Imagine having no mouth and your only instinct is to spend the next few hours or days trying to mate before you starve to death or get eaten by fish. Such is the plight of the adult mayfly (order Ephemeroptera).
Mayflies start out with a pretty comfortable life. As hemimetabolous insects, the eggs hatch into naiads (aquatic nymphs) that vaguely resemble wingless adults. Naiads spend several months to two years crawling around on the bottom of streams, rivers, ponds, or lakes feeding and growing (Triplehorn and Johnson 2005). They molt often as they rapidly outgrow their exoskeletons. It’s all relatively mundane. Once they’re ready to leave the water, however, things get dramatic.
The naiads go to the surface of the water and molt into a winged sub-adult stage (the subimago). It’s at this point that they suddenly become very attractive to freshwater fish. Large numbers of these subimagoes are consumed by fish, for which they’re an important food source (fishermen refer to subimagoes as “duns” and often use live or fake duns as bait).
Subimagoes that survive the fish fly towards land, resting on plants for a short period before they again molt into fully reproductive adults (the imago stage, called “spinners” by fishermen). While they gain functional genitalia, they lose functional mouthparts. Since they can no longer eat they then have a very limited amount of time to live, normally only a day or two (Triplehorn and Johnson 2005). At that point mating is the only thing on their very simple minds. Adults often emerge at around the same time and engage in mating flights (Triplehorn and Johnson 2005). Shortly thereafter, females lay eggs in the water and the adults all die from starvation and exhaustion.
Some mayflies like those of the genus Hexagenia occur in plague-like numbers near lakes. Mating flights can be dense enough to appear on Doppler weather radar, and they frequently blanket every surface near the shore. I’ve walked along Lake Erie near Port Clinton and Sandusky, Ohio in May where you often crush scores of them with every step.
The individual shown above occurs in less impressive numbers, and farther from lakes. I found it in a forest along the River Raisin in southeast Michigan. By examining the wing veination and caudal filaments (the “tails”) I keyed this mayfly to the family Baetiscidae (armored mayflies). In this family there is only one genus (Baetisca) that occurs in North America (Triplehorn and Johnson 2005). They’re called “armored” mayflies because the naiads have enlarged and fused thoracic sclerites that cover part of their abdomens.
Triplehorn, C.A. and N.F. Johnson. 2005. Borror and DeLong’s Introduction to the Study of Insects. Seventh Edition. Thomson Brooks/Cole, Belmont, CA.