On the fracture resistance of dragonfly wings.

The biological success of insects is attributed to evolution of their wings. Over 400 million years of evolution, insect wings have become one of the most complex and adaptive locomotor structures in the animal kingdom. Although seemingly fragile, they satisfactorily perform their intended function under millions of cycles of repeated stress without failure. However, mechanistic origins of wing resistance to failure remain largely unknown. Most of our understanding of biomechanics of insect wing and flight is based on computer simulations and laboratory experiments. While those studies are needed to reveal certain aspects of wing design, a full understanding can be achieved only by linking obtained data with results of studies in natural conditions. In this study, we tracked the initiation and progression of wing damage of dragonflies in their natural habitats. By quantifying wing area loss over the flight season, we aimed to find a link between the wing structure and accumulated damage. Our results showed that dragonfly wings are exceptionally damage tolerant. Even at the very end of the flight season, the mean wing area loss does not exceed 1.3% of the total wing area. Crack termination, deflection, bifurcation and bridging are the mechanisms that raise the resistance of wings to fracture. This study suggests that insect wings are adapted not only for flight efficiency, but also for damage tolerance. Hence, they should be studied not only from the perspective of aerodynamic performance, but also from that of fracture mechanics.