Exuviae in the context of "Exoskeleton"

Ecdysis is the moulting of the cuticle in many invertebrates of the clade Ecdysozoa. Since the cuticle of these animals typically forms a largely inelastic exoskeleton, it is shed during growth and a new, larger covering is formed. The remnants of the old, empty exoskeleton are called exuviae.

After moulting, an arthropod is described as teneral, a callow; it is "fresh", pale and soft-bodied. Within one or two hours, the cuticle hardens and darkens following a tanning process analogous to the production of leather. During this short phase the animal expands, since growth is otherwise constrained by the rigidity of the exoskeleton. Growth of the limbs and other parts normally covered by the hard exoskeleton is achieved by transfer of body fluids from soft parts before the new skin hardens. A spider with a small abdomen may be undernourished but more probably has recently undergone ecdysis. Some arthropods, especially large insects with tracheal respiration, expand their new exoskeleton by swallowing or otherwise taking in air. The maturation of the structure and colouration of the new exoskeleton might take days or weeks in a long-lived insect; this can make it difficult to identify an individual if it has recently undergone ecdysis.

Sclerotin is a component of the cuticle of various Arthropoda, most familiarly insects. It is formed by cross-linking members of particular classes of protein molecules, a biochemical process called sclerotization, a form of tanning in which quinones are enzymatically introduced into the cuticle, and react with terminal and lysine-related amino groups in the proteins to form strong links between the molecules. The resulting material greatly increases the rigidity of an insect's chitinous exoskeleton, which is otherwise fairly soft. It is particularly prominent in the thicker, armoured parts of insect and arachnid integument, such as in the biting mouthparts and sclerites of scorpions and beetles.

As it matures, freshly formed sclerotin becomes a hard, horn-like substance with a range of yellow-brown colors. As animals adapted to life on land, increasingly diverse needs for organic stiffening components arose (as opposed to mineral stiffening components such as calcium carbonates and phosphates). Among the invertebrates, this need was met largely by the development of sclerotins and other cross-linked proteins that allowed insects to adapt to existence on the land and later to develop wings.