The long-legged flies, family Dolichopodidae, may rank as the most conspicuous group of insects you’ve never noticed. They’re diverse, active, often colorful, widely distributed, and sometimes amazingly numerous. And they spend much of their time in the open.
But these flies range in size from just under a centimeter in length down to just a few millimeters, putting them just below the size threshold at which most casual observers are likely to notice them.
Still, if you’ve been anywhere outdoors lately, you’ve come within inches of one. And with just a little focus of your attention, I guarantee you can find them at this time of year in nearly any habitat where you choose to look. They seem most active in the morning and late afternoon; our local members of this family, anyway, seem to avoid intense sun.
This is a family that you can definitely find, with careful enough observation, in your yard or garden. Many of the long-legged flies that live around human habitation are colorful ones; in many cases, their entire bodies are metallic green, and a lot of species also have patterns of dark spots or bars on their wings. They truly are long-legged, and compared to some other insect families anyway, relatively smooth and free of spines.
Dolichopodids are much smaller than the familiar “green-bottle flies” — likewise metallic green, but much more robust, and actually a type of blowfly — that show so much interest in your rubbish. Body shape is also very different between these two kinds of flies: the blowfly is stout, while the long-legged flies generally taper to a narrow end on their abdomens. But aside from these metallic green blowflies, there isn’t much else that looks like a long-legged fly.
Long-legged flies spend much of their time on the exposed top sides of broad leaves. Hosta leaves seem to be a particular favorite in our yard, with forsythia running a respectable second, but any kind of broad and relatively hairless leaf will do. (When you’re as small as these flies are, even the finest fuzz on the surface of a leaf represents a major encumbrance.)
Sometimes these flies will simply perch quietly on a leaf, probably waiting for a potential mate to come by. At other times, they dash frantically around, likely searching for prey. They run amazingly quickly for such seemingly frail insects. But in any case, their fondness for being in the open make these flies easy to spot once you calibrate your eyes for finding objects just a few millimeters long.
Males and females in a single long-legged fly species often differ in minor but obvious ways. In some cases, for example, the males develop little flag-like appendages on their legs. Among the Vineyard species, some evidently have males with a fine extension on the antennae, perhaps as long as the body of the insect.
Long-legged fly species often have energetic courtship rituals as well, sometimes carried out in the air, sometimes taking the form of chases and tumbling on the surface of a leaf.
With just a very few exceptions, long-legged flies are predators both as larvae and as adults. Given their size, they necessarily take small prey: springtails, aphids, tiny worms, recently hatched spiderlings. But they have efficient mouth parts that methodically chop up and ingest food items. Like many insect predators, long-legged flies don’t seem to be very fussy: Some species may specialize in one type of prey, but in general, it seems like long-legged flies happily take whatever they feel they can manage.
How many species of long-legged flies occur on the Vineyard is anybody’s guess. But the number could be fairly large. Some 7,500 species in this family have been described worldwide, with about 1,400 known in North America. Given the small size of these flies and fact that many species are very similar to one another, both of those figures surely reflect just a fraction, and likely a small fraction, of the actual numbers.
Given their bright colors and active behavior, it must be just their small size that keeps long-legged flies from being better known. In that sense, these charismatic flies represent a large class of arthropods: All the ones just a little too small for most humans to notice. Long-legged flies are a worthy quarry, if you’re interested in starting to hone your perception.