Chitin is non living and non-cellular and is secreted by the underlying epidermis. It is a protein-carbohydrate compound. It is composed of several different substances. It is really made up of an outer waxy layer, a middle horny layer and an inner flexible layer. These layers are modified among the various arthropods and also among the different parts of the body. In some, the chitin is soft and permeable (aquatic insects) while in others it forms a veritable coat of armour (terrestrial insects). Between joints and between segments, it is flexible and thin to permit free movements. It is harder in crustaceans where it is infiltrated with calcium salts. In general its structure is admirably adapted for protection of delicate internal organs, for attachment of muscles, for serving as levers and centres of movement, and for preventing the entrance and loss of water.
Although hard protective structures are common among other groups of animals none have used them so effectively as the arthropods. Chitin in arthropods is used as biting jaws, for grinding in the stomach, as lenses of the eyes, for sound production (e.g. grasshoppers), as sensory organs (antennae and antennules), for copulatory organs, organs of defence, and for ornamental purposes.
A chitinous exoskeleton limits the size and weight of arthropods. In order to increase in size or grow it has to shed its outer shell at intervals and grows larger. This process is called ecdysis or moulting. The presence of chitin helps arthropods to adapt to different variety of terrestrial habitats.