If you run into Bombylius major — and it’s easy to do, since this species is common on Martha’s Vineyard — you might find yourself puzzling over what, exactly, you’re looking at. About a half-inch long, plump and fuzzy with an almost spherical abdomen, Bombylius sure looks like a bee. But a closer look, if you manage one, tells a different story: this species has stubby antennae and only one pair of wings, physical traits that land Bombylius squarely in the fly department.
It’s not too surprising, then, that Bombylius major belongs to a family known familiarly as the “bee-flies,” due to their uncanny resemblance to bees. (The genus name “Bombylius” echoes “Bombus,” the genus to which our true bumble-bees belong). Bee-flies are a diverse and hugely successful division within the flies, with many thousands of species known (just the single genus Bombylius has more than 250 members). There are surely scores, and maybe hundreds, of bee-fly species on the Vineyard alone, but like so many groups of insects, our bee-flies have never been seriously studied.
Known commonly as the big bee-fly or greater bee-fly (despite the fact that its of middling size for this family), Bombylius major occurs naturally across much of the world. In addition to the traits I’ve already mentioned, this insect sports distinctly two-toned wings, with a dark leading half sharply distinguished from a transparent trailing half. The legs of Bombylius are long and delicate, and protruding from the front end is a fearsome-looking spike that might suggest a blood-sucking habit.
But don’t be alarmed: adult greater bee-flies are not the least bit aggressive, and they limit their diet to pollen and nectar taken from flowers. Given their early spring flight season on the Vineyard, these foods are often taken from blueberry and mayflower blossoms (about all that’s in bloom when Bombylius is at its peak of activity). The long proboscis, effectively just a straw, allows this fly to drink nectar from the deep, bell-like flowers of these plants.
If adults are benign, the larvae of the greater bee-fly are tough customers. Like most members of its large family, Bombylius is a parasite of ground-nesting bees. Female flies lay their eggs near or even in the burrow of a ground-nesting bee. I’ve never seen this behavior, but some accounts suggest that the eggs are flicked toward the bee burrow while the fly is airborne, aiming for the burrow like pilots releasing aerial bombs. The bee-like appearance of Bombylius may help it approach bee burrows without alarming the bees, and of course it may also deter would-be predators from attacking the fly.
Upon hatching, the fly larva bides its time while the bee industriously lays her own egg and provisions the burrow with pollen for the young bee to eat. As the larval bee nears maturity, the fly larva attaches to it, gradually devouring it as an external parasite. The maturing fly then overwinters in the bee burrow, tunneling its way to the surface in the spring before emerging as a fuzzy adult, ready to repeat the cycle.
Pollination is a beneficial role played by insects, of course, and it would be easy to frown on Bombylius because it preys on bees. But the greater bee-fly is itself a pretty fair pollinator, with its hairy body carrying pollen effectively from one flower to another. And there is presumably competition among bee species for pollen and suitable nest sites; bee parasites like Bombylius help keep bee populations in balance, ensuring that no single bee species grows so common that it squeezes out its relatives.
Greater bee-flies occur quite widely around the Island. But given their close association with ground-nesting bees, these flies are most common in areas with the dry, sandy soils that bees find most congenial for burrow construction. Fire lanes in Correllus State Forest are the easiest place to find Bombylius, and on a warm, sunny day in late April, I encounter one of these excellent flies every few yards.
Their small size and unobtrusive coloration make them a bit hard to spot, as does their active flight pattern: hover, dart, hover, nearly always within a foot or so of the ground. From time to time, you’ll spot a bee-fly visiting a flower, perching delicately on the flower’s rim and sometimes even continuing to beat its bicolored wings while that long proboscis probes the flower like a hypodermic needle.
Equipped, like other flies, with compound eyes that wrap around a good portion of the head, the greater bee-fly has remarkable vision and is very wary of moving objects (like people). When disturbed, they don’t usually move very far, but it is next to impossible to sneak up on one, and a pair of binoculars is almost essential if you hope to get a good look at this species. It took some crawling, but a high point of this past weekend was finally getting within camera range of one of my favorite flies!