On the Indentation Hardness of Insect Cuticle

Abstract

The indentation hardness of locust cuticle was measured using a Vickers diamond applied to the surface of the cuticle. A number of areas of cuticle showed similar hardness values, approximately 24 kg mm−2, for the dehydrated cuticle but the mandibles (29.8 kg mm−2) and the dorsal mesothorax (33.3 kg mm−2) were harder. The mandibles have localized areas of hardness which are related to their function. It is suggested that the hardness of the dorsal thorax may be related to its function as an energy storing structure in flight. The measured hardness of cuticle depends on the hydration of the material but relative differences between cuticles from different parts of the locust persist whether tested fresh or after dehydration. Hardness is correlated with the dihydroxyphenol content of cuticle (unlike stiffness), conversely it is not correlated with the hydrophobicity of the matrix proteins (unlike stiffness). It is emphasized that hardness and stiffness are very different mechanical properties.