Epicuticle

The outermost layer of a cuticle is called epicuticle; it forms a continuous layer covering the complete cuticular surface. Seldom more than 2 |lm thick, it is responsible for the waterproofing properties of the cuticle. Electron microscopy shows that the epicuticle can be subdivided into several layers, of which the inner epicuticle, also called the dense layer, is the thickest. It is covered by the thin, outer epicuticle, sometimes called the cuticulin layer, which is assumed to be responsible for the mechanical stiffness of the epicuticle. The inner and outer epicuticle are composed of polymerized lipids and protein, and they contain no chitin. These two layers remain poorly characterized because they are difficult to purify, dissolve, and degrade.

The outer epicuticle is covered by a waterproofing wax layer, containing complex mixtures of extractable lipids, secreted during the molting process from integumental oenocytes and epidermal cells. This layer is again covered by a protective cement layer, secreted immediately after ecdysis from glands in the integument.

The extractable lipids in the wax layer have been characterized for several insect species. They appear to be species-specific mixtures of a wide range of lipids, including normal and branched, saturated and unsaturated hydrocarbons, fatty acids, alcohols, esters, sterols, and aldehydes. Differences in lipid composition have been used to discern closely related insect species. The epicuticular lipid composition also can vary between instars and sex of the same species, and these lipids often play an essential role in recognition and communication between insects.

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