Effect of mechanical stress on insect exoskeleton

Abstract

A typical feature of many biological materials is their ability to adapt to mechanical load. Bone remodeling for example is stimulated by deformation, trees can react to increased load by formation of compression or tension wood. This ability allows for efficient investment of building material, as it is only deposited when biomechanically needed. However, it is still not known whether cuticle exoskeletons of insects, one of the most common biological materials, also show the ability to remodel under increased mechanical load. To investigate the hypothesis, that the insect exoskeleton, as a biological material, reacts to long-term increased mechanical load, four studies were conducted:1) How to measure insect cuticle biomechanical properties? 2) How can cuticle morphology be visualized in high detail? 3) If and how does insect cuticle react to long term applied mechanical load? 4) If and how does insect exoskeleton react under mechanical and light induced stress? Results show direct experimental evidence that increased mechanical load affects the biomechanical properties of an insect exoskeleton for the first time. The exo- and endocuticle were visualized using a novel staining method for high-resolution X-ray microtomography. Comprehensive biomechanical measurements show that up to 3g load the Young’s modulus and bending strength of cuticle increase. Higher gravitational loads, however, decreased insect survival rate and body mass and endocuticle thickness. These findings are not only a starting point for fundamental questions regarding the proximate mechanisms behind this ability of cuticle exoskeletons; however, also add important context to the discussion on general ultimate factors in the evolution of adaptive biological materials.