The forewing of a black cicada Cryptotympana atrata (Hemiptera, Homoptera: Cicadidae): Microscopic structures and mechanical properties.

Insects in nature flap their wings to generate lift force and driving torque to adjust their attitude and control stability. An insect wing is a biomaterial composed of flexible membranes and tough veins. In this paper, we study the microscopic structures and mechanical properties of the forewing of the black cicada, Cryptotympana atrata. The thickness of the wing membranes and the diameter of veins varied from the wing root to the tip. The thickness of the wing membranes ranged from 6.0 to 29.9 μm, and the diameter of the wing veins decreased in a gradient from the wing root to the tip, demonstrating that the forewing of the black cicada is a nonuniform biomaterial. The elastic modulus of the membrane near the wing root ranged from 4.45 to 5.03 GPa, which is comparable to that of some industrial membranes. The microstructure of the wing vein exhibited a hollow tubular structure with flocculent structure inside. The "fresh" sample stored more water than the "dry" sample, resulting in a significant difference in the elastic modulus between the fresh and dried veins. The different membrane thicknesses and elastic moduli of the wing veins near the root and tip resulted in varied degrees of deformation on both sides of the flexion line of the forewing during twisting. The measurements of the forewing of the cicada may serve as a guide for selecting airfoil materials for the bionic flapping-wing aircraft and promote the design and manufacture of more durable bionic wings in the future. RESEARCH HIGHLIGHTS: The distribution of the wing vein diameter and the wing membrane thickness indicated that the forewing of Cryptotympana atrata is composed of heterogeneous materials. The wing membrane and the outer wall of the wing vein are the layered structure with multilayer fibers, which has a great significance for improving the ability of the forewing to sustain aerodynamic loads. The elastic modulus of the wing membrane near the wing root is in the range of 4.45-5.03 GPa, which is comparable to that of membranes manufactured by industries. This is a suitable reference for selecting materials for making bionic aircraft wings. We proved that the elastic moduli of the "fresh" and "dry" wing veins differ greatly compared with those of the wing membrane. Because the wing vein microstructure exhibits an internal hollow tubular structure with flocculent structure inside, the "fresh" sample stores more water than the "dry" sample. The wing membrane near the wing root is thicker and reinforced by the main wing vein with a high elastic modulus. This renders the region near the wing root difficult to deform. The membrane far from the wing root is thinner and the elastic modulus of the nearby wing veins is smaller, making them more flexible.