Aerial contests, sexual selection and flight morphology in solitary pompilid wasps

Aerial contest competition has proven to be a challenging phenomenon to interpret in many territorial insects. Because the duels often consist of elaborate and/or high speed ascending maneuvers, the hypothesis that they are settled due to asymmetries in flight performance is intuitively appealing. We evaluated this hypothesis by contrasting differences in known morphological determinants of flight performance between (1) residents vs. non-residents of the territorial wasp, Hemipepsis ustulata and between (2) H. ustulata vs. a non-territorial relative, Pepsis thisbe. In the first contrast, resident male H. ustulata were seen to be larger, and had a tendency for reduced wing loading, but they did not possess greater flight musculature or wing aspect ratios (i.e., more elongated wings) than their non-resident counterparts. In the second contrast, male H. ustulata exhibited clearly greater flight musculature and greater sexual dimorphism in this parameter (males more muscular), and also exhibited a slight tendency for greater wing loading and smaller aspect ratios than males of the patrolling species P. thisbe. Interestingly, although size is linked with territorial success in H. ustulata, males of this species were not larger than male P. thisbe, nor did the former species exhibit greater sexual size dimorphism. These results do not support the hypothesis that the repeated ascending contests of H. ustulata require, and select for, a high acceleration design. However, the observed intraspecific patterns of flight musculature suggest that high acceleration is favored in males of the perching species, perhaps for the ability to intercept passing receptive females.