exoskeleton

exoskeleton (cuticle) The horny skeleton that encloses the body of all Arthropoda. It is secreted by an underlying cellular layer (the hypodermis). The structure is complex. Immediately above the epidermis is the Schmidt layer; above that an endocuticle of chitin; and above that an exocuticle of tanned chitin which is frequently dark in colour. There is a thin epicuticle (the outermost layer) covered with wax or grease. Internal to the epicuticle is the procuticle, comprising an outer exocuticle and an inner endocuticle. Both are composed of a complex glycoprotein formed from the binding of chitin with protein; the molecular structure of the exocuticle is further stabilized by phenolic tanning and additional cross-linkages. The precise nature of these layers–whether they are tanned, lamellar, etc.–varies from species to species, and also from place to place in the exoskeleton of an individual insect, according to its species. In Crustacea, the procuticle is also impregnated with mineral salts, calcium carbonate, and calcium phosphate. Cuticle is relatively impermeable to water and has a high strength-to-weight ratio, two characteristics important for the successful conquest of the land by the arthropods. Ingrowths of the cuticle, called ‘apodemes’, provide insertion sites for muscles. The division of the cuticle into separate plates facilitates movement: the plates are connected by the so-called arthrodial membrane, which comprises thin, flexible, untanned cuticle. Periodic moulting of the cuticle (ecdysis) permits body growth.