Among the most important substances in exoskeleton is chitin. Chitin is a protein made up of long molecules bound together in bundles giving exoskeleton the unique combination of strength and flexibility. Changes in the nature of the proteins binding the chitin molecules determine the final exoskeleton structure. Where the protein binding the chitin is not sclerotized, the chitin is more flexible and paler. This is why newly moulted insects are often pale and soft.
Chitin is similar in structure to plant cellulose, but is much tougher.
In crustacea , calcium carbonate is incorporated into the exoskeleton for an even tougher armour. But such armour is heavier and thus found mostly in aquatic arthropods where buoyancy helps ease the load. The exoskeleton provides a waterproof layer to prevent water loss. Most can adjust their breathing holes in the exoskeleton to minimise water loss. Many also secrete an additional layer of wax to further prevent water loss.