Macro-micro synergistic mechanisms of Curculio rostrum against buckling

Slender biological structures, which are widely observed in nature, exhibit exceptional properties and biological functions. A notable example is the Curculio rostrum that can drill into fruits for feeding and egg-laying without buckling. However, it remains unknown how the slender and curved Curculio rostrum resists buckling. In this article, we propose an elastic rod model for the rostrum that has multi-layered microstructures reinforced by helical chitin fibres. Its buckling behaviour is investigated through the combination of theoretical analysis and numerical simulations. The critical buckling load is derived and verified by experiments. It is found that despite the coupled twisting deformation induced by the helical microstructure, the originally curved rostrum exhibits a two-dimensional buckling mode. The critical buckling load for this two-dimensional buckling is much higher than for three-dimensional buckling, owing to the relatively large stiffness of the outermost layer. The macroscopic straight-curved shape helps the rostra to achieve similar buckling resistance capability among individuals, thereby enabling them to sustain the same feeding habits. Our results reveal that the macroscopic shape and helical microstructure synergistically contribute to the buckling resistance of Curculio rostra. This work could also inspire the bionic design of high-performance slender structures and soft robots.