While many species use elaborate displays to attract mates — peacocks flaunt their feathers, deer grow imposing antlers — female dance flies (Empididae) take a slightly different approach. Though they puff up their bodies, darken their wings and sprout extra hair to appear more fertile than they really are, it’s all an illusion

And the males might be evolving sharper vision to see through the deception.

The aptly named dance fly is known for its intricate courtship rituals. To the naked eye, a swarm of these tiny insects may seem like a chaotic buzz of movement, but their aerial displays follow a structured pattern. Females perform elaborate maneuvers to attract a mate while males closely observe their traits to assess reproductive fitness.

Males are particularly drawn to females with thick, hairy legs and swollen bellies —suggesting an abundance of eggs and better chances of carrying on the genes. But the females are masters of trickery, using clever tactics to exaggerate their appearance.

“They swallow air to inflate their abdomen and look larger when they swarm,” explains Axel Wiberg, researcher at Stockholm University and lead author on a 2024 study on the correlation of evolving physical traits of the male and female dance flies. “They’ve also developed hair on their legs and larger, darker wings to enhance the illusion.”

Beyond visual deception, courtship involves another key element: gifting. Males capture prey, wrap it in a silken cocoon and present it to the chosen female.

For the most part, this could be the only food she will eat in her lifetime as females can’t feed themselves.

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CAPTION: Axel Wiberg IMAGE: Elina Immonen

This mating ritual happens with several males, and once the female has eaten enough high protein “gifts,” she can lay eggs.

It does seem, however, the gaming goes both ways.

Some males eat part of the gift before wrapping it or simply give a ball of silk with no gift inside at all. Show up empty-handed, however, and suffer rejection.
But while there’s trickery happening from both genders, the females’ ruse may be losing its effectiveness.

Researchers at the University of Gothenburg and Stockholm University have found that male dance flies are evolving better eyesight — allowing them to detect which females are truly the most fertile and which are simply putting on a show hoping for a snack.

In certain dance fly species, there is a correlation of females with big bellies and thick, hairy legs to larger eye facets on the males. This indicates the males have evolved over time to suss out the most fertile females.

“For generations, male flies with the largest facets have been favored in mating, and their genes are passed on. We see this as an evolutionary system where the development of different traits in male and female flies has been alternating,” says Luc Bussière, evolutionary biologist at the University of Gothenburg and co-author on the study.

CAPTION: Luc Bussière IMAGE: Olof Lönnehed

How can this dance continue to evolve the species?

Gothenburg University’s publication of the findings says that the females likely developed characteristics over time as the males became better at detecting the ruse.
So, what happens next as each gender continues to evolve to beat out the other?

“Perhaps the female develops new traits that make it harder for the male to judge her size. Or she may develop entirely new strategies to gain an evolutionary advantage. For example, we see that in the species with the largest facets in the male, the female also has slightly enlarged facets, but on the underside of the eye.

We don’t know what this means, but perhaps it helps the female to see an approaching male more quickly and thus find an advantageous position in the swarm,” Wiberg tells KUST Review.

The paper was published in the International Journal of Organic Evolution.