Dance Flies: Bigger Abdominal Air Sacs Attract More Males

Dance flies (Rhamphomyia longicauda) have some interesting features. Females have expandable, air-filled sacs under their abdomen as well as hairy legs. A multinational research team has found that males are attracted to females with bigger abdominal air sacs. The flies belong to the family Empididae and are also known as balloon flies and as dagger flies. The latter name is derived from the piercing mouthparts of some species in the family.

A female Ramphomyia longicauda
Katja Shulz, CC BY 2.0 License

What Is a Dance Fly?

The term “dance flies” is sometimes used as a general term for insects in the family Empididae as well as those in the family Hybotidae. The name comes from the behaviour of the insects during their mating ritual. The flies form a swarm flying around in a dance-like motion. The swarms generally develop near sunset and can be enjoyable to watch. Rhamphomyia longicauda is found in the northeastern part of North America. It lives in wooded areas that contain wetlands or bodies of water and swarms near water.

Rhamphomyia longicauda

Rhamphomyia longicauda has a black upper surface. The head is small but has large orange or red eyes. The abdominal air sac of the female is also orange. The insect has a slender body with a long abdomen. The species name of the insect is derived from two Latin words meaning long and tail.

The females have two interesting features that differentiate them from the males. One is their abdominal air sacs. The other is the presence of stiff hairs (technically pinnate scales) forming a fringe on their legs. The fringe is most noticeable on the middle and hind legs. Though the hairs have an advantage, as described below, according to the BugGuide site they make it more likely that the female will become trapped in spider webs.

Swarming and Mating

Researchers from the University of Toronto in Canada and the University of Stirling in Scotland have made some intriguing discoveries about the mating swarm of the species. The females swallow air to fill their abdominal sac just before they take flight. Once they have taken to the air, they pull their legs in and up so that they one leg is folded on each side of the abdomen. This adds to the impression that the females have a huge abdomen relative to their body size. One researcher says that the behaviour makes a female look like a helicopter.

When a male and female have made the decision to mate, they leave the swarm. The researchers have observed that the males prefer females with bigger air sacs. Though we can’t tell what an insect is thinking, researchers believe that the male “thinks” that the females who are faking the appearance of a bigger abdomen contain a larger group of mature eggs that are ready to be fertilized. The female’s behaviour is actually a form of subterfuge. The researchers have also discovered that if two females have abdominal air sacs of the same size, a male choses the one with the hairier legs. The protruding scales that form the hairs likely increase the impression that the female has a large abdomen.

An Exchange of Gifts

The male flies and the larvae feed on smaller insects. The adult females don’t catch their own prey but depend on food brought to them by males. A male gives a female a bit of dead insect prey in exchange for copulation. The female mates with multiple males and gets multiple meals, enabling her to survive and produce offspring.

Some people may not think much about flies, except when the insects become a nuisance. They are interesting animals, though, especially some species, such as dance flies. I think they are worth studying.

References

Male dance flies favour females with bigger abdomens from the phys.org news service

Rhamphomyia longicauda facts from BugGuide