During the summer, swarms of dance flies gather on the banks of the Credit River, where they perform a swirling mating ceremony in mid-air, but while in typical insect mating systems females choose their mates, in the case of the dance fly, it’s males who are the “choosier” sex.
Female dance flies suck in air through their mouth parts and inflate their abdomen. Males find larger female dance flies attractive, so females create an illusion by inflating the abdomen and tucking in their fringed legs. They tuck their “fringed” legs close to their abdomen and go to mingle among other, competing, female flies.
“Once inflated, the females pull up their legs alongside the abdomen and enter an all female flying swarm to compete for ideal positions within the swarm with other female ‘inflate-a-mates.’ Males carrying nuptial gifts of small dead bugs enter the swarm and pick the largest of the inflated females to mate with,” explained Dr. Darryl Gwynne.
Gwynne is a UTM biology professor, who focuses his research on the distinct behavioural and structural differences between the sexes-the factors that control “femaleness” and “maleness”-in most animal species.
He looks at so-called role reversals in insects. These occur when females experience strong sexual selection. In the case of the dance flies, the prey that the male provides generates competition between females.
“I got to thinking about Darwin’s original ideas about sexual selection which came from his observations of “sexual dimorphism”-[for example] elaborate plumage or weaponry in males but not females. If there is strong sexual selection on females in some of these groups showing nuptial feeding behaviour then perhaps there are sexually selected devices or ornaments in females?,” said Gwynne. Contrary to Darwin’s original idea, females can also exhibit elaborate features under the pressure of sexual selection. Female dance flies are a case in point. They have the equivalent of a “peacock’s tail.”
Gwynne’s original work revolved around role-reversed behaviour in katydids. Katydids, commonly known as “long horned grasshoppers” or “bush crickets” belong to the grasshopper family Tettigoniidae.
Katydid males secrete a large edible spermataphore resembling a cheesy glob, which females eat while the male inseminates them. Gwynne found that when food levels were high, males competed for access to females. But when food levels were low, the mating gift acquired a high value. Roles reversed and females competed for access to males.
Insect mating systems rarely involve parental care from the male. The male invests in the offspring by providing a mating “gift.” “There is a bewildering diversity of mating gifts: a menu that includes proteinaceous substances that are absorbed in the female’s genital tract to gifts “taken orally” by the females, such as enormous protein gifts attached to sperm packages, prey items collected by the male, to gifts or regurgitated nectar in some Australian wasps,” said Gwynne.
The only known example of females giving mating gifts to males is a water bug in Australia. Gwynne suggested that the reason for this exception might be, “that sexual selection on males is so strong that it has selected for attractive food gifts as a way to get mates.”
