Even on Martha’s Vineyard, robber flies have a bad name

A female Sachem (approximately 5/8 of an inch long) photographed a few weeks ago. Will her offspring add to the number of adults here in September? — Photo by Matt Pelikan

Flies — the insect order Diptera — have a public relations problem, based largely on the habits of species that either bite people (like the deer fly) or display a fondness for eating nasty stuff (like the common house fly). Our attitude toward flies isn’t really fair: of the many, many thousands species of flies in the world, relatively few bite humans, and for every species that swabs dog droppings, there is one that dines on flowers. Still, I’d almost admit to a measure of justice in the accusatory common name of the family Asilidae, known as “robber flies.”

These predatory insects have never been systematically studied on the Vineyard, so hard information about their diversity here is lacking. Based on what little I’ve learned by watching them over the past few years, it seems likely that the Island has a dozen or so species, and perhaps many more than that. World-wide, this is a fairly diverse group of insects, with more than 7,000 species known. Something like 1,000 of these occur in the United States, though many of those have limited geographical ranges or are highly specialized in terms of habitat preference.

Robber flies are easy to recognize as a group, although some species are startlingly close mimics of bees or other types of insects, and precisely identifying almost any robber fly is challenging. The typical one is large for a fly (one of our species is about an inch and a half long), with a heavy thorax, large compound eyes, spiny legs, bristly facial hairs, and — here is where the robber part comes in — a long, stout proboscis extending down from the face. Catching a prey item on the wing, a robber fly envelopes its victim in its spine-covered legs and jams that proboscis into the victim’s body (usually aiming, it seems, for the thorax or head). The proboscis functions at first like a syringe, injecting a paralyzing venom as well as digestive enzymes into the prey. After these chemicals have had some time to work, the proboscis acts as a straw: the robber fly sucks up the dissolving flesh of its victim.

If we can overcome our human bias against such dining habits, though, robber flies, like any other group of insects, turn out to be fascinating to watch and important participants in the natural world. These are, by and large, fairly tame insects that are easy to get a good look at. Indeed, some species of robber flies seem to have a fondness for perching on bare human skin, perhaps because it’s warm and easy to get a grip on.

Not to worry if a robber fly lands on you, however. Larger species are said to be capable of a fairly painful bite in self-defense, but like most predatory insects and spiders, they have no inclination at all to bite you unless you’re harming them. I view a robber fly on my arm as an opportunity to get a good look.

While some robber flies specialize in particular types of prey, all the species I’ve run into on the Vineyard appear to be quite happy eating anything they can catch. But these flies are notable for the size of the targets they’ll tackle: they rarely waste their time on small targets, and it is not unusual to see a robber fly feeding on an insect as large as itself. (There is at least one documented record of a robber fly, a large species in the southern U.S., capturing and killing an adult hummingbird!).

A robber fly’s venomous saliva surely contributes to its prowess. But these are powerful insects for their size, wrestlers as well as aerialists, and if you disturb a robber fly while it’s eating, it simply flies off with its meal rather than abandoning it.

Of course it’s unfair to condemn robber flies, or any other insect, for their dining habits. (And trust me, there are many insects that do far more grisly things than any robber fly does.) Moreover, one can hardly blame these animals for their Sci-Fi-creature looks, since insects evolve for efficiency, not aesthetics.

Like every one of our native species, robber flies have a role to play in the intricate web of life and death that makes our ecosystem work. Highly refined predators that routinely engage in dramatic and complex behavior, robber flies deserve a better name.