Ecdysis (from Ancient Greek: ἐκδύω - ekduo - to take off, strip off) is the moulting of the cuticula in arthropods and related groups (Ecdysozoa). Since the cuticula of these animals is also the skeletal support (the exoskeleton) of the body and is inelastic, it is shed during growth and a new, larger covering is formed. The old, empty exoskeleton is called an exuvia (or "exuvium").
After moulting, an arthropod is described as teneral; it is "fresh", pale and soft-bodied. Within one or two hours, the cuticle hardens and darkens following a tanning process similar to that of the tanning of leather. It is during this short phase that the animal grows, since growth is otherwise constrained by the rigidity of the exoskeleton. Growth of the limbs and other parts normally covered by 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.
Ecdysis may also enable damaged tissue and missing limbs to be regenerated or substantially re-formed, although this may only be complete over a series of moults, the stump being a little larger with each moult until it is of normal, or near normal size again.
In preparation for ecdysis, the arthropod becomes inactive for a period of time, undergoing apolysis (separation of the old exoskeleton from the underlying epidermal cells). For most organisms, the resting period is a stage of preparation during which the secretion of fluid from the moulting glands of the epidermal layer and the loosening of the underpart of the cuticle occur.
Once the old cuticle has separated from the epidermis, the digesting fluid is secreted into the space in between them. However, this fluid remains inactive until the upper part of the new cuticle has been formed.
While the old cuticle is being digested, the new layer is secreted. All cuticular structures are shed at ecdysis, including the inner parts of the exoskeleton, which includes terminal linings of the alimentary tract and of the tracheae if they are present.
Then, by crawling movements, the pharate animal pushes forward in the old integumentary shell, which splits down the back allowing the animal to emerge. Often, this initial crack is caused by an increase in blood pressure within the body (in combination with movement), forcing an expansion across its exoskeleton, leading to an eventual crack that allows for certain organisms such as spiders to extricate themselves.
Moulting in insects
Each stage in the development of an insect between moults is called an instar, or stadium. Endopterygota tend to have few instars (4-5), while other insects such as Exopterygota can have anywhere up to 15. Endopterygota insects have more alternatives to moulting, such as expansion of the cuticle and collapse of air sacs to allow growth of internal organs.
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