tergum; if the tergum bears any sclerites, those are called tergites. The ventral region is called the sternum, which commonly bears sternites. The two lateral regions are called the pleura (singular pleurum) and any sclerites they bear are called pleurites. Within entomology, the term glabrous is used to refer to those parts of an insect's body lacking in setae (bristles) or scales. Chemical composition Chemically, chitin is a long-chain polymer of a N-acetylglucosamine, which is a derivative of glucose. The polymer bonds between the glucose units are β(1→4) links, the same as in cellulose. In its unmodified form, chitin is translucent, pliable, resilient and tough. In arthropods and other organisms however, it generally is a component of a complex matrix of materials. It practically always is associated with protein molecules that often are in a more or less sclerotised state, stiffened or hardened by cross-linking and by linkage to other molecules in the matrix. In some groups of animals, most conspicuously the Crustacea, the matrix is greatly enriched with, or even dominated by, hard minerals, usually calcite or similar carbonates that form much of the exoskeleton. In some organisms the mineral content may exceed 95%. The role of the chitin and proteins in such structures is more than just holding the crystals together; the crystal structure itself is so affected as to prevent the propagation of cracks under stress, leading to remarkable strength. The process of formation of such mineral-rich matrices is called biomineralization. The difference between the unmodified and modified forms of chitinous arthropodan exoskeletons can be seen by comparing the body wall of say a bee larva, in which modification is minimal, to any armoured species of beetle, or the fangs of a spider. In both those examples there is heavy modification by sclerotisation. Again, contrasting strongly with both unmodified organic material such as largely pure chitin, and with sclerotised chitin and proteins, consider the integument of a heavily armoured crab, in which there is a very high degree of modification by biomineralization. Moulting As mentioned, the integument, and in particular the cuticle, is the product of the single layer of columnar or cuboidal epithelial cells attached to the basement membrane. The cuticle provides muscular support and acts as a protective shield as the insect develops, but it is not in itself cellular, so once established it cannot grow and offers little scope for maintenance, renewal or increase