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The procuticle is the major portion of the exoskeleton of an insect (and various other arthropods); its exact composition and structure may differ somewhat among different taxa, but certain aspects can be generalized:
- When first secreted by the epidermis, it is soft, pliable, and pale, as much of the chemical composition is chitin.
- The chitin molecules are intertwined to form microfibrils, and the microfibrils are often laid down in parallel to form layers ("lamina").
- The microfibrils are embedded in a protein matrix, and it is the chemical process of cross-linking between these protein chains (called sclerotization) that causes hardening and darkening of the outer portion of the procuticle; this sclerotized layer is called the exocuticle to differentiate it from the unsclerotized portion below it, or endocuticle.
- When sclerotization is complete, the regions of exocuticle form rigid sclerites, and are separated by membranous areas of endocuticle which retain their flexibility.
- The exocuticle cannot be digested during the molting cycle (see ecdysis), but the endocuticle can be resorbed, so only the exocuticle and epicuticle are shed.
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