Structure and tensile properties of the forewing costal vein of the honeybee Apis mellifera†
In this study, we investigated the morphological features and tensile properties of the forewing costal vein of the honeybee (Apis mellifera) under fresh, dry and in vitro-time varied conditions. The costal vein is composed of an outer sub-vein and an inner vein starting from the wing base to nearly 50% of the wing span and then they are fused into one vein extending to the wing tip. Confocal laser scanning microscopy revealed that the outer sub-vein with red autofluorescence is stiffer than the inner one with green autofluorescence, and the membrane in the gap between the sub-veins exhibited a long blue-autofluorescence resilin stripe. Considering the irregular cross-sectional shape of the costal vein, cross-sections of the tested specimens after tensile failure were analysed using scanning electron microscopy, to precisely calculate their cross-sectional areas by a customized MATLAB program. The Young's modulus and tensile strength of fresh specimens were ∼4.78 GPa and ∼119.84 MPa, which are lower than those of dry specimens (∼9.08 GPa and ∼154.45 MPa). However, the tensile strain had the opposite relationship (fresh: ∼0.031, dry: ∼0.018). Thus, specimen desiccation results in increasing stiffness and brittleness. The morphological features and material properties of the costal vein taken together represent a tradeoff between both deformability and stiffness. Our study provides guidance for material selection and bionic design of the technical wings of flapping micro aerial vehicles.