A typical arthropod exoskeleton is a multi-layered structure with four functional regions: epicuticle, procuticle, epidermis, and basement membrane. Of these, the epicuticle is a multi-layered external barrier that, especially in terrestrial arthropods, acts as a barrier against desiccation. The strength of the exoskeleton is provided by the underlying procuticle, which is in turn secreted by the epidermis. Arthropod cuticle is a biological composite material, consisting of two main portions: fibrous chains of alpha-chitin within a matrix of silk-like and globular proteins, of which the most well-known is the rubbery protein called resilin. Although the cuticle is relatively soft when first secreted, it soon hardens in a poorly understood process that involves sclerotization and/or tanning mediated by hydrophobic chemicals called phenolics. Different types of interaction between the proteins and chitin leads to varying mechanical properties of the exoskeleton.
In addition to the chitino-proteinaceous composite of the cuticle, many crustaceans, some myriapods and the extinct trilobits further impregnate the cuticle with mineral salts, above all calcium carbonate, which can make up up to 40% of the cuticle. This can lead to great mechanical strength.
In addition to the chitino-proteinaceous composite of the cuticle, many crustaceans, some myriapods and the extinct trilobits further impregnate the cuticle with mineral salts, above all calcium carbonate, which can make up up to 40% of the cuticle. This can lead to great mechanical strength.