Ecdysis is the moult

Moult

In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...

ing of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa

Ecdysozoa

Ecdysozoa is a group of protostome animals, including Arthropoda , Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes...

, comprising the arthropod

Arthropod

An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s, nematode

Nematode

The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

s, velvet worms, horsehair worms

Nematomorpha

Nematomorpha is a phylum of parasitic animals that are superficially morphologically similar to nematode worms, hence the name. They range in size in most species from long and can reach in extreme cases up to 2 metres, and in diameter...

, rotifer

Rotifer

The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

s, tardigrade

Tardigrade

Tardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773...

s and Cephalorhyncha. Since the cuticula of these animals often forms an inelastic exoskeleton

Exoskeleton

An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

, it is shed during growth and a new, larger covering is formed. The remnants of the old, empty exoskeleton are called exuvia

Exuvia

Exuviae is a term used in biology to describe the remains of an exoskeleton and related structures that are left after ecdysozoans have moulted...

e.

After moulting, an arthropod is described as teneral, a callow; it is "fresh", pale and soft-bodied. Within one or two hours, the cuticle

Cuticle

A cuticle , or cuticula, is a term used for any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticles" are non-homologous; differing in their origin, structure, function, and chemical composition...

 hardens and darkens following a tanning process similar to that of the tanning

Tanning

Tanning is the making of leather from the skins of animals which does not easily decompose. Traditionally, tanning used tannin, an acidic chemical compound from which the tanning process draws its name . Coloring may occur during tanning...

 of leather. It is during this short phase that the animal expands, 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. Some arthropods however, especially large insects with tracheal respiration, expand their new exoskeleton by swallowing or otherwise taking in air. The maturation of the structure and coloration of the new exoskeleton might take days or weeks in a long-lived insect; this can raise problems in trying to identify the species when a specimen has just recently undergone ecdysis.

Ecdysis may also enable damaged tissue and missing limbs to be regenerated

Regeneration (biology)

In biology, regeneration is the process of renewal, restoration, and growth that makes genomes, cells, organs, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. At its most...

 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.

Process

In preparation for ecdysis, the arthropod becomes inactive for a period of time, undergoing apolysis

Apolysis

Apolysis is the separation of the cuticula from the epidermis in arthropods and related groups . 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. During this process, an...

 (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.
Then, by crawling movements, the pharate animal pushes forward in the old integumentary shell

Integumentary system

The integumentary system is the organ system that protects the body from damage, comprising the skin and its appendages...

, 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

Exoskeleton

An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

, leading to an eventual crack that allows for certain organisms such as spider

Spider

Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s to extricate themselves.
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 trachea

Invertebrate trachea

The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....

e if they are present.

Insects

Each stage in the development of an insect between moults is called an instar

Instar

An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...

, or stadium. Endopterygota

Endopterygota

The Endopterygota, also known as Holometabola, are insects of the subclass Pterygota which go through distinctive larval, pupal, and adult stages. They undergo a radical metamorphosis, with the larval and adult stages differing considerably in their structure and behaviour...

 tend to have few instars (4-5), while other insects such as Exopterygota

Exopterygota

The Exopterygota, also known as Hemipterodea, are a superorder of insects of the subclass Pterygota in the infraclass Neoptera, in which the young resemble adults but have externally-developing wings. They undergo a modest change between immature and adult, without going through a pupal stage...

 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.

The process of moulting in insects begins with the separation of the cuticle from the underlying epidermal cells (apolysis

Apolysis

Apolysis is the separation of the cuticula from the epidermis in arthropods and related groups . 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. During this process, an...

) and ends with the shedding of the old cuticle (ecdysis). In many of them it is initiated by an increase in the hormone

Hormone

A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

 ecdysone

Ecdysone

Ecdysone is a steroidal prohormone of the major insect molting hormone 20-hydroxyecdysone, which is secreted from the prothoracic glands. Insect molting hormones are generally called ecdysteroids. Ecdysteroids act as moulting hormones of arthropods but also occur in other related phyla where they...

. This hormone causes:

• apolysis
  Apolysis
  Apolysis is the separation of the cuticula from the epidermis in arthropods and related groups . 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. During this process, an...
  
   – the separation of the cuticle from the epidermis
  Squamous epithelium
  In anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells...
  
  
• secretion
  Secretion
  Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
  
   of new cuticle materials beneath the old
• degradation of the old cuticle

After apolysis

Apolysis

Apolysis is the separation of the cuticula from the epidermis in arthropods and related groups . 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. During this process, an...

, 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.

Spiders

Spiders generally change their skin for the first time while still inside the egg sac, and the spiderling that emerges generally looks fairly recognisably like the adult. However, there may be confusion in trying to identify the species if one does not realise that it is a juvenile one is working with. Even if one does know this, it may not be possible to identify it with any confidence, since most keys refer to adult characters.

The number of moults varies, both between species and genders, but generally will be between five times and nine times before the spider reaches maturity. Not surprisingly, males generally being smaller than females, the males of many species mature faster and do not undergo ecdysis as many times as the females before maturing.

Members of the Mygalomorphae

Mygalomorphae

The Mygalomorphae, , are an infraorder of spiders. The latter name comes from the orientation of the fangs which point straight down and do not cross each other .-Description:...

 generally are very long-lived, sometimes 20 years or more, and typically they moult annually even after they mature.

Spiders stop feeding some time before moulting, usually several days. The physiological processes of releasing the old exoskeleton from the tissues beneath typically cause various colour changes, such as darkening. If the old exoskeleton is not too thick it also may be possible to see new structures, such as setae, from outside. However, nervous contact with the old exoskeleton is maintained till a very late stage in the process.

Commonly the new, teneral exoskeleton is wrinkled because it has to accommodate a larger frame than the previous instar

Instar

An instar is a developmental stage of arthropods, such as insects, between each molt , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions, colors, patterns, or...

, while fitting into the previous exoskeleton until it has been shed.

Most species of spiders hang from silk during the entire process, either dangling from a drop line, or fastening their claws into webbed fibres attached to a suitable base. The discarded, dried exoskeleton typically remains hanging where it was abandoned once the spider has left.

To open the old exoskeleton, the spider generally contracts its abdomen (opisthosoma

Opisthosoma

The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma . It is a distinctive feature of the subphylum Chelicerata...

) to supply enough fluid to pump into the prosoma

Cephalothorax

The cephalothorax is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. The word cephalothorax is derived from the Greek words for head and thorax...

 with sufficient pressure to crack it open along its lines of weakness. The carapace

Carapace

A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

 lifts off from the front, like a helmet, as its surrounding skin ruptures, but it remains attached at the back. Now the spider works its limbs free and typically winds up dangling by a new thread of silk attached to its own exuviae, which in turn hang from the original silk attachment.

At this point the spider is a callow; it is teneral and vulnerable. As it dangles, its exoskeleton hardens and takes shape. The process may take minutes in small spiders, or some hours in the larger Mygalomorphs. Some spiders, such as some Synema

Synema

Synema may refer to:* Synema a botanical genus name that is synonymous to Mercurialis,* Synema, a genus of spiders in Thomisidae...

species, members of the Thomisidae (crab spiders) mate while the female is still callow, during which time she is unable to eat the male.

Eurypterids

Eurypterid

Eurypterid

Eurypterids are an extinct group of arthropods related to arachnids which include the largest known arthropods that ever lived. They are members of the extinct order Eurypterida ; which is the most diverse Paleozoic chelicerate order in terms of species...

s are a group of chelicerates that became extinct

Extinction

In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

 in the late Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

. They underwent ecdysis in a similar manner to extant chelicerates, and most fossils are thought to be of exuvia

Exuvia

Exuviae is a term used in biology to describe the remains of an exoskeleton and related structures that are left after ecdysozoans have moulted...

e, rather than cadavers.