COMBES, S.A.; DANIEL, T.L.: Dynamic bending in insect wings: origins and consequences of structural complexity

Insect wings are complex, flexible structures displaying both inhomogeneous and anisotropic mechanical properties. While several studies have addressed the complexity of insect wing structure from a qualitative standpoint, we lack an understanding of how wing structure affects dynamic wing bending, and thus aerodynamic force generation, during flight. Our measurements of spatial variation in wing flexural stiffness in the hawkmoth, Manduca sexta, show that stiffness declines exponentially in both the spanwise and chordwise directions. We also find a large structural anisotropy, with spanwise flexural stiffness approximately 100 times greater than chordwise stiffness in wings from a wide range of insect species. We use a finite element model (FEM) based on a Manduca wing to explore the origins of this anisotropy, and find that wing veins, particularly leading edge veins, generate significant anisotropy. We then use FEM to create dynamically flapping wings that mimic the inhomogeneity and anisotropy measured in real wings, and complement this modeling with measurements on actual Manduca wings flapped with similar kinematics. We find that an inhomogeneous, anisotropic wing develops strain (and thus wing curvature) in the distal regions of the wing when flapping, while a homogeneous wing does not. Surprisingly, a sustained pressure load equal to the weight of the animal distributed over the surface of the wing has little effect on the patterns of strain. This suggests that dynamic wing shape in Manduca sexta may be largely determined by the interaction between wing structure and the inertial mechanics of elastic waves, and less affected by fluid and other pressure loading on the wing.