May is a glorious time of year. Spring is in full swing with its colorful blooms, fragrant scents and melodic birdsong. But anyone who hikes near a surging stream can tell you it is also the season of the pesky black fly.

Tom Murray CC BY-ND-NC 1.0

There are over 1,500 species of black fly worldwide, with about 150 species found in North America. The black fly family, Simuliidae is in the order Diptera, the true flies. These stocky little flies are between one-eighth- and one-quarter-inch in length. Most accounts refer to them as being humpbacked in appearance. But what you are most likely to remember about them is their annoying habit of swarming about your head as you travel beside flowing water or as you sweat while digging soil in your garden.

Black fly females lay their eggs in swift flowing streams and rivers, either at the edge of the water or directly onto its surface. Black fly young require these highly oxygenated waters to survive. The hatching larvae attach themselves en masse to submerged rocks and branches by means of hooks or suckers at the tips of their abdomens. (What appears to be an immersed mat of moss may actually be a mass of black fly larvae.) Though anchored to one spot, they are able to feed by filtering tiny organic particles, such as bacteria and diatoms, through their feathery mouthparts as the food drifts past. Once ready to pupate, the larvae will form cocoons and complete their metamorphosis to adult flies while still underwater. In late spring and early summer the adult black flies will emerge – and that’s when our troubles begin.

Male black flies feed only on nectar, and while the females will also sip flower nectar we know them best when they create those painful welts on our skin as they suck our blood. Both mammals and birds are fair game to a female black fly.

Thankfully, our local black fly population is fairly small and the season is short, so we should consider ourselves lucky. Regions to the north of us where mountain streams produce prodigious numbers of these flies have an unofficial “black fly season,” a time best spent indoors. And many a New Hampshire car carries a bumper sticker paying tribute to this very successful insect – it proclaims the black fly New Hampshire’s state bird.