The tough polysaccharide that forms a major part of an insect exoskeleton is chitin.
Chitin is a long-chain polymer built from repeating units of N-acetylglucosamine, a nitrogen-containing derivative of glucose. In insects, it is woven into the procuticle with proteins; the resulting layered composite gives the outer body both strength and flexibility. A thin waxy epicuticle helps reduce water loss, but that outermost layer itself contains little or no chitin.
An exoskeleton cannot expand continuously as an insect grows, so the insect periodically molts. It separates from the old cuticle, forms a larger replacement underneath and then sheds the former covering. Fresh cuticle is often soft and pale before it hardens. Chitin is not unique to insects: it also occurs in crustacean shells and fungal cell walls. It is often compared with cellulose because both are structural polysaccharides, while keratin is a protein found in materials such as hair, feathers and nails.