The wings of insects are complex structures adapted to insect flight by millions of years of selection. The wings beat many times every second and must withstand the stress and deformation caused by the force on the wings. The structure of insect wings and their components are inspiration to engineers seeking to fabricate new materials with unique properties.
A group of scientists* have studied the structure and function of dragonfly wing veins through a combination of microscopy and modeling. The wing veins provide the wing with rigidity and resistance to deformation as the wing beats against the air. The wing vein is hollow in the center allowing fluid to be pumped through the wings and keep the cuticle hydrated. Anyone who has collected and curated insects knows that the wings of live insects are flexible and resistant to tearing; Desiccated wings of pinned insects become stiff, brittle and break if handled.
The wing cuticle including the veins are secreted by epidermal cells. In the membranous areas of the wings the cells withdraw and the upper and lower cuticle of the wing fuse. In the wing vein a layer of epidermal cells remains between the hollow vein interior and the cuticle. In dragonflies, the wing vein is not round, but oval, longer in the direction perpendicular to the wing and shorter in the plane of the wing. This shape gives the vein more resistance to force applied to the wing during beating.
Microscopy shows that the cuticle of the vein has at least 6 distinguishable layers. A waxy outer epicuticular layer under 1 micron thick coats outside. Three layers of exocuticle follow, with the outermost heavily sclerotized to provide much of the stiffness and rigidity. The endocuticle layer is thicker than the other layers combined and contains a lot o resilin, the flexible resilient protein found in insect joints. The layer separating the endocuticle from the epidermis is thin with a series of layers. Thus, the wing vein is not a solid uniform material but a composite of layers with very different properties.
Dragonflies beat their wings about 30 times a second. The force of air against the wing causes some degree of wing deformation on the down stroke. These wing deformations can be important to the aerodynamics of insect flight. A wing that deforms slightly on the downstroke must rapidly return to its initial state once the force is removed.
Modeling suggests two important contributions of the resilin-containing endocuticle. It contributes to the flexibility of the wing veins and the rapid return to its initial state after deformation. Without the thick resilin-containing endocuticular layer the wing returning from its deformed state would overshoot the desired position and oscillate around that position. The resilin accommodates the overshoot force by compression, thus damping the movement of the wing, allowing the wing to rapidly reset in position for the next down stroke.
*Rajabi H, Shafei A, Darvizeh A, Dirks J-H, Appel E, Gorb SN. 2016. Effect of microstructure on the mechanical and damping behaviour of dragonfly wing veins. R. Soc. open sci. 3: 160006.