Sexual Size Dimorphism and Flight Behavior in Cicada Killers, Sphecius speciosus

Abstract

I examined the size and mass allocation patterns of the two sexes of cicada killers (Sphecius speciosus) in relation to the demands of their flight behavior. Females were nearly 2.5 times the size of males, but there was no difference in the ratio of flight muscle mass to body mass (FMR) between the sexes. Female wasps captured and carried cicadas that were on average 88% heavier than their own body mass, and nearly all were heavier than they should have been able to take off with. Loading of foraging females resulted in a three-fold reduction in FMR as compared to the unloaded state, lower even than the marginal flight muscle ratio for Hymenoptera. Cicadas procured by female cicada killers were not significantly different in body mass or sex ratio from those collected at random. Cicadas had flight muscle ratios that were essentially the same as that of female cicada killers. Females lack sufficient force production to take off with an average cicada, but compensate behaviorally by repeatedly climbing trees and descending under power in the direction of their burrows. Cicada killers of both sexes have relatively high FMRs. Females may have the capacity to capture cicadas in flight. For males high FMR may be important in territorial defense and pursuit of females. Males can be produced at substantial energetic savings to the parent female, and still retain high maneuverability.