Bibio femoratus

For millions of March flies, it’s party time.

Bibio femoratus — Matt Pelikan

“Swarms”: The word keeps cropping up in phone calls and emails, asking me to explain the astonishing explosion of black flying insects that has been apparent at various places around the Island. A sport practice session at the high school almost had to be shut down, I was told, because the masses of airborne bugs made it hard to breathe. Other people arrived home to find their cars peppered with mashed insects.

Once I saw a picture of the culprit, identification was pretty easy. The insects, about a centimeter long and sporting bright red femora (the bug equivalent of thighs), proved to be a type of fly, Bibio femoratus. (The species is widespread and common, but evidently not enough so that it has earned a common name.) Bibio is in the family Bibionidae, also known as “March flies,” because many members of the group are on the wing so early in the season.

These flies, while a nuisance at this outrageous level of abundance, are harmless; adults have weak mouthparts and eat little more than a bit of pollen, and the larvae, likewise unequipped for biting or stinging, live in the soil as “detritivores” — that it, consumers of decaying debris. Careful observers noted a neat little twist to these flies: the sexes differ quite dramatically in appearance, with the males sporting huge eyes and clear wings, while the females have small eyes and nearly black wings.

No one, myself included, can recall seeing such swarms of this insect on the Vineyard (indeed, I had never encountered this species at all until this spring). But such massive aggregations are well-documented in this species, occurring when a mass emergence takes place and the newly active adults apply themselves to mating. More broadly, mating swarms are standard operating procedure in many types of insects, and it’s worth taking a look at this phenomenon.

Mating, of course, is a critical point in the life cycle of any species that engages in sexual reproduction, and there is nearly infinite variety in the procedures insects have evolved to get the job done. Any approach has both advantages and risks, but over the evolutionary history of a species, one particular procedure takes hold as the most reliable way to make the next generation happen.

In the case of mating swarms, a couple of very strong advantages apply. If a species can convene in huge numbers, the odds are good that high percentage of the available females will be able to mate. That’s a good thing, maximizing the number of young produced. Moreover, if thousands or millions of individuals abruptly congregate in the same place, no predatory species is going to be able respond quickly enough and in large enough numbers to eat them; by overwhelming any possible predator, the swarming insects guarantee that at least some of them will survive mating to lay eggs.

Swarms may also be competitive situations, especially if numbers of males and females differ dramatically. Members of the surplus sex may need to outlast rivals in order to get a chance to mate, and the endurance this requires means that mostly healthy, vigorous individuals — that is, ones with good genes — are the ones that mate.

There are risks as well to the swarming lifestyle. If everybody emerges on a day that happens to feature a late-season snowstorm, for example, an insect species can experience disastrously low reproduction that year. But the practice of swarming wouldn’t have persisted in the species that use it unless, on balance, it was an effective approach.

March flies belong to the most primitive group of flies, along with midges, gnats, mosquitoes, and crane flies. Among this group, reproductive swarms seem to be the rule rather than the exception. Many of these insects have aquatic larvae, and adults swarm in wet areas or even open waters. Larvae of other species, like Bibio, live in the soil, and swarms of these species may occur nearly anywhere.

Really, all that’s needed for a good mating swarm is a lot of individuals (easy to achieve, because most insects lay tons of eggs) and a way to synchronize the emergence of adults. This, too, is pretty easy: For aquatic species, a certain water temperature may trigger swarming, ensuring that most of the individuals in a body of water emerge at the same time. Day length would be an effective trigger as well, since all the individuals in a given area would experience the same duration.

I have no idea what Bibio uses to trigger its synchronized emergences, but whatever it is, it works. Also, clearly, something about conditions on the Vineyard over the past year favored a very high survival rate in a fly that is normally not very common. Any species needs a way to get males and females together. Humans use nightclubs and dating apps. March flies and their relatives simply take this basic idea to a logical extreme, building their entire life history around plans for one huge party.