Insect wing flexibility improves the aerodynamic performance of small revolving wings

Insect wings are flexible, elastically deforming under loads experienced during flapping. The adaptive value of this flexibility was tested using a revolving wing set-up. We show that the wing flexibility of the beetle <i>Batocera rufomaculata</i> suppresses the reduction in lift coefficient that is expected to occur with a reduction of wing size compared to rigid propeller blades. Moreover, the scaling of wing flexibility with size is intra-specifically tuned through changes in wing-vein cross-section, resulting in smaller wings achieving proportionally larger chordwise deformations compared to larger wings, when loaded with aerodynamic forces. These elastic deformations control the separation of flow from the wing as a function of angle-of-attack, as evidenced by the turbulence activity in the flow field directly beneath the revolving wings. The study underlines the contribution of flexibility to control the flow over insect wings through passive wing deformations without the need for input or feedback from the nervous system.