In some parts of the world, mayfly's "hatch" all at once. That is, the naiads or nymphs, crawl from the water where the adults laid eggs and molt for the 2nd to final time into the subimago or sub-adult stage. Poor flyers and not fully, sexually mature, in less than a day, they molt the final time, here amassing in some places by the millions to mate, lay eggs, and die.