Given the roughly 110 families of flies (the order Diptera) in North America, and my twice-monthly publication schedule for this column, I figure I could spend the next four years or so writing about nothing but flies. One family per column: I’d enjoy that! Flies are fascinating, with their astonishing diversity and the bizarre life histories of how some of them have evolved. And best of all, flies are poorly known: Every scrap of knowledge I gain feels like a triumph. But I expect that my readers, not to mention my editors, would rebel before I had even made it through the Nematocera. So I’ll devote this column to the fly family that’s currently on my mind, and then I promise to move on (for the short term, at least) to more charismatic critters.
That family, Heleomyzidae, is an interesting one, though it seems rather poorly represented on the Vineyard. And in many ways, it’s a good representative of the whole order: What’s true for Heleomyzidae is true for many other groups.
You can start, for instance, with the basic taxonomic status of the family, which is untidy in the extreme. Some experts argue for subsuming all of Heleomyzidae into some other family; other experts argue that the subfamilies of Heleomyzidae are so different from one another that all should be elevated to family level. An excellent handbook of flies by the entomologist Stephen Marshall describes Heleomyzidae as “a bit of a grab-bag family made up of a number of distinct lineages of dubious relationships.” I’m sorry to say that this situation is not unusual among flies.
On the Vineyard, at least, we avoid most of the complexity of this family. As far as I know, only four species representing two subfamilies have been found here. There are surely other representatives present, but again, like many fly families, Heleomyzids are small, secretive, and hard to spot. In the absence of a focused study, which seems unlikely, it will take years before enough haphazard observations have trickled in for any convincing picture of our Heleomyzids to emerge.
While Vineyard records for this family span the full year, the bulk of my own observations have come during late fall or early winter. Again, while this schedule isn’t typical of flies, it isn’t rare for this order, either: Many species of flies are amazingly hardy, with adults of some species even surviving through the winter at our latitudes. Some of these flies, seeking warmth, turn up indoors.
Indeed, our Heleomyzids appear to associate closely with humans. Virtually all of my records actually come from inside a building, and a high percentage of my Heleomyzid photographs show the insect perched on a window screen. Again, this is neither rare among flies nor inexplicable: The larvae of many flies, including many Heleomyzids, live in damp, dark settings rich with decaying matter, and ideal examples of such conditions can be found around human dwellings in compost bins, potted plants, or well-mulched garden beds. Prime fly habitat!
Expanding on the idea of rot and moisture, many Heleomyzids associate with fungi, especially as a food source and substrate for larvae. Some members of the genus Sullia, for example, are said to associate with truffles, and harvesters searching for those valuable fungi can reportedly find their quarry by spotting swarms of Sullia.
So don’t make the mistake I did and imagine that “Heleomyzid” means these flies are sun-lovers. The “Heleo-” (not “Helio-”) part is actually a Greek root referring to marshes, so you might think of this family as the “swamp flies.” These are small, shade-loving insects, ranging up to about a half-inch in length, and many Heleomyzids have bodies with some combination of gray and yellowish coloration.
Reflecting the taxonomic chaos of Heleomyzidae, there are few characteristics that define the family, and despite the hard work I’ve put in on flies, I often mistake Heleomyzids for members of several other groups. But while it’s not an ironclad rule, one trait is common enough in Heleomyzids, especially in the subfamilies known to occur on the Vineyard, to be useful: The leading edge of the wing, called the “costa” by entomologists, is equipped with a row of robust bristles.
Almost all flies have bristles along the costa, but in Heleomyzidae, these can be robust, unmissable on a specimen, obvious in clear photographs, and occasionally even visible on a fly in the field.
In any event, so far I know the Vineyard boasts two species in the genus Orbellia (one has dark legs, the other doesn’t); one species (I have no idea which one) in the genus Pseudoleria, with yellow legs and especially robust costal spines; and the seemingly common and presumably fungus-loving Suillia quinquepunctata, with a mostly yellow body. So keep an eye out for a Heleomyzid sharing your living space this winter. And take a moment to be grateful that I don’t plan to write another 109 consecutive fly columns!