On a spring evening along the River Thames, thousands of mayflies perform what may be one of Earth’s oldest dances: a steep vertical climb, a flip, and a slow skydive back down. Scientists have finally figured out why, because apparently 300 million years wasn’t enough time for the insects to leave a note.
The males use this bizarre up-and-down flight pattern to tell who’s male and who’s female. By staying vertical, they avoid flying horizontally above the swarm, which is the female’s signature move. In simulations, males stopped pursuing any target that dropped beneath the horizon - a helpful filter, since males will otherwise try to mate with a beach ball.
“The problem is that the males have almost no filter,” says Samuel Fabian, a research fellow at the University of Oxford. “You can give them a beach ball - which, as far as I’m concerned, looks quite different from a female mayfly - and males will go right up to that much larger object and try to mate with it.”
This is critical because mayflies only live a few hours to a few days, so they can’t afford to waste romantic energy on beach balls. The dance helps them pass on their genes, which is especially urgent since many of Britain’s 51 mayfly species are declining - another casualty of the so-called “insect apocalypse.”
A 2019 global review estimated 40% of the world’s insects are declining, with more than one in ten species potentially lost by century’s end. From 2015 to 2021, the nonprofit WildFish found that Britain’s chalk streams had lost 41% of their mayfly species compared with 1998.
“Pollution, sediment runoff, reduced river flows and rising water temperatures are all eroding the conditions these insects depend on,” says Janina Gray, head of science and policy at WildFish.
Fabian encourages Britons to enjoy the ancient spectacle while they can. “These are quite urban places with lots of traffic, but they’re still hanging on and they’re still doing this dance that they have probably been doing since before Britain was separated from mainland Europe.”