They cause traffic disturbances and nearly black out skies with their massive swarms of depravity. After they're all sexed out, the females drop the fertilized eggs in the water and carry on with the rest of their lives.
"I'm going to business school!"
Of course, "the rest of their lives" isn't terribly long in this case, because ...
Their Stupid Defect:
They have no freaking mouths. They can't eat.
Adult mayflies do not develop working mouths. Their digestive organs are completely filled with air, which is the saddest thing we've imagined all week. As soon as they go through metamorphosis and reach adulthood, they begin starving to death. It would be bad enough if they were born without mouths, but the fact that they actually lose it when they grow up proves that nature has gone out of its way to screw them.
In a manner of speaking.