Wild Side: Dance flies

Want to mate? Dinner first.

A male dance fly in the genus Rhamphomyia. —Matt Pelikan

I’ve written before about my growing fascination with flies — that is, the insect order Diptera — and I’m sorry to report that the problem is only growing worse. Every time I’m in the field these days, it seems I run across another startling instance of the games evolution has played with this enormous group.

The latest case would be the dance flies, members of the family Empididae. These are, for the most part, tiny insects, only a few millimeters long. And their habits — they spend a good portion of their time flying busily in swarms, like gnats or midges — combine with their size to make them hard to detect and observe.

But as I develop the knack for noticing them, I’m finding them to be quite common on the Vineyard, though fiendishly difficult to photograph successfully. And as I research their life histories, I’ve learned of some truly bizarre behavior.

First, it’s worth noting that Empididae is a huge, diverse family. Some 3,200 species are known around the world, varying widely in appearance and ecology, and experts are unanimous that this represents just a small percentage (perhaps 10 percent) of the actual number of species. Clearly, a lot of work remains to be done on this group! North America has roughly 450 known species. How many occur on the Vineyard is anybody’s guess, but I’d be shocked if it were less than 50.

Dance flies are so called because of elaborate courtship rituals that take place in the flying swarms I mentioned earlier. Observing such a swarm, it would be easy to assume that the insects are simply percolating at random, with little interaction among individuals. But in fact, researchers have shown, these “dances” are highly structured, and highly evolved behavior takes place within them.

The rules vary from species to species. For one thing, each type appears to have a preferred setting for swarming (some prefer sunny spots over low vegetation, for example, while others prefer to swarm over open water). And the details of how males and females interact within the swarm also vary.

But the general gist, for a lot of dance flies, is that males collect presents (in the form of a small, edible prey item, often an even smaller insect), quite literally gift-wrap them (using silk produced by glands on their legs), and offer them to females as a sort of bribe. If a female likes the looks of a present, she and that male peel off, perch outside the swarm, and mate.

In many cases, females apparently aren’t capable of feeding on their own; these wedding presents offered by males represent the only food the female will eat as an adult, and the protein in the offering is needed for the female to produce eggs.

Some males – so typical! – try to game the system, eating part of their gift before they wrap it up, or offering just a ball of silk with nothing edible inside it. In both cases, the males are hedging their bets, probably accepting a lower chance of mating in exchange for less investment in effort and energy.

But in any event, there is clearly an elaborate interplay going on, with males and females in some primitive way calculating their return on investment for various courses of action. When the game is over, the individuals that have mated most successfully presumably show, on balance, the instincts that are best suited to ensure the perpetuation of their particular species.

Dance flies are related fairly closely to the larger, more familiar robber flies; the two groups often have very hairy bodies and distinct “necks,” and come equipped with a piercing proboscis they use to attack prey items. As you’d expect for visual hunters, both groups have proportionately large eyes and maneuver deftly when in flight. Dance fly larvae live in a variety of settings, sometimes highly specialized ones: Some, for example, are aquatic, while others are reportedly found exclusively in bird nests.

The males of many dance fly species have conspicuously enlarged segments on their legs, housing the glands that produce the silk used in their mating rituals. While this isn’t universal within the family, it seems common enough among Vineyard dance flies to be a useful cue for recognizing them.

Unless you actively study flies, the odds are good you’ve never knowingly seen a member of Empididae. These flies are simply below the size threshold for attracting our attention, and if you have encountered dance flies, you’ve likely dismissed them as generic gnats (which in fact are an unrelated group). But dance flies are widespread and numerous, carrying out their curious rituals right under our noses, an example of how much gets missed by obtuse human perception.