The allometry of flight in hoverflies

Due to physical scaling laws, size greatly affects animal locomotor performance. Natural selection may favour scaling relationships between traits and size that improve performance in crucial behaviours such as foraging, mating or escaping from predators. To understand the evolution of scaling relationships, it is essential to study how both morphology and locomotion scale with size. Here, we examine the relative importance of morphological and kinematic changes in mitigating the consequence of size on aerodynamics force production in flying insects, focusing on hovering flight of hoverflies (Syrphidae). We compared the flight biomechanics and aerodynamics of eight hoverfly species varying 20-fold in body masses (from 5 to 100 mg). We used stereoscopic high-speed videography to quantify wingbeat kinematics, we characterized wing morphology with high-resolution photography, and modelled the aerodynamics using Computational Fluid Dynamics (CFD). Our study reveals no effect of body size on wingbeat kinematics among the eight species, suggesting that morphological changes rather than kinematic changes may compensate for weight support. Our CFD simulations confirm that the variations in wing morphology, and not kinematics, allow species of different sized to generate weight support. Based on these results, we show smaller hoverflies have disproportionately larger wingspan, wing chord and non-dimensional second moments of area to mitigate the reduction in weight support with decreasing size. Altogether, these results suggest that the hovering flight of hoverflies underpins highly specialised wingbeat kinematics, which have been conserved throughout evolution, and instead wing morphology has changed throughout the apparent evolutionary miniaturization of hoverflies.