Relationship between structure, material property and function in locust cuticle
Insect cuticle is one of the many biological composites. However, what makes it different from others is the wide range of its mechanical properties. The elastic modulus of insect cuticle, for example, covers a range of more than eight orders of magnitude. Why do cuticle properties vary so dramatically? With the aim to address this question, researchers have used a set of different testing methods to measure properties of cuticle specimens, which were selected from various body parts, across a variety of insect species and often preserved/prepared in different ways. However, almost all the factors mentioned earlier can influence obtained data. Hence, the literature data cannot be simply compared with each other, and no solid conclusion can be drawn regarding the biomechanical mechanisms that underlie the property variations in the cuticle. To fill this gap in the literature, this thesis focuses on two key questions. First, how do the mechanical properties of insect cuticle differ in a single species when all testing conditions are kept constant? Second, what are the biomechanical mechanisms behind the wide range of cuticle properties? Using a combination of scanning electron microscopy (SEM), micro-computed tomography (micro-CT), confocal laser scanning microscopy (CLSM) and nanoindentation, I performed one of the most comprehensive studies to date, and simultaneously investigate the microstructure, sclerotization and the elastic modulus of locust cuticle. I have shown that, in the desert locust Schistocerca gregaria, the elastic moduli of tibiae, femora and compound eyes range from 0.5 GPa to 8 GPa. This property change can be explained almost fully by the differences in the microstructure and sclerotization of the investigated specimens. My results suggest that, in most of the cases, sclerotization determines the difference between the elastic moduli of different body parts, whereas anisotropic properties of each body part are controlled by their microstructure, in particular fiber orientation. However, it is still possible that a cuticle specimen that is not as sclerotized as others shows a stiffness as high as more sclerotized specimens. This is in contradiction to the common understanding and is particularly observed in the hind femur, which consists of a cuticle that is less sclerotized than those of the fore and mid femora but can still reach a stiffness higher than others in certain directions. This, therefore, suggests that the interaction between the microstructure and sclerotization may not be as simple as previously thought. I expect that my results help to better understand the complicated structure-materialfunction relationship in insect cuticle.