Differences in velocity between the sexes and stages of courtship in a dance fly

Abstract

Dance flies (Diptera: Empididae: Empidinae) are famous for their elaborate mating behaviour, which can be broken down to three distinct stages in adults: hunting, swarming, and mating. In many species, males hunt for prey that they present as nuptial gifts to recipient females in swarms, after which the pair will copulate in the air or after landing on nearby vegetation. Dance flies are also famous for the female sexual ornaments that occur in approximately a third of the species in the family, which challenges theories on the nature of ornament evolution. The ornamentation can take the form of feathery legs, inflated abdomen, or enlarged or darkened wings, and are mostly theorized to alter their silhouette to make them appear larger. The progress in studying mating systems in dance flies has been hampered by the fact that dance flies do not behave as expected in lab environments and by the speeds of natural swarms. Some prior hypotheses for sexual selection include scramble contest where the fastest individuals gain access to the opposite sex and mate choice where individual velocity is a preferred trait. The differences in velocity could also be due to load lifting constraints caused by the extra weight of the caught prey the males carry or as a response to predation avoidance or hunting success. In this thesis I use high speed video and tracking software to observe and quantify sexual differences in swarming behaviour including the flight paths and velocities of the sexes at different stages of courtship (hunting, swarming, and mating). I showcase the possibility of using high speed video when studying dance flies and I show that swarming males fly slightly faster than swarming females, and hunting males fly slightly slower than swarming females. The fact that hunting males are slower than the swarming participants is inconsistent with the hypothesis that the velocity differences are based on hunting success or predation avoidance, but wholly consistent with sexual selection.