A beautiful, delicate creature - and the epitome of "ephemera."
Mayflies spend the majority of their lives in an aquatic environment, emerging as a flying insect in the early summer:
The final moult of the naiad is not to the full adult form, but to a winged subimago that physically resembles the adult, but which is usually sexually immature. The subimagos are generally poor fliers, and typically lack the colouration patterns used to attract mates. The subimago eventually moults to the full adult, making mayflies the only insects where a winged form undergoes moulting.They live for only a day or two. Some people are offended by the fact that when the mayflies emerge, they do so by the tens of millions, coating lakeside vegetation (and cabins and vehicles and anything in the riparian area), and offering game fish enough free nutrition from their dead bodies that fishing declines for a measureable period.
The photo enlarges to wallpaper with a click (photo credit me).
Addendum: Reposted from 2009 to add this awesome radar image of a massive emergence of mayflies along the Mississippi River valley near La Crosse, Wisconsin:
Image from the NOAA website, where there are also a dozen or so still images of the effects of the massive swarm on ground structures (especially those lit at night like gas stations). The Star Tribune and other local newspapers also documented the phenomenon and noted that the masses of mayflies were responsible for a three-car accident on a local highway bridge, but note that the emergence is a favorable sign of local water quality:
By the 1980s, mayflies, stoneflies, caddis flies and even some species of fish had disappeared from the Mississippi River from the Twin Cities area all the way to Lake Pepin. But modern sewage treatment facilities and regulations on disposal of toxic chemicals have brought mayflies, and a healthy food chain, back to most waterways.