Wild Side: Mysterious fly

This early-season species is happiest on bare sand.

Tabuda varia is a type of fly that is quite common on the Island. — Matt Pelikan

As a student of insects, I spend a lot of time contemplating the daunting slopes of my personal Everest of ignorance. The class Insecta comprises 30 orders, roughly 1,000 families, and upward of a million described species (to say nothing of the undescribed ones). Meanwhile, I feel like I have a meaningful acquaintance with, say, two or three families, perhaps 10 genera, and a few dozen species.

I have, in short, some work to do. It’s reassuring, then, to occasionally run across an insect that nobody else knows anything about either. An example is on the wing right now on Martha’s Vineyard, a peculiar fly named Tabuda varia.

Like so many insects, Tabuda varia doesn’t have a common name, and it is rarely encountered, or at least rarely noticed, even by insect enthusiasts. The massive database at iNaturalist.org — 62 million records compiled by about 3,750,000 users worldwide — contains only a half-dozen records for this species (one of those is mine), ranging from Northern Florida and Georgia to Southern New England. Bugguide.net, also massive, holds only five records, spanning the same geographical range. In terms of information on this species, neither site provides more than the bare bones of its taxonomy, which is also all that Wikipedia can come up with.

Tabuda varia, though, happens to be rather common on the Vineyard (and on Nantucket, too, my limited time over there suggests). I find it reliably every year, sometimes noting a dozen or more in a day, with multiple individuals often concentrated in a relatively small area. And between my personal experience, inference from the photos posted on iNat and Bugguide, and the sparse information I’ve gleaned about Therevidae, the family Tabuda is part of, I can say at least a little about this mysterious fly.

For one thing, it’s an early-season species, with New England records generally falling between April and June. For another, it is at its happiest on bare, mineral sand: Every Tabuda I’ve found has been perched on the ground, on sand free of vegetation, and the 11 individuals I’ve mentioned on “citizen science” sites appear to be doing exactly the same thing.

I have to admit this sedentary behavior is a bit disappointing. The usual common name for the family Therevidae is “stiletto flies,” a name that conjures up dramatic visions of stealth and violence. And Therevids in general resemble, and are closely related to, the robber flies (family Asilidae), which really are stealthy and violent predators, ambushing their prey in mid-air and dispatching them with injections of potent venom.

Larval stiletto flies apparently are predators, elongated maggots living in the soil and opportunistically grabbing whatever smaller arthropods they encounter. And the adults are certainly capable of flight; they “sit tight” when you approach them, but once disturbed, they dart away with zeal and alacrity, never to be seen again.

But I am sorry to report that adult stiletto flies are not predators of any sort; indeed, nobody seems to know what they eat, although their occasional presence on flowers suggests they may take pollen or nectar. The adults, as far as I can tell, do nothing at all except sit on the sand.

Tabuda varia, ranging from a half to three-quarters of an inch long, shows the typical Therevid structure of an elongated body (more so in females than in males). Perhaps this shape is where “stiletto” comes from? As with the related robber flies, the body and legs of this species are hairy and heavily spined. And they perch with a characteristic posture, front end elevated, tip of the abdomen close to the ground. The wings, folded over the back when the insect is perched, are clear with dark speckles and dark veins that stand out dramatically. The signature of Tabuda varia is a fine set of antennae, covered in fuzz and projecting well out in front of the insect. This species is not easy to detect, given its small size and sluggish behavior, but once seen, it’s among our easiest flies to identify to the species level.

An excellent handbook on flies by Stephen A. Marshall reports that Australia, of all places, is the global center of Therevid diversity. (Alas, Marshall doesn’t even mention the genus Tabuda.) But as a family, stiletto flies are globally distributed and tolerably diverse, with some 1,600 species worldwide and about 150 in North America.

One peculiarity of the family seems to be that, like Tabuda varia, most species make a veritable religion out of specific perching habits. As diligent as Tabuda is about perching on bare sand, for example, the other Therevid I’ve found on the Vineyard, Ozodiceramyia notata, rarely sits anywhere other than at the tip of a broad leaf on an herbaceous plant.

Why? I have no idea. And there a million other insect species that I know even less about. As I said, I need to get to work.