not work out well for the host. The fly larva completes its development inside the husk of its unwilling foster parent, eventually emerging as an adult to start the process again.

Host species vary widely across the family Conopidae, from beetles to other flies. But many conopids, including all the species I’ve found on the Vineyard, target bees or wasps.
Adult thick-headed flies, although some possess a dauntingly long proboscis and many resemble stinging insects, are harmless to anything other than their targeted hosts. They typically turn up on flowers, an affinity that serves two purposes. First, it brings the flies to same resource that their victims are attracted to, and second, the flies themselves take nectar from flowers as food.

I’ve never seen a conopid attack its victim, but the process is said to be dramatic. The female fly either grapples with its victim in the air, or knocks it to the ground by diving into it in flight, like a falcon attacking a bird. Once the two insects are in contact, the fly uses a pair of flanges on the underside of its abdomen much like a bottle opener: It pries apart two segments of the abdomen of the bee or wasp, and the fly then deposits a single egg in the resulting gap in the victim’s armor.

The genus Myopa is now on the wing; I found my first one of the season on April 28, as it visited periwinkle flowers (and presumably searched for bees) outside my office off Lambert’s Cove Road. Our two myopa species resemble each other: Myopa clausa, apparently rare here since I’ve found it only once, is a gorgeous, red-bodied fly with clear wings. Myopa vicaria, appreciably more common and widespread, is less brightly colored and has dark spots on its wings. Given the timing of their activity as adults, they likely specialize in parasitizing solitary, burrowing bees, many of which are also early-season insects.

Somewhat later in the year appears the drab genus Zodion, tiny like Myopa but darker and with bold lengthwise stripes on their thorax. I’ve been unable to determine what species (there could be more than one) of Zodion occurs here; this is one genus that probably requires a specimen rather than just a photograph for reliable ID.

Finally, the larger Physocephala tibialis is on the wing in July and early August. Its resemblance to wasps is striking: The fly has a wasp-like narrow waist, and this species is easy to overlook among the many species of solitary wasps that visit flowers during midsummer. Apparently our most common, and certainly our most conspicuous conopid, Physocephala tibialis has half-dark wings that are quite distinctive, and its antennae, fairly short and bladelike, are quite different from the threadlike antennae of a wasp.

With bees, like many other pollinators, quite famously facing population declines, and with thick-headed flies preying largely on bees, does it follow that we should mash every conopid we come across? I’d say surely not. For one thing, our conopids are respectable native species, entitled to a place in our ecosystem. For another, these flies, by virtue of their fondness for flowers, are pollinators themselves, and as such serve a useful ecological function.

And finally, parasitism is part of the reality of life as a bee; thick-headed flies are far from the only insects that prey on or parasitize bees, and every bee species has evolved in the presence of the selective pressure applied by parasitism. If bee parasites were removed from the system, our bees, in subtle but significant ways, would cease to be the same species. The relationship between parasite and host, in other words, is an integral part of the nature of each species.

All in all, thick-headed flies illustrate a lot of what I find fascinating about flies in general: Surprisingly beautiful insects, yet rarely noticed, they engage in dramatic behavior right beneath our noses. Keep an eye out for them.