When a person says dance fly, most people would likely think of a dancing housefly. However, there is actually such an insect that goes by this name, though it is very different from the common housefly. Dance flies, also called balloon flies or dagger flies, are predatory Dipterans that are members of the family Empididae. Empids are characterized by their long, pointy mouth parts and their extravagant mating rituals. If a fly can be charming, dance flies are certainly the ones that fit the bill. During the mating time, swarms of male dance flies will gather together and perform for nearby females while carrying delicate, specially crafted silk balloons. These balloons are called nuptial gifts and a successful male will pass his balloon to his partner during mating and they will often tumble to the ground together.
Nuptial gifts in the form of silk balloons are unique to this family. Often, these balloons will contain a protein rich meal for the female to consume whilst mating. Typically, the larger the prey item, the more time the male gets to mate and the more sperm he can deliver. Other insects such as scorpionflies show this behavior as well, but they do not construct balloons to carry the prey items as dance flies do.
Jennifer Sadowski and her team (1998) have shown that an interesting phenomenon has evolved in certain species of dance flies, more specifically, Empis snoddyi. What they’ve learned is that males of this species no longer offer prey to their female mates, but just the silken balloon, which has no nutritional value and serves no purpose for the female other than to hold during copulation. This is called an “empty” nuptial gift. Although females will accept gifts of no direct or indirect benefit to themselves, they can still be choosy about who they take as their partners. This study found that there is an inverse correlation between balloon size and body size within successful males. It appears that female choice over generations has selected for a smaller balloon size and bigger male body size. Sadowski et al. noticed that the most successful males weren’t necessarily the ones with the smallest balloons, though. They observed that males with large body sizes and intermediate sized balloons took the most mates. There are a number of reasons why this may be. When sexual selection favors female choice, there are often signs that females look for in a mate that are indicative of their health and good genetics. In dance flies, females will chase their chosen male out of the swarm before deciding to mate with him. This could possibly be an assessment of his flying ability and fitness. It makes sense, then, that males with larger balloons would have a more difficult time performing at the top of their game. The bigger a balloon is, the more cumbersome it might be and could hinder a male’s flight displays. On the other end of the spectrum, a female requires a gift to mate and if a balloon is so small that it is not readily visible to her, she may ignore the male carrying it. As we can see from these animals, the courtship and mating process of insects can be complex, elaborate, and elegant. It isn’t just the more charismatic animals like mammals that can show fascinating sexual behavior. Next time you go outside and notice the insects around you, consider the intricacies of their intimate behaviors and marvel at the great variation of life on this planet.
1. Sadowski, Jennifer A., Allen J. Moore, and Edmund D. Brodie III. “The evolution of empty nuptial gifts in a dance fly, Empis snoddyi (Diptera: Empididae): bigger isn’t always better.” Behavioral ecology and sociobiology 45.3-4 (1999): 161-166.
2. Cumming, Jeffery M. “Sexual selection and the evolution of dance fly mating systems (Diptera: Empididae; Empidinae).” The Canadian Entomologist 126.03 (1994): 907-920.