Biology and physics of locust flight. V. Strength and elasticity of locust cuticle

Abstract

Elastic deformations of the cuticle play a major role in the energetics of flying locusts but the literature provides no relevant information about the elastic properties of any arthropod cuticle. The results are therefore discussed both in relation to locust flight and in relation to strength and elasticity of organic materials in general. InSchistocerca gregariaForskal there are two types of elastic cuticle, ordinarysolid cuticleandrubber-like cuticle. The characteristic material in the latter type is a newly discovered protein rubber,resilin. Samples of both were studied under static and dynamic conditions. The tensile properties of solid cuticle from various parts of the body (hind tibia, pleural wall, forewing) are similar to those of oak wood and of synthetic resins reinforced with cellulose; the static coefficient of elasticity (dcr/de) is 800 to 1000 kg/mm²and the tensile strength 8 to 10 kg/mm², corresponding to an ultimate extension of 2 to 3 %. At moderate loads, the tensile and compressive moduli are of equal magnitude, but it is argued that the effect of tanning (hardening) is to increase the compressive strength and modulus rather than the tensile properties. Static loading results in lasting deformation. The dynamic modulus is of the same magnitude as the static modulus (forewing), at least up to 5 kg mm^-2s^-1and, provided the tension does not exceed 0-5 kg/mm², the loss factor is less than 0.1. The rubber-like sample (prealar arm) consists of parallel lamellae of chitin (0.2μ thick) glued together by sheets of resilin (about 3μ thick). It behaves like a solid when extended in the direction of the lamellae but otherwise like a rubber, the elastic modulus being 0.2 kg/mm². The swelling pressure of resilin does not play any direct role but swelling alters the geometry and, to a small extent, the elastic modulus. It is suggested that the animal controls the stiffness of its rubber-like structures by altering the swelling equilibrium chemically which, in a model experiment, is done by blocking the free amino groups. Rubber-like cuticle does not encounter any permanent deformation which is attributed to the known lack of flow of pure resilin. Within the biological rate of deformation (up to 6 unit lengths per second), the dynamic stiffness remains within 4 % of the static value and the loss factor is only 0.03, i.e. less than for other natural or synthetic rubbers. A three-component model of arthropod cuticle is suggested. It accounts for the enormous differences in mechanical properties between adjacent parts and also for the fact that strict structural and developmental continuity is observed between the parts. It has three components: (1) crystalline chitin, (2) a rubber-like protein which may act as a deformable matrix and which entraps, (3) water-soluble proteins which can undergo proper tanning.