Changes in wing morphology rather than wingbeat kinematics enabled evolutionary miniaturization of hoverflies

Due to physical scaling laws, size greatly affects animal locomotor ability and performance. Whether morphological and kinematic traits always jointly respond to size variation is however poorly known. Here, we examine the relative importance of morphological and kinematic changes in mitigating the consequence of size on aerodynamic force production for weight support in flying insects, focusing on hovering flight of hoverflies (Syrphidae). We compared the flight biomechanics, aerodynamics, and morphology of eight hoverfly species varying from 5 to 100 mg. Our study reveals no effect of body size on wingbeat kinematics among species, suggesting that morphological rather than kinematic changes may compensate for the reduction in weight support associated with an isometric reduction in size. Computational fluid dynamics simulations confirmed that variations in wing morphology, and not kinematics, allow species of different sizes to generate weight support. We specifically show that relatively larger wings and aerodynamically more effective wing shape have evolved in smaller hoverflies, mitigating the reduction in aerodynamic weight support with decreasing size. Altogether, these results suggest that hovering flight of hoverflies underpins highly specialised wingbeat kinematics, which have been conserved throughout evolution; instead, wing morphological adaptations have enabled the evolutionary miniaturisation of hoverflies.