A tough, insoluble polysaccharide (carbohydrate; as starch, inulin, or cellulose) that is the main constituent of the arthropod exoskeleton which is also found in the cell walls of most fungi.

The outer covering of arthropods, the cuticle, is impregnated in its outer layers with chitin, which makes the exoskeleton more rigid.

It is associated with protein to give a uniquely tough yet flexible and light skeleton, which also has the advantage of being waterproof.

The chitinous plates are thinner for bending and flexibility or thicker for stiffness as required. The plates can not grow once they are laid down and are broken down at each molt.