In late spring, we had hundreds of these float by our cottage dock. They look like some kind of shell that something has hatched from. We have never seen them before in all our years here at the cottage. What the heck are they?—Don Judson, Stony Lake, Ont.
Those are the shed “skins” (exoskeletons) of mayflies, specifically burrowing mayflies, says Wynne Reichheld, a biological sciences MSc student at Brock University in St. Catharines, Ont. “All mayflies start their lives in the water,” they explain. “Most lay their eggs on the surface, where they sink to the bottom before hatching. The aquatic nymphs stay in the water until they reach adulthood.” At which point—surprise!—a large number can emerge at the same time. “It’s similar to cicadas.”
Growing up involves a lot of transformations for mayfly nymphs—up to 40 in the case of some burrowing mayfly species. Since the skins you saw were floating on the surface of the water, Reichheld suspects that they were most likely from the bugs’ final transformation into winged, adult forms (called “duns”). “Some mayflies are sexually mature at this point and will find a mating swarm, lay their eggs, and die. Other mayflies undergo one more adult moult to reach sexual maturity. This final stage is where they become ‘spinners,’ ” says Reichheld. “If you find a mayfly skin outside the water, it’s most likely from a dun becoming a spinner.”
Cottage Q&A: Can bugs fly up between floorboards?
Okay, well, now we’ve solved the mystery of the floating skin. If you’re wondering why these bugs have earned the name “burrowing,” it’s because as nymphs, they bury themselves, probably to avoid predators. “Most nymphs use their front legs to dig, while pushing themselves forward with their back legs into soft sediment on lake bottoms or clay in river banks,” says Reichheld. They can burrow down in a few seconds. And they’re not the only clever bugs who do this. Caddisflies also use the strategy. “Although I believe they usually bury themselves butt-first,” says Reichheld. We assume they also prefer backing into parking spaces.
Got a question for Cottage Q&A? Send it to answers@cottagelife.com.
This article was originally published in the Spring 2026 issue of Cottage Life.
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