Among the most impressive members of the family Asilidae — that is, the robber flies — the genus Laphria poses something of a puzzle on Martha’s Vineyard. We’ve got vast amounts of seemingly suitable habitat. But this large genus (there are more than 60 species known from North America) is represented here by just four species, none of which appears common.
Sometimes known as “bee-mimic robber flies,” the species in the genus Laphria are for the most part good-size, stoutly built, hairy insects with black-and-yellow coloration that does indeed make them resemble bees. They won’t fool experienced observers, but beginners are often hoodwinked into mistaking these insects for odd-looking bees.
The resemblance has no doubt evolved for good reason. For one thing, mimicking a stinging insect can be a useful strategy for self-defense. And for another, a fair percentage of Laphria species take bees as a significant or even dominant portion of their prey. Looking bee-like themselves may help Laphria catch its preferred target by surprise.
I said “none of which appears common” up above for a reason: A shortage of observations doesn’t necessarily equal actual scarcity. Visual hunters, robber flies in general are wary and easily disturbed. I suspect that for every Laphria I actually see, a good number of other ones spot me first, and simply bail out before I detect them.
Even if they don’t flee, robber flies, including Laphria, can be hard to spot. Their preferred foraging method is patient perch-hunting. Poised on a leaf or twig tip, a robber fly will sit motionless until a suitable prey item flies past. The attack that follows is fast and efficient, but until a robber fly attacks, its stillness makes it hard to detect.
But regardless of their actual status, three of the Vineyard’s four documented Laphria species are known here from just a handful of records (two of them from just single observations, as far as I know). All of our species are associated with woodland of various types, perhaps because Laphria larvae commonly live under the bark of rotting wood, where they prey on other tunneling insects.
Our most common representative of this genus, Laphria champlainii, is actually a species that can be considered truly rare in most of its modest range in the Northeast. This is because the habitat this fly prefers — scrub oak barrens — is a scarce one, from the regional perspective. On the Vineyard, however, scrub oak abounds, especially on the dry soils of the outwash plain.
All the local records for this species that I’m aware of (a total of 11) come from the eastern half of Correllus State Forest. The Vineyard accounts for fully one-third of all the records of this species in the iNaturalist platform, and looking at how those records are distributed, it’s clear that the scrub oak barrens of the Cape and Islands represent the core range of this specialized predator.
The prevailing management of the State Forest may benefit this species; at the very least, current management makes the fly much easier to find. Mowed periodically, the fire lanes in Correllus feature fairly low vegetation. In areas rich in scrub oak, the transition from mowed to unmowed vegetation along the lanes results in a sort of low cliff of scrub oak shrubs overlooking the mowed sections of the lanes.
Leaves on the uncut scrub oak provide the favored hunting perches for L. champlainii, offering these predators panoramic views of the shorter vegetation in front of them. The way I find this species is by walking slowly along that scrub oak edge, looking for perched Laphrias and hoping that if I flush one before I spot it, I’ll be able to mark where it lands and make a stealthier approach.
This very tactic recently produced the only local record I know of for Laphria grossa, closely related to champlainii, and apparently similar to that species in habits. I was searching for champlainii and stumbled over grossa instead. A massive insect, well over an inch long, grossa favors bees as prey, and can handle the largest bumble or carpenter bees with ease.
Represented here by just a single record, from Tom Chase’s naturalized backyard in Oak Bluffs, Laphria thoracica prefers somewhat moister, shadier habitat. I expect it’s more common here than the single record suggests. And the Laphria canis complex, a group of nearly inseparable, closely related species that are atypical for Laphria in being relatively hairless, is known here from three records, all from the terminal moraine of Chilmark and West Tisbury.
I expect all of these species are overlooked here, more than truly rare. And I also expect that other members of the genus Laphria occur here, and await discovery. But despite their size and aggressive hunting methods, these robber flies can be surprisingly stealthy. And I suspect that a tendency toward cannibalism, which simultaneously provides a meal and removes a competitor, may help keep Laphria rather thin on the ground. I’ll keep looking.
