Many species of adult mayflies emerge in May as the name implies. Some mayfly species, such as the Brown Drake, Ephemera simulans, emerge is large numbers. Tree leaves and other surfaces near the lake can be covered by Mayflies.
The synchronized timing of mayfly mass emergence has two purposes. First, mayflies do not feed as adults and do not live long. By timing the emergence, finding mates is easier because of the dense populations. Second, mass emergences allow most of the mayflies to escape predation by fish. A few unlucky individuals become meals for fish, but the remainder flutter to safety while the fish are distracted by other individuals.
During my trip to Michigan in early June, I found large numbers of mayflies along the shores of Lake Leelanau including brown drakes. Mayflies spend most of their life as aquatic larvae. Brown Drake larvae burrow in coarse sand or pebbles at the bottom of a lake. They feed on small pieces of plants or microscopic animals present in the water.
Like all mayflies, mature Brown Drake larvae will come to the surface to molt. A larva molts first to an immature form with stubby wings called a sub-imago or a dun. The sub-imago is a poor flyer, but can fly shortly after molting. It flies well enough to make the shore. The sub-imago then undergoes a molt to a true adult with longer wings and superior flight ability. The adult stages, called spinners, will mate and lay eggs. The spinners have a brief life, often only one or two days.