Let us prey

Why choose venomous victuals when you could sup on a butterfly?

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For this robber fly, supper will be a bumblebee. Sort of a mixed blessing, if one is not careful. (Photo by Matt Pelikan).

Predation of one insect by another is not surprising: entire orders of insects, like the dragonflies, are predatory; and among flies, beetles, and many other orders, some families have turned their back on eating plants and evolved into hunters. What surprises me is that, amid the vast variety available to an insect that eats other insects, some predators court danger by specializing in prey that fights back.

An example that I’ve been fascinated by lately is a dark gray robber fly called Proctacanthus (like most insects, it isn’t well enough known to have a common name). About an inch and a quarter long, Proctacanthus ranks among the largest robber flies found on the Vineyard, and although it seems to be rather solitary and territorial, it also seems to be fairly common and widespread. And, as far as I can tell, all it eats are bees and wasps.

Like all robber flies, Proctacanthus is harmless to people but bad news for other insects. Robber flies specialize in aerial ambush, picking flying insects out of the air, piercing their bodies with a sharp beak, and injecting the prey with chemicals that paralyze it and break down its tissues. The robber fly then, quite literally, drinks its dinner, using its pointed mouthparts like a straw to slurp up its liquefied prey. When the fly has drawn as much nourishment as it can from its victim’s body, it drops the empty husk and resumes hunting.

Suitable for their predatory habits, robber flies are powerful fliers, and their legs are equipped with long spines that help them capture and hold their victims. While many robber flies are generalists in terms of diet, others specialize. One species of robber fly is a notorious slayer of butterflies; another eats spiders right out of their webs. But none seem quite as wedded to taking dangerous prey as Proctacanthus.

The first Proctacanthus I ever found was clutching a bumblebee. A while later, I found one eating a black-and-white ichneumon wasp in a woodland clearing. Passing the same spot two hours later, I found what I feel sure was the same robber fly eating a different ichneumon wasp, this one bright orange. Almost every Proctacanthus I see is eating something when I find it; and every Proctacanthus I’ve seen with food has been consuming something that stings. When you’re not much larger than a wasp yourself, the possibility of death or serious injury from a sting is very real. So I found myself wondering how and why this fly evolved to live so dangerously.

The “why” is easy. Bees and wasps are a fine choice to specialize in, if you can avoid getting stung. Few other insects are looking to tangle with a wasp, so there is not much competition for prey. Because bees and wasps are abundant and diverse, they represent a plentiful, season-long source of food. And the fondness of bees and wasps for taking pollen or nectar from flowers makes them easy to find: a typical tactic for Proctacanthus seems to be to stake out a flower-rich area, wait for a bee to come close, and nail it.

Surely the size of Proctacanthus helps it successfully vanquish stinging insects. The wealth of spines lining its powerful legs undoubtedly help it keep a firm grip on its prey, and a fine layer of dense hair that covers parts of the fly’s body may offer a little protection against stings. But it was not until I saw Proctacanthus actually hit a prey item that I figured out its secret.

A female Proctacanthus had been hanging around my yard for a day or two; I had disturbed it several times and had seen it eating a tiny, iridescent green bee. Then as I was watching a yellow-jacket taking pollen from a flower, the wasp was suddenly swept out of my field of view by a swooping robber fly. Holding its prey, the fly landed briefly on a twig, quickly adjusted its grip on the yellow-jacket, and flew to another perch in a lilac bush.

I approached carefully, so as not to disturb the fly, but even so, I was enjoying close looks within about 15 seconds of its initial attack on the wasp. And at that point, the wasp already appeared to be totally paralyzed by the fly’s venom! Wrapped in the fly’s spiny arms, the yellow-jacket was utterly inert; the fly, with its beak embedded in the back of the wasp’s thorax, already appeared to be eating.

Perhaps all robber flies are equipped with venom that effective. But for Procatcanthus especially, the ability to quickly paralyze its prey is vital. Tackling wasps may sound dangerous. But with the right tactics and a fast-acting venom, Proctacanthus makes it look safe and easy, paralyzing its victims before they realize they’ve been attacked.