Arthropods are covered with a tough, resilient
integument or
exoskeleton of
chitin. Generally the exoskeleton will have thickened areas in which the chitin is reinforced or stiffened by materials such as minerals or hardened proteins. This happens in parts of the body where there is a need for rigidity or elasticity. Typically the mineral crystals, mainly
calcium carbonate, are deposited among the chitin and protein molecules in a process called
biomineralization. The crystals and fibres interpenetrate and reinforce each other, the minerals supplying the hardness and resistance to compression, while the chitin supplies the tensile strength. Biomineralization occurs mainly in
crustaceans; in insects and
Arachnids the main reinforcing materials are various proteins hardened by linking the fibres in processes called
sclerotisation and the hardened proteins are called
sclerotin.