Why do bugs swarm over water? - Luke M. Jacobus

Like so many winged insects, the world’s roughly 4,000 mayfly species take to the skies for their annual reproductive ritual. These flights happen above every kind of freshwater body on Earth, barring Antarctica and a few remote islands. But whether the breeding mayflies are large Hexagenia limbata, tiny Paracloeodes minutus, or the thousands of species in between, their time on the wing will be brief. Very brief. For most mayflies, adulthood lasts roughly one day. And for some species, it’s only a matter of minutes. This isn’t because they’re all eaten up by predators—though they are a favorite food of many fish, birds, and larger insects. Rather, this abridged adulthood is a natural part of their life cycle.

A close look at these insects reveals they've ditched functional mouths and digestive systems. Without these innards taking up space, mayfly bodies can store more eggs, with some species producing up to 12,000 per individual. This adaptation is essential to mayfly survival since relatively few of these unborn flies are likely to reach adulthood, and even fewer will successfully reproduce themselves. In most of these flights, male mayflies form dancing swarms, which female mayflies move through until a male grasps them for some mid-air mating. Some all-female populations can reproduce without joining in the dance, using a process called parthenogenesis to produce viable offspring without the help of a male. And recent research suggests some mayfly populations may even switch back and forth between these two forms of reproduction.

But regardless of how the eggs are made ready, the female will deposit them on or under the water, and promptly perish. This might sound like a tragically brief existence. And even mayflies’ scientific name, Ephemeroptera, refers to their stunningly short lifespan. But for centuries, fishermen suspected that mayflies live much longer than this short flight. And in 1681, scientists finally caught up to their fishing friends with a published account of the full mayfly life cycle. When the time is right, these eggs will hatch. Depending on the kind of mayfly, this can happen as late as several months after the egg is laid, or even before it’s completely left the mother’s body.

But whenever their egg opens up, the mayfly inside quickly finds its way into fresh water, and it’s in this aquatic, nymph stage that mayflies spend the majority of their lives. Inhabiting rivers, small streams, ponds, lakes, and other places with clean fresh water, most mayfly species stay in this nymph stage and feed underwater for nearly an entire year. In fact, some species live as nymphs for up to two years before they metamorphose into their winged adult state. A year is a long time for such a small creature to stay alive. So to survive and thrive, mayfly species have a wide variety of habits and corresponding body forms.

Sprawlers are flat enough to stick to surfaces despite the strength of a current, while clingers accomplish the same feat with suckers and claws. Other species are swimmers, crawlers, or borers. One of the most noticeable groups are the burrowing mayflies; sizable specimens which use their legs, tusks, and a scoop on their heads to dig U-shaped tunnels. In these muddy dwellings, the nymphs use their abdominal gills to accelerate water through their burrow, filtering it for food along the way. After they metamorphose, adult swarms can get so large they appear on weather radars like rain.

Regardless of how they've adapted, each unique species plays an essential role in their ecosystem. Plus, nearly all mayflies are a staple of freshwater fish diets, and since they ingest the smallest particulate matter in their environment, mayflies are also very sensitive to water pollution. In fact, scientists can tell a lot about a water body’s health based on which mayfly species they find. If they don't find mayflies at all, it's usually an indication that something is wrong. So the next time you see a swarm of mayflies over a pond, bridge, or park fountain, remember that their short aerial dance means a healthy world exists below the surface.