You might swat at a swarm buzzing about, thinking they’ll bite. But they don’t: by the time mayflies are at the flying stage, they can’t bite.
Related to dragonflies and damselflies, adult mayflies have slender bodies, two or three long, stiff filaments extending off the pointed rear end, and two pairs of wings (although in some species the second pair of wings is reduced or absent). In fact, adult mayflies look rather like small, delicate dragonflies that hold their wings up over their backs instead of off to the side.
According to a 2007 paper by M.D. Meyer and W.P. McCafferty in the American Entomological Society transactions journal, Oregon is home to 142 species of mayflies. Mayflies are a large, old group: over 3,000 species of mayflies are found worldwide; there are mayfly fossils over 300 million years old.
While the basic life cycles of those myriad species vary somewhat in their lengths and timing, the basic pattern for mayfly lives is the same.
Mayfly eggs hatch in the freshwater streams where they were laid. The newly-hatched “nymphs” are long and slender, with six legs near the head end and three stiff filaments off the hind end, but lacking wings. Mayfly nymphs have gills and live in water, crawling along or clinging to or burrowing in the bottom. Most nymphs feed on the algae and detritus (rotting organic matter) that coat the rocks and sediment on the bottom and sides of the stream or lake, scraping their food off the substrate with their mandibles. Some mayfly nymphs also feed on tinier animals.
Mayflies are nymphs for one season or up to two years, depending on species and conditions, molting their skins up to 50 times as they grow. Mayfly larvae then metamorphose into a sub-adult (“subimago” or “dun”) stage, a very short phase when they look like adults, but aren’t ready to mate. Another molt and the subimagos become full adults (“imagoes” or “spinners”). Mayflies are the only group of insects that molt when they have wings.
As in a few other insects, such as crane flies, the mouthparts of adult mayflies aren’t functional. Mayfly adults can do only one thing: reproduce. Adult mayflies swarm near the waters where they grew up, seeking mates for an in-air alliance. In some places in America’s upper Midwest, a good year for the mayflies can mean those swarms are large enough to be tracked by the same radar systems that track tornadoes.
Very shortly after successfully mating -- sometimes within hours -- the males die and the females lay eggs then die. A mayfly may spend less than 0.14 percent of its life as an adult -- less than one seventh of one percent. (We humans spend about 80 percent of our lives as biological adults.)
What happens to those gigantic swarms of mating mayflies when the participants die? Sometimes the mass of the tiny insects is so great they form a slippery layer on bridges and roads, causing a major traffic hazard.
In any case, the sheer volume of biomass swirling through the air and settling on the ground is a major windfall for predators and scavengers. Bats, swallows, swifts, and others feed on airborne mayflies; insects, fish, and others feed on the fallen.
As testimony of their edibility, adult and sub-adult mayflies are common models for fishing lures. Fly fishers know that the best fly is one that matches the current “hatch,” the adults that are swarming and falling into the water at that time. There are a lot of options, as different mayfly species take to the air at different times, from early spring to autumn (not just May!).
As key algae grazers and detritus feeders, mayfly nymphs are primary consumers of many freshwater environments, keeping the algae in check and cleaning out the detritus. Certain other insects, amphibians, birds, and fish -- such as dragonfly nymphs, frogs, dippers, and trout -- eat mayfly nymphs.
Too, mayfly larvae are sensitive to pollution and low oxygen levels, making them good indicators of clean water.
Whether witnessing an overwhelming swarm or a modest, creek-sized hatch, there are many reasons to appreciate mayflies -- but no reasons to swat them.