With swollen abdomens and fuzzy legs, female flies attempt to enhance their size to attract food from potential mates. In response, male flies have improved their vision to see through this ruse. Research from the Universities of Gothenburg and Stockholm indicates that this dynamic is part of an ongoing evolutionary battle where both genders strive to outwit each other.
With swollen abdomens and fuzzy legs, female flies attempt to enhance their size to attract food from potential mates. In turn, male flies have sharpened their vision to see through this ruse. A recent study conducted by researchers from the Universities of Gothenburg and Stockholm indicates this ongoing evolution is characterized by both sexes trying to outsmart one another.
This study marks the first instance where researchers have demonstrated that male flies can also develop traits that allow them to successfully pass on their genes, despite the deceptive adaptations seen in females. In various species of dance flies, there exists a notable link between the female flies’ ornamentation and the size of the eye facets in males. This suggests that males improved their vision to better identify the most suitable female in a gathering.
Dead insect as a nuptial gift
“In species where female flies exhibit hairy legs, we frequently observe enlarged eye facets in male flies. Therefore, we suspect there is a relationship between the two,” notes Luc Bussière, an evolutionary biologist at the University of Gothenburg.
When mating, dance flies engage in a complex interaction. They have a lifespan of only 1-2 weeks during which they must reproduce by laying eggs that turn into larvae. To enhance their chances of passing on their genes, male flies seek out females they believe have the most eggs. During mating, males present a gift, usually a dead insect, to the females. Since females seldom hunt for themselves, they rely on the protein from the insect to produce eggs, which means that without presenting a gift, the male cannot mate.
How does a younger female fly, lacking eggs, attract males for food?
“Females employ various characteristics and tricks to imitate being egg-laden. They can swallow air to expand their abdomen, have developed hairy legs, and possess larger, darker wings to appear more substantial when they gather in swarms,” explains Axel Wiberg, a researcher at Stockholm University.
The behavior of animals mimicking and adapting to deceive the opposite sex is not unprecedented. Earlier studies have indicated that male flies have a preference for plumper females over thinner ones during mating. Recent findings reveal that males have enhanced their eyesight to dodge deception.
Sexual play drives species evolution
By capturing male flies and analyzing their eye facet sizes, researchers found that the upper eye facets were larger than those on the lower sides in some male species. Since male flies approach females from below when mating, it is theorized that this eye development assists them in determining if a female is genuinely egg-heavy or merely pretending.
This sexual competition may catalyze evolutionary changes within the species. Over generations, males with more prominent eye facets have been more successful in mating, passing on their genes. This reflects an evolutionary mechanism where the development of characteristics in male and female flies alternates over time,” states Luc Bussière.
The female flies in the study exhibit various traits—some have only slightly darker wings, while others possess both darker wings and hairy legs, along with the ability to inflate their abdomen. These traits likely didn’t emerge all at once; rather, they evolved as males became increasingly adept at seeing through the females’ pretenses.
What’s the next move for females?
“Females may evolve new traits that complicate males’ assessments of their size, or they might develop new strategies for gaining an evolutionary edge. For instance, in species where males have the largest eye facets, females also exhibit slightly enlarged facets, but on the underside of their eyes. The reason behind this is still uncertain, but it may help females spot approaching males more quickly, assisting them in positioning themselves advantageously within swarms,” says Axel Wiberg.
Facts: The life cycle of a dance fly
Adult dance flies lay their eggs during the summer, which then develop into predatory larvae in the soil. The larvae hunt and consume other organisms, and in the northern hemisphere, they likely enter the pupal stage before winter. In spring, these pupae complete their metamorphosis into adult flies. Flies emerge at different intervals throughout the summer, with most species having a flight period that lasts only a few weeks. During this time, males and females congregate in swarms to mate. In almost all dance fly species included in the study, males present nuptial gifts, typically in the form of insects, to attract females in the mating swarm. While mating, the female consumes the male’s gift as he inseminates her. Mating occurs multiple times with different partners. After mating and receiving sufficient food from males, the female can then lay multiple eggs in the soil, starting the next generation.
