Go thou to the ant: A comparative biomechanical analysis of locomotion in Hymenoptera (Hexapoda)

Abstract

Although ants are conceived of as paragons of social complexity, it may be their locomotory capacity that truly sets them apart from other Hymenoptera. Based on our comparative kinematic analysis of Formicidae for level, straight-line locomotion in a broad phylogenetic context, we observe that ants are distinctly capable runners. No sampled hymenopteran paralleled the body-scaled speed of ants. Relative stride lengths for ants were longer than other sampled taxa despite short ground contact durations relative to swing durations. With respect to spatial gait patterns, ants had relatively narrow hindleg and broad midleg step-widths on average, possibly enhancing speed and turning ability. Ants were able to extend their propulsive pair of legs, those of the metathorax, extremely far posterad relative to other sampled taxa, and had a distinct locomotory posture, with a high ground clearance and the femorotibial joints raised above their backs. Despite the unique modifications of their coxotrochanteral articulations, ant forelimbs were largely unremarkable with respect to our quantified variables. Sawflies, in contrast, had extremely wide and perhaps inefficient foreleg stances, and were observed for the first time to have what appears to be a dominant tetrapodal gait pattern, which raises unexpected questions about the early evolution of the Hymenoptera. Finally, we observed variability in attachment abilities and no consistent pattern of leg liftoff sequence across the sampled taxa. Our results establish locomotory evolution in the Hymenoptera as a functionally and structurally variable system with numerous directions of future research, particularly for phylogenetic comparison across wing-monomorphic and wing-polymorphic lineages. Summary statementThis work establishes a comparative phylogenetic approach to hymenopteran kinematics, demonstrating that ant locomotory capacity is derived and observes, unexpectedly, that the sampled sawflies (symphyta) never used a tripod gait.