In arthropods, the integuement, or external "skin", consists of a single layer of epithelium ectoderm from which arises the cuticle, an outer covering of chitin the rigidity of which varies as per its chemical composition.
The cuticle is made up of two layers; the epicuticle, which is a thin, waxy water-resistant outer layer containing no chitin, and a layer beneath it, the procuticle. This is chitinous and much thicker than the epicuticle, and consists of two layers, the outer exocuticle and the inner endocuticle. The tough and flexible endocuticle is built from numerous layers of interwoven fibrous chitin and proteins, while the exocuticle is more rigid and may be sclerotized. The exocuticle is greatly reduced in many soft-bodied insects, especially the larval stages (e.g., caterpillars).
There are four principal regions of an insect body segment: tergum or dorsal, sternum or ventral and the two pleura or lateral. Hardened plates in the exoskeleton are called sclerites. Sclerites that are subdivisions of the major regions are tergites, sternites and pleurites, for the respective regions tergum, sternum and pleuron.
Chemically, chitin is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose. In its unmodified form, chitin is translucent, pliable, resilient and quite tough. In arthropods, however, it is often modified, becoming embedded in a hardened proteinaceous matrix, which forms much of the exoskeleton. In its pure form it is leathery, but when encrusted in calcium carbonate it becomes much harder. The difference between the unmodified and modified forms can be seen by comparing the body wall of a caterpillar (unmodified) to a beetle (modified).
From the embryonic stages itself, the integument, which consists of a layer of columnar or cuboidal epithelial cells give rise to the external cuticle and an internal basement membrane, referred to as endocuticle. The cuticle provides muscular support and acts as a protective shield as the insect develops. However since it cannot expand with the insect, the external sclerotised part of the cuticle is periodically shed in a process called "moulting". As the time for moulting approaches, most of the exocuticle material is reabsorbed. In moulting, first the old cuticle separates from the epidermis (apolysis). Enzymatic moulting fluid is released in between the old cuticle and epidermis which separates the exocuticle by digesting the endocuticle and sequestering its material for the new cuticle. When the new cuticle has formed sufficiently, the epicuticle and reduced exocuticle are shed in ecdysis.:16–20
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