Wild Side: Robber flies

From robust to tiny, these furry-legged fellows can be hard to find.


Among my favorite groups of insects are the robber flies, the predatory members of the family Asilidae. Ranging in length from a half-inch or so to well over a full inch, these formidable hunters typically ambush flying prey from a perch, piercing their victim with a stout beak and injecting saliva that both paralyzes the prey and contains enzymes that digest it. Lacking chewing mouthparts like all flies, robber flies literally drink their meals, sucking up broken-down tissues through the same beak they used to kill their prey.

Although it’s a good-size family, with some 7,500 species worldwide and more than 1,000 in North America, Asilidae does not seem especially well represented on the Vineyard. My personal photo archive contains only about 20 species, and very little other information currently exists that could expand that number. But it’s a safe assumption that I’ve missed some — perhaps many — species that occur here, and so I keep a sharp eye out for these agile aerialists.

I hit the perfecta on June 21, finding two new species within a few feet and a few seconds of each other as I worked the scrub oak barrens of Correllus State Forest. Both species turned out to be unusual ones, uncommon, or at least infrequently observed, and with very little known about the specifics of their natural histories. Conveniently, these two species fall close to the extremes of robber fly size, and in their appearance, they illustrate the range of characteristics this family can show.

Let’s start with the big one, an inch-long, massive bumblebee mimic that proved to be called Laphria champlainii (it is scarce enough that it has no common name). While we have robber flies with longer bodies on the Vineyard, we only have one species I’m aware of that approaches this beast in sheer bulk and intimidating appearance. In addition to its size, L. champlainii (like most members of its large genus) is prodigiously hairy: Black or yellow fuzz covers its entire body. The resemblance to a stinging bee presumably helps deter attacks by other predators, and may also help this species bushwhack bumblebees, which may well figure among its menu items.

I mistook this insect for Laphria grossa, a similar species I’ve documented previously on the Vineyard. I did note that this individual turned up in a much drier setting than did the small number of L. grossa that I’ve found. So I was not entirely surprised, when I posted photos of this insect in a Facebook robber fly group, to learn that I botched the ID. The dry habitat proved to be a meaningful clue: L. champlainii associates strongly with scrub oak barrens wherever it occurs. There are also subtle differences in appearance between this species and L. grossa: The first segment of the abdomen is yellow in champlainii, black in grossa, and the latter species has a constriction at the base of the abdomen that champlainii lacks. Such is the level of detail on which insect ID depends!

Because it is so rarely observed, little is known about the specific habits of L. champlainii. Like other members of the genus, champlainii probably spends its larval stage in decaying wood or under bark, feeding on wood-boring or tunneling larvae or beetles. As an adult, it probably rests on the tip of a scrub oak leaf (like my individual was doing), waiting for flying prey to pass by. Some sources opine that bees and wasps are favored prey of Laphria in general; but most of the photos I’ve found of this genus with prey shows them eating flies of various kinds. In all likelihood, they are generalists, taking whatever happens to pass by. Such powerful predators can probably handle nearly any arthropod prey, from soft-bodied leafhoppers up to well-armored beetles.

At the other extreme was the tiny Taracticus octopunctatus (again, too poorly known to have acquired a common name, but “octopunctatus” refers to a set of white crescents on the abdomen). A bit under a centimeter long (sources give 8 mm as the average length), Taracticus is as delicate as Laphria is robust, and as hairless as Laphria is fuzzy. Only the shape and position of the eyes and the presence of stout bristles on the lower legs tipped me off to the fact that this insect was a robber fly at all. But it’s a beautiful creature, especially its eyes, with flashing, iridescent highlights.

Taracticus seems to occur sparingly across much of the Northeastern U.S., roughly from Kansas to Maine. I have no idea whether it is truly uncommon, or simply overlooked because of its small size. Again, little has been published on its habitats, but the subfamily it belongs to is noted for its penchant for preying on bees. They would surely have to be small ones, given the diminutive scale of Taracticus.

The range of variation found among robber flies is a source of endless fascination for me, even as my ignorance about this group is a source of frustration. Encountering two new species in a single outing makes me think I still have lots of work to do in order to fully account for the Vineyard members of this lethal family.