Ecdysis is the
moultIn biology, moulting signifies 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 In biology, moulting (or molting, also known as sloughing, shedding or for...
ing of the cuticula in
arthropodAn arthropod is an invertebrate that has an exoskeleton , a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s and related groups (
EcdysozoaThe Ecdysozoa are a grouping of protostome animals, including the 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...
). Since the cuticula of these animals is also the skeletal support (the
exoskeletonAn exoskeleton is an external skeleton that supports and protects an animal's body, in contrast to the internal endoskeleton of, for example, a human. Some animals, such as the tortoise, have both an endoskeleton and an 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
exuviaExuviae is a term used in biology to describe the remains of an exoskeleton that is left after an arthropod has moulted...
(or "exuvium").
After moulting, an arthropod is described as
teneral; it is "fresh", pale and soft-bodied. Within one or two hours, the
cuticleA cuticle , or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Cuticles are non-homologous, differing in their origin, structure and chemical composition.-Human anatomy:In human anatomy, cuticle ...
hardens and darkens following a tanning process similar to that of the
tanningTanning is the process of making leather, which does not easily decompose, from the skins of animals, which do. Often this uses tannin, an acidic chemical compound. Coloring may occur during tanning....
of leather. It is during this short phase that the animal grows, since growth is otherwise constrained by the rigidity of the exoskeleton.
Ecdysis may also enable damaged tissue and missing limbs to be
regeneratedIn biology, an organism is said to regenerate a lost or damaged part if the part regrows so that the original function is restored.Regenerative capacity is inversely related to complexity: in general, the more complex an animal is the less regeneration it is capable of. Whereas newts, for example,...
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
apolysisApolysis 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.
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
tracheaTerrestrial arthropods have evolved an open respiratory system composed of spiracles, tracheae, and tracheoles to transport metabolic gasses to and from tissue. Some terrestrial woodlice have evolved pseudotrachea, a system which is also called corpus alatum, and is made up of air tubes that...
e if they are present.
Then, by crawling movements, the pharate animal pushes forward in the old
integumentary shellThe 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
exoskeletonAn exoskeleton is an external skeleton that supports and protects an animal's body, in contrast to the internal endoskeleton of, for example, a human. Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton...
, leading to an eventual crack that allows for certain organisms such as
spiderSpiders are air-breathing chelicerate arthropods that have eight legs, and chelicerae modified into 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.
Moulting in insects
Each stage in the development of an insect between moults is called an
instarAn instar is a developmental stage of arthropods, such as insects, between each moult , 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 or changes in the number of...
, or stadium.
EndopterygotaThe 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
ExopterygotaThe 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 (
apolysisApolysis 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
hormoneA hormone is a chemical released by one or more cells that affects cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms...
ecdysoneEcdysone 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 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 epidermisIn anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous cell...
- excretion
Excretion is the process of eliminating waste products of metabolism and other non-useful materials. It is an essential process in all forms of life. It contrasts secretion, where the substance may have specific tasks after leaving the cell....
of new cuticle beneath the old
- degradation of the old cuticle
After
apolysisApolysis 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
epicuticleThe epicuticle is the outermost portion of the exoskeleton of an insect ; its exact composition and structure may differ somewhat among different taxa, but certain aspects can be generalized:...
is secreted. This prevents them from digesting the new
procuticleThe procuticle is the major portion of the exoskeleton of an insect ; its exact composition and structure may differ somewhat among different taxa, but certain aspects can be generalized:...
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
epicuticleThe epicuticle is the outermost portion of the exoskeleton of an insect ; its exact composition and structure may differ somewhat among different taxa, but certain aspects can be generalized:...
resist digestion and are hence shed at ecdysis.
External links