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The Exoskeleton

An insect's exoskeleton (integument) serves not only as a protective covering over the body, but also as a surface for muscle attachment, a water-tight barrier against desiccation, and a sensory interface with the environment. It is a multi-layered structure with four functional regions: epicuticle, procuticle, epidermis, and basement membrane.

The epidermis is primarily a secretory tissue formed by a single layer of epithelial cells. It is responsible for producing at least part of the basement membrane as well as all of the overlying layers of cuticle. The basement membrane is a supportive bilayer of amorphous mucopolysaccharides (basal lamina) and collagen fibers (reticular layer). The membrane serves as a backing for the epidermal cells and effectively separates the hemocoel (insect's main body cavity) from the integument.

The procuticle lies immediately above the epidermis. It contains microfibers of chitin surrounded by a matrix of protein that varies in composition from insect to insect and even from place to place within the body of a single insect.

As the procuticle forms, it is laid down in thin lamellae with chitin microfibers oriented at a slightly different angle in each subsequent layer. In some parts of the body, procuticle stratifies into a hard, outer exocuticle and a soft, inner endocuticle.

Differentiation of exocuticle involves a chemical process (called sclerotization) that occurs shortly after each molt. During sclerotization, individual protein molecules are linked together by quinone compounds. These reactions "solidify" the protein matrix, creating rigid "plates" of exoskeleton known as sclerites. Quinone cross-linkages do not form in parts of the exoskeleton where resilin (an elastic protein) is present in high concentrations. These areas are membranes -- they remain soft and flexible because they never develop a well-differentiated exocuticle.

The epicuticle is the outermost part of the cuticle. Its function is to reduce water loss and block the invasion of foreign matter. The innermost layer of epicuticle is often called the cuticulin layer, a stratum composed of lipoproteins and chains of fatty acids embedded in a protein-polyphenol complex. An oriented monolayer of wax molecules lies just above the cuticulin layer; it serves as the chief barrier to movement of water into or out of the insect's body. In many insects a cement layer covers the wax and protects it from abrasion.

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