When I visited the river this morning I found it littered with thousands of upon thousands of dead mayflies.
These insects live in the river, then moult and emerge as a winged insect in a massive rush to procreate in their 24-hour lifespan as an adult, laying the eggs that will lead to next year’s batch. Apparently, for our stretch of the river, the emergence happened over the last couple of nights, though in some years I’ve seen them emerge in late June.
You can learn more on Wikipedia and at mayfly.org.
“The lifespan of an adult mayfly is very short and varies depending on the species. The primary function of the adult is reproduction; the mouthparts are vestigial, and the digestive system is filled with air.
“It often happens that all the mayflies in a population mature at once (a hatch), and for a day or two in the spring or fall, mayflies will be everywhere, dancing around each other in large groups, or resting on every available surface. … Because of its short lifespan, the mayfly is called one-day or one-day fly in some languages…. “
Wikipedia [2012]
David Kotz 