Singing cicada advertises a singles bar for flies

When a cicada sings, it doesn’t just attract its own species – for parasitic flies, a chirping cicada is a pick-up joint for potential mates, according to a new study.

Females of the parasitic fly Emblemasoma erro attach themselves to cicadas when it’s time to give birth so the maggots can feed off their host.

Previous studies have shown that female parasitic flies find cicada hosts by eavesdropping on their mating calls. What remains a mystery, however, is why these highly sensitive ears are found on male parasitic flies.

“Hearing is a multi-functional sense in insects,” says Brian J Stucky, a biologist at the Florida Museum of Natural History and author of the new paper. “[For E. erro] hearing may have originated as a means of finding a host but has become useful in another way as well.”

Stucky observed E. erro over three years at test sites in Kansas, US, luring them with a recording of the calls of their preferred host cicada, Neotibicen dorsatus. 

He trapped any flies that arrived in response to the call and determined their sex. Some females were tested to see if they carried any larvae.

According to the results, published in the Journal of Insect Science, more than 30% of the flies attracted by the calls were male. And some 30% of the females had no larvae onboard, suggesting many of the flies had another motive for sniffing out a host.

Stucky also observed the behaviour of the flies lured by the cicada song, and found that males tended to pursue one or more flies once they arrived at the site of the loudspeaker.

When another fly appeared in the male’s line of sight, regardless of its sex, the male exhibited behaviours associated with attempts at mating. Several of these interactions resulted in copulation.

The results, Stucky says, demonstrate the possibility that cicada calls lure in parasitic flies not just as hosting for larvae, but as a meeting place for males and females looking to mate.

“Travelling to calling cicadas is an expenditure of both time and energy, and it is difficult to imagine any other compensatory benefit to either males or nongravid [non-larvae-bearing] females besides finding mates,” the paper explains.

According to the paper, E. erro is the first acoustic parasitoid for which there is strong evidence that hearing plays a significant role in the lives of the adult males.

Stucky acknowledges the possibility that female flies express pheromones to attract males once at the site, but the fact that males don’t appear able to distinguish between males and females when choosing mates renders this unlikely.