Ecdysis

For other meaning of "moult", see moult (disambiguation).

Series of pictures showing the progression of ecdysis in Callinectes sapidus, the blue crab

Ecdysis is the moulting of the cuticula in arthropods and related groups (Ecdysozoa). Since the cuticula of these animals is also the skeletal support of the body and is inelastic, it is shed during growth and a new covering of larger dimensions is formed. In preparation for ecdysis, the arthropod will become inactive for a period of time. During this time, the arthropod will undergo apolysis. Then, by crawling movements, it pushes forward in the old integument, which splits down the back allowing the animal to emerge. Often, this initial crack within the integumentary shell will be 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. For most organisms, the resting period is used as a stage of preparation in the secretion of fluid from the moulting glands of the cellular layer and the loosening of the underpart of the cuticula occurs. Following the shedding of the old cuticula, a new layer is secreted during a second period of inactivity. All cuticular structures are shed at ecdysis, including the endoskeleton, which includes terminal linings of the alimentary tract and of the tracheae if they are present.

Other reasons for moulting are damaged tissue and missing limbs. Over a series of moults, a missing limb can be regenerated, the stump being a little larger with each moult until it is of normal size again.

Moulting in insects

Each stage in the development of an insect between moults is called an instar, or stadium. Higher insects tend to have fewer instars (4-5) than lower insects (anywhere up to ~15). Higher insects have more alternatives to moulting, such as expansion of the cuticle and collapse of air sacs to allow growth of internal organs.

A cicada moulting

A Luna moth caterpillar just after ecdysis. The old exoskeleton is visible behind it.

The process of moulting in insects begins with the separation of the cuticle from the underlying epidermal cells (apolysis) and ends with the shedding of the old cuticle (ecdysis). In many of them it is initiated by an increase in the hormone ecdysone. This hormone causes:

apolysis - the separation of the cuticle from the epidermis
excretion of new cuticle beneath the old
degradation of the old cuticle

After apolysis, moulting fluid is secreted into the space between the old cuticle and the epidermis (the exuvial space), this contains inactive enzymes which are activated only after the new epicuticle is secreted. This prevents them from digesting the new procuticle as it is laid down. The lower regions of the old cuticle - the endocuticle and mesocuticle - are then digested by the enzymes and subsequently absorbed. The exocuticle and epicuticle resist digestion and are hence shed at ecdysis.

Miscellanea

Related to the word ecdysis is ecdysiast, a euphemism for a striptease dancer.

External links

[Crayfish Moult] Very rare crayfish moulting video
PLOS primer: [How the Ecdysozoan Changed Its Coat]
[Scorpion (Centruroides gracilis) moulting] - several pictures.

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