How does a slender tibia resist buckling? The effect of material, structural and geometric characteristics on the buckling behaviour of the hindleg tibia in the postembryonic development of the stick insectCarausius morosus

During their lifespan, the long and narrow tibiae of the stick insect Carausius morosus (Sinéty, 1901) experience substantial compressive loads. The mechanical load on the tibiae increases as the weight of the insect rises. The increase in the body weight is accompanied by a notable increase in the insect's body size and, accordingly, by an increase in the length of the tibiae. These changes can both raise the risk of buckling of the tibiae. In this study, we track changes in the material and geometric properties of the hindleg tibia of C. morosus during growth. The results show that although buckling (either by Euler buckling or local buckling) is the dominant failure mode under compression, the tibia is well capable of maintaining its buckling resistance in each postembryonic developmental stage. This is found to be essentially the result of a compromise between the increasing slenderness of the tibia and its increasing material stiffness. The use of an optimal radius to thickness ratio, a soft resilin-dominated core, and chitin fibres oriented in both longitudinal and circumferential directions are presumably additional strategies preventing buckling of the tibia. This study, providing the first quantitative data on changes in the biomechanical properties of cuticle during the entire life of an insect, is expected to shed more light on the structure-property-function relationship in this complex biological composite.