Dioctria hyalipennis

On the hunt with the glassy-winged robber fly.

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The business end of the Dioctria hyalipennis, or glassy-winged robber fly, including the prominent beak that robber flies use to puncture their prey. –Photos: Matt Pelikan

In my effort to study insects, the goal is simply to learn at least one new thing every day. Over time, in theory, the result will be … slightly less ignorance. Too many species! Too little time!

This is why I have special fondness for the species that occur in my own yard. It’s not a flashy mix: With a scant quarter-acre in the suburban sprawl around metropolitan Oak Bluffs, I mainly find common, widely distributed insects here. And a high percentage of them — close to half — are non-native ones, introduced accidentally or on purpose from other continents.

But the advantage of sharing real estate with these insects is that I can observe them frequently, often year after year, sometimes for as long as I care to watch. With repeated exposure, I learn far more than just the basics of how to identify a species; I can see how they hatch, eat, grow, mate, reproduce, and die. There are always more questions! But with these “roommate” insects, I feel like I can actually get acquainted.

This past month has been a good one, in particular, for watching the tiny robber fly Dioctria hyalipennis. (This fly has no common name, though its species name translates to something like “glassy-winged,” and I’ll use that impromptu name in this column.) Robber flies, which are all predatory, run large. (Our biggest genus, Proctacanthus, grows to almost an inch and a half in length, and happily preys on insects up to the size of fat bumblebees!) Dioctria, though, is at the small end of the spectrum, barely half an inch long, and slightly built as well.

This is a fairly common species, with a range encompassing Eastern North America and Western Europe. On the Vineyard, they’re by no means everywhere, but with an eye trained to spot them, I find a few virtually every day during their June flight period. They are as likely, maybe even more likely, in settled areas than in high-quality natural habitat. And they are somewhat gregarious, so if you find one, there are likely others nearby.

Glassy-winged robber flies, like most of their relatives, are primarily “perch hunters.” They park themselves on a leaf or twig and simply wait for prey to come within striking range. But each robber fly species has a preferred set of victims, suited to the size and characteristics of the robber fly itself. And each preferred set of victims dictates subtle variations on the perch-hunting strategy.

For Dioctria, the ideal hunting perch is three or four feet off the ground, and they cheerfully use both conifers and broad-leafed shrubs like blueberry. This species rarely occupies the highest or outermost branches of a plant; rather, it picks a spot set back slightly into the outline of the plant, combining a good vista with a measure of concealment.

Perched Dioctria are not easy to spot: Most of the ones I find have been flushed into flight by my motion. It’s not hard to track a glassy-winged robber in the air, though. They’re slow by the standards of this family, and they rarely fly far once disturbed, often returning to the same perch they just left.

Not just smaller than most robbers, Dioctria is also appreciably less spiny. This made sense when I discovered that soft-bodied, defenseless insects make up the bulk of the prey of this species: small moths, leafhoppers, fruit flies, and the like.

Perched glassy-winged robbers are constantly alert, and visibly track any object that flies near them, adjusting their stilt-like legs and pivoting their heads to keep the target in view. Fast-moving insects are left alone, but when a suitable target putters past, Dioctria launches and zeroes in on its victim. As with most robbers, a high percentage of attacks connect.

The process is too fast to watch. The robber grasps its victim and rams pointed mouthparts into its body, injecting a blast of enzymes that both paralyze the prey and begin to break down its tissues. Robbers, with spindly necks, surely don’t have neck muscles that allow them to “peck” their way through a victim’s exoskeleton; I surmise that they grasp the victim with their legs and pull it against their beaks, with the hairs and spines on a robber’s legs being there to assure a firm enough grasp to do this.

In any event, by the time the robber lands with its meal, just seconds after launching its attack, the prey is usually immobilized and the robber fly is already sucking out its tissues. It doesn’t take Dioctria long to drain a typical victim: after five minutes or so, the emptied husk is tossed overboard, and the robber fly resumes scanning for its next meal.

A Piper Cub in a family of fighter planes, the glassy-winged robber fly may be the shortest, lightest, least well-defended, and slowest-flying member of this group. You might think those traits would be disadvantages in a predator. But a lot of dead leafhoppers argue otherwise. Dioctria is perfectly adapted to find and catch a class of prey that is both abundant and vulnerable.