A functional comparison of swimming behavior in two temperate forest ants (Camponotus pennsylvanicus and Formica subsericea) (Hymenoptera: Formicidae)

Abstract

Water is a dangerous and often lethal obstacle for small terrestrial animals like insects. However, some ants survive this hazard by efficiently traversing the water surface. Swimming performance (velocity, acceleration, and efficiency) differs dramatically among ant species, yet the factors that affect performance remain unclear. Here, we investigate the relative importance of behavior and morphology to swimming performance using a comparative study of two temperate forest ant species having superficially similar morphology: Camponotus pennsylvanicus (De Geer, 1773) (Hymenoptera: Formicidae) and Formica subsericea (Say, 1836) (Hymenoptera: Formicidae). We also investigated how water surface tension and ant morphology enable some F. subsericea workers to walk across the water surface, whereas others swim partially submerged. Leg ablation experiments demonstrated that both species use their forelegs for propulsion and hind legs as stabilizers, whereas their midlegs effect both propulsion and stabilization during swimming. C. pennsylvanicus workers swam faster than F. subsericea workers, probably reflecting the larger body size and longer relative foreleg length of C. pennsylvanicus workers. F. subsericea workers that walked on water were somewhat smaller than swimming conspecifics, and no workers were able to walk on water when surface tension was reduced with ethanol. Collectively, these results and those of related studies suggest that, within a clade of ants, differences in swimming performance arise mainly from subtle morphological differences. The importance of ant ecology and cuticular chemistry in this context remains to be explored.