The evolution of empty nuptial gifts in a dance fly, Empis snoddyi (Diptera : Empididae): bigger isn't always better

Adaptive female choice is thought to have led to the evolution of nutrition ally valuable nuptial gifts in many insect species. However, in several dan ce fly species, males offer and females accept "empty gifts" with no nutrit ional value. In the species studied here, Empis snoddyi Steyskal, males pro duce empty balloons comprised of hundreds of silk bubbles and form mating s warms that females approach to investigate males. Males within the swarm en gage in agonistic interactions. The empty balloon has been hypothesized to be an indicator of male condition such that males with larger balloons are predicted to have higher mating success and be more successful in male-male interactions than males with smaller balloons. We examined the role of mal e body size and balloon size in the context of intersexual and intrasexual selection. We found that neither male body size nor balloon size affected t he outcome of pairwise male-male interactions. Using multiple-regression te chniques, we found significant linear selection for increasing male body si ze and decreasing balloon size associated with mating success, a surprising result given a positive relationship between male body size and balloon si ze. A visualization of selection showed the highest peak of male mating suc cess for larger males with intermediate-size balloons. These results can be explained by a trade-off between long-range attraction of females using la rge balloons and close-range attraction of females via improved flying effi ciency associated with smaller balloons. Both male body size and balloon si ze are important components in determining male mating success; however, th e empty balloon does not appear to play a typical role as a sexually select ed ornament.