As far as I know, these insects live in Hungary's river Tisza and the surrounding smaller rivers only. Having spent years in larva state digging around in the riverbed, they emerge just for 1-2 hours to mate, and then die. In fact, they have no digestive tract in that state. A few evenings every June, mayflies by the millions swarm on and above the river. This year's swarming was unfortunately not as intense as two years ago.