Wild Side: Fuzzy fly mystery

Identifying an elusive bee fly caught on film.

A bee fly, Sparnopolius confuses, on Sept. 15 in the Manuel F. Correllus State Forest. — Matt Pelikan

A typical field season generates more questions than answers for me, producing a lamentable sense of backward progress: As the years go by, questions accumulate, and I feel like I know steadily less about that natural world. So when an answer does come – a mystery species identified, a question of ecology answered – I seize it and hold it close as a moment when I actually feel like I’m learning something.

Such a moment occurred the other day, as I reviewed photos from an outing in Correllus State Forest. For years now, I’ve been getting glimpses in September of a tiny bee fly, perched momentarily on flowers or darting around a clump of blooming asters. The elusive behavior of this insect made a decent photograph, or even a decent look, impossible.

I persevered, stalking each example I found in the hope that my luck would eventually change. And finally, on September 15, I got my break. Conditions weren’t promising; a brisk wind had the leaves and flowers bobbing, and although the fly I found was sitting tight on a wild indigo seed pod, photographs were impossible until a sudden calm allowed a dozen quick shots.

I should explain that bee flies (the family Bombyliidae) are among my favorite insects. They are indeed bee-like, with plump, fuzzy bodies and a penchant for taking nectar from flowers. Many species possess a long, pointed proboscis in front, and while this organ looks daunting, it is in fact nothing more than a straw. These flies can’t bite, and the proboscis, if present, is just a device for reaching the nectaries at the bases of flowers.

In general, the bee fly family exhibits some of the most interesting reproductive biology to be found in the insect world. Larvae are generally parasites on other insects. But the identities of the host, and the methods adult bee flies use for getting their eggs into the right place, are highly variable. One common springtime species locates the burrows of solitary bees, dropping eggs right at the mouth of its host’s nest burrow. Another early-season species that parasitizes tiger beetles swoops repeatedly at the opening of a larval tiger beetle burrow, squeezing out an egg with each swoop like a tiny dive-bomber.

But although attractive and fascinating, as a family, bee flies are frustratingly hard to study and identify. Highly visual insects, they are wary and difficult to approach. Many species patrol almost constantly for larval hosts, flitting erratically a foot or so off the ground, never holding still long enough to allow a photograph. And once you have a photograph, you often find that what it shows doesn’t suffice for a firm identification; probably half of my bee fly photos are identified only to genus, and for another healthy percentage, subfamily is the best I can do. The details that matter for precise identification just don’t show up in photos.

My mystery fly, when I reviewed the photos I had managed to take, proved to belong to one of the long-proboscis clans. Like a lot of its relatives, it was essentially unmarked but covered with luxurious, yellowish hair. As I did more research, the picture became clearer. The date, for example, was helpful: most of our fuzzy yellow bee flies belong to a genus that turns out to be active only in the spring (no doubt reflecting the seasonality of their host species). And on studying my photos, I noticed some tiny details that ultimately pinned down the ID.

The proboscis on the Mystery Fly was rather short by bee fly standards, and it appeared wrinkled, almost twisted. The antennae were notably long, with their final section tapering from a stout base to a fine point. And on the wings, a particular vein (yes, insect ID often comes down such details) swept sharply forward toward the wing’s leading edge, a pattern subtly but distinctly different from that on the wings of other genera.

The details pinned the fly down to the genus Sparnopolius, which has only five members worldwide and only one that occurs in the eastern United States. So I had my answer: Sparnopolius confusus. With a range roughly centered on the Great Lakes and St. Lawrence River valley, this beast seems scarce in southern New England and likely qualifies as a sort of Vineyard specialty.

Sparnopolius uses June beetles as its larval host, it’s said. But I find no information on what June beetles, specifically, are used; how the fly finds the beetles; or what life stage the fly parasitizes (given the date, I suspect it’s the beetle’s larvae that are parasitized, but that’s a guess). I have only a vague sense of what habitat this fly prefers, or what flowers it finds most attractive.

So I guess I spoke too soon. I may have put a name to the Mysterious Autumn Bee Fly. But that discovery spawned a series of new questions I now need to answer, so once again, I’m falling behind.