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: An exoskeleton, in contrast to an endoskeleton, is an external anatomical feature that supports and protects an animals body. ... Orders See taxonomy Insects are invertebrates that are taxonomically referred to as the class Insecta. ... Subphyla and Classes Subphylum Trilobitomorpha Trilobita - trilobites (extinct) Subphylum Chelicerata Arachnida - spiders,scorpions, etc. ...

• 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.