Wild Side: Stopped in my tracks

Laphrias, new to me on Martha’s Vineyard, are massive robber flies.

One of the largest of our robber flies, Laphria thoracica uses its size and strength to subdue difficult prey including bees and beetles. —Matt Pelikan

After a quarter-century or so of steadily growing interest in insects, I’m not easily impressed. Big bugs, tiny bugs, colorful bugs, bugs with insanely peculiar life histories — I’ve learned to expect the unexpected.

Still, once in awhile I come across something that stops me in my tracks, with size, coloration, or sheer ferocity beyond the range of normal. A good example would be my recent first-ever encounter with the massive robber fly Laphria thoracica.

Among the vast majority of insects not widely enough known to have earned a common name, this predatory fly is the quintessential member of a genus sometimes referred to as “the bee-like robber flies.” The name doesn’t really fit a good number of the roughly 65 North American members of this genus. But by and large, Laphria is large, hairy, and yellow-and-black — surprisingly similar to a typical bee.

The genus can’t be very common on the Vineyard. While I’ve run into some Laphrias on the mainland, this was the first time I’ve encountered one on the Island. And these are simply not insects I’d forget seeing! Nearly an inch long, L. thoracica is robust in build, with thick legs, a massive body, and a long, daunting proboscis — a protruding mouth part that is central to this fly’s predatory behavior.

Like all robber flies, Laphria captures its prey in flight, clutching the unfortunate victim in a net of spiny legs and driving that proboscis in like a stiletto. Having pierced its prey, the robber instantly injects a squirt of saliva that combines toxins with enzymes that break down protein. The toxins immobilize the victim, usually within seconds. And as the enzymes liquefy the muscles and organs of the prey, the robber fly literally drinks its dinner, repurposing its lethal proboscis as a straw.

As you might expect, given its size and structure, Laphria thoracica specializes in tackling large or hard-to-kill prey. Bees, up to and including our largest bumblebee species, are a particular favorite of this robber fly. I imagine the sting of even a small bee could be a serious problem for a robber fly. But once seized by L. thoracica, a bee rarely gets a chance to fight.

But thoracica also preys on beetles, the tough shells of which often defy the efforts of lesser robber flies. The proboscis on Laphria thoracica is long and stout even by robber fly standards, and these predators have an instinctive knack for planting that weapon in the seam between a beetle’s two sets of wings. Avoiding the beetle’s armor, the robber fly jams its beak into the soft underlying body of the beetle, rapidly subduing it.

The similarity between this fly and a bumblebee is remarkably close — when I encountered my specimen recently, I had shot a half-dozen frames of what I thought was a bumblebee before I realized something was fishy. The head was too small, the antennae too short … and then I knew what my subject had to be.

Their resemblance to bees, of course, is not accidental. Mimicking bees presumably makes it easier for L. thoracica to bushwhack its favorite targets, which assume they’re in the company of another bee until it’s too late to react. The bee-like look may also protect Laphria from its own predators, which might leave the robber fly alone on the assumption that it’s armed with a stinger.

I found my thoracica in a friend’s backyard in Oak Bluffs. In an ongoing habitat-improvement program, the friend has installed a massive “water feature” in his yard, populating the pond shore with wetland plants. It’s a rich floral setting, with a wide variety of native plants present, and something always in bloom. As such, it’s fine bee habitat, and hence, a reasonable spot to find Laphria.

As imposing as Laphria and our other robber flies may be, they’re surprisingly timid animals when it comes to their own safety. Equipped with acute vision (a necessity for aerial hunting), robber flies readily detect approaching threats; sudden movements invariably cause a perched robber fly to take off and disappear.

But with care, they’re fairly easy insects to approach closely, observe, and photograph. And while one would undoubtedly deliver a painful jab in self-defense if you were crushing it, I think they need to fully envelop their prey in order to bite it — the process may be more a matter of hugging their prey upward against their proboscis than of stabbing downward with that organ. So a robber fly just perched on your arm might not have the leverage needed to pierce your skin even if it wanted to (which it wouldn’t).

I can’t guarantee you’ll be able to find Laphria thoracica. But if you pay attention, you can surely spot some of the lesser members of its predatory family. And larger or small, these exquisitely evolved killing machines will reward your attention.