Last updated
Adult Rhopalomyia solidaginis fly, emerging from pupal case Rhopalomyia solidaginis eclosion.jpg
Adult Rhopalomyia solidaginis fly, emerging from pupal case
Process of ecdysis of a cicada. Cicada molting animated-2.gif
Process of ecdysis of a cicada.

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. [1] The remnants of the old, empty exoskeleton are called exuviae. [2]


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. [3] 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 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.

Ecdysis allows damaged tissue and missing limbs to be regenerated or substantially re-formed. Complete regeneration may require a series of moults, the stump becoming a little larger with each moult until it is a normal, or near normal, size. [4]


The term ecdysis comes from Ancient Greek : ἐκδύω (ekduo), "to take off, strip off". [5]


In preparation for ecdysis, the arthropod becomes inactive for a period of time, undergoing apolysis or 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, a digesting fluid is secreted into the space between them. However, this fluid remains inactive until the upper part of the new cuticle has been formed. Then, by crawling movements, the organism pushes forward in the old integumentary shell, which splits down the back allowing the animal to emerge. Often, this initial crack is caused by a combination of movement and increase in blood pressure within the body, forcing an expansion across its exoskeleton, leading to an eventual crack that allows for certain organisms such as spiders 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 tracheae if they are present.

Physiology of ecdysis


Aeshna cyanea freshly slipped L2.jpg
Moulting (ecdysis) in southern hawker, Aeshna cyanea

Each stage of development between moults for insects in the taxon endopterygota is called an instar, or stadium, and each stage between moults of insects in the Exopterygota is called a nymph: there may be up to 15 nymphal stages. Endopterygota tend to have only four or five instars. Endopterygotes 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) and ends with the shedding of the old cuticle (ecdysis). In many species it is initiated by an increase in the hormone ecdysone. This hormone causes:

After apolysis the insect is known as a pharate. Moulting fluid is then secreted into the exuvial space between the old cuticle and the epidermis, this contains inactive enzymes which are activated only after the new epicuticle is secreted. This prevents the new procuticle from getting digested 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.


Female crab spider Synema decens, teneral after final ecdysis, still dangling from drop line, about to be mated, opisthosoma still shrunken Crab spider female in ecdysis; Genus Synema, Family Thomisidae 5725s.jpg
Female crab spider Synema decens , teneral after final ecdysis, still dangling from drop line, about to be mated, opisthosoma still shrunken
Complete process of a spider moulting.

Spiders generally change their skin for the first time while still inside the egg sac, and the spiderling that emerges broadly resembles the adult. The number of moults varies, both between species and sexes, but generally will be between five times and nine times before the spider reaches maturity. Not surprisingly, since males are generally 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 are very long-lived, sometimes 20 years or more; they moult annually even after they mature.

Spiders stop feeding at some time before moulting, usually for 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 may be possible to see new structures, such as setae, from outside. However, contact between the nerves and the old exoskeleton is maintained until a very late stage in the process.

The new, teneral exoskeleton has to accommodate a larger frame than the previous instar, while the spider has had to fit into the previous exoskeleton until it has been shed. This means the spider does not fill out the new exoskeleton completely, so it commonly appears somewhat wrinkled.

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) to supply enough fluid to pump into the prosoma with sufficient pressure to crack it open along its lines of weakness. The carapace 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 species, members of the Thomisidae (crab spiders), mate while the female is still callow, during which time she is unable to eat the male. [6]


Eurypterids are a group of chelicerates that became extinct in the Late Permian. They underwent ecdysis similarly to extant chelicerates, and most fossils are thought to be of exuviae, rather than cadavers. [2]

See also

Related Research Articles

Metamorphosis Profound change in body structure during the postembryonic development of an organism

Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some insects, fish, amphibians, mollusks, crustaceans, cnidarians, echinoderms, and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior. Animals can be divided into species that undergo complete metamorphosis ("holometaboly"), incomplete metamorphosis ("hemimetaboly"), or no metamorphosis ("ametaboly").

Skin Soft outer covering organ of vertebrates

Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.

Strepsiptera order of insects

The Strepsiptera are an endopterygote order of insects with nine extant families that include about 600 described species. They are endoparasites in other insects, such as bees, wasps, leafhoppers, silverfish, and cockroaches. Females of most species never emerge from the host after entering its body, finally dying inside it. The early-stage larvae do emerge because they must find an unoccupied living host, and the short-lived males must emerge to seek a receptive female in her host.

Arachnid Class of arthropods

Arachnida is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Spiders are the largest order in the class, which also includes scorpions, ticks, mites, harvestmen, and solifuges. In 2019, a molecular phylogenetic study also placed horseshoe crabs in Arachnida.

Moulting process by which an animal routinely casts off a part of its body

In biology, moulting, or molting, also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body, either at specific times of the year, or at specific points in its life cycle.

Instar A developmental stage of arthropods between moults

An instar is a developmental stage of arthropods, such as insects, between each moult (ecdysis), 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, changes in the number of body segments or head width. After moulting, i.e. shedding their exoskeleton, the juvenile arthropods continue in their life cycle until they either pupate or moult again. The instar period of growth is fixed; however, in some insects, like the salvinia stem-borer moth, the number of instars depends on early larval nutrition. Some arthropods can continue to moult after sexual maturity, but the stages between these subsequent moults are generally not called instars.

Stratum corneum outermost layer of the epidermis

The stratum corneum is the outermost layer of the epidermis. There has been a long-standing belief in dermatology that the stratum corneum consisted of dead cells (corneocytes), devoid of biological activity and function. The stratum corneum is now understood to be live tissue that performs protective and adaptive physiological functions including mechanical shear, impact resistance, water flux and hydration regulation, microbial proliferation and invasion regulation, initiation of inflammation through cytokine activation and dendritic cell activity, and selective permeability to exclude toxins, irritants, and allergens. This layer is composed of 15–20 layers of flattened cells with no nuclei or cell organelles. Their cytoplasm shows filamentous keratin. These corneocytes are embedded in a lipid matrix composed of ceramides, cholesterol, and fatty acids. Their properties depend on the component ratio of the three major components.

Apolysis The first process of molting, characterized by the detachment of the old cuticle from the underlying epidermal cells.

Apolysis is the separation of the cuticle from the epidermis in arthropods and related groups (Ecdysozoa). Since the cuticle 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 arthropod becomes dormant for a period of time. Enzymes are secreted to digest the inner layers of the existing cuticle, detaching the animal from the outer cuticle. This allows the new cuticle to develop without being exposed to the environmental elements.

The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa, trochanter, femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus, patella.

<i>Polygonia interrogationis</i> species of insect

Polygonia interrogationis, the question mark, is a North American nymphalid butterfly. It lives in wooded areas, city parks, generally in areas with a combination of trees and open space. The color and textured appearance of the underside of its wings combine to provide camouflage that resembles a dead leaf. The adult butterfly has a wingspan of 4.5–7.6 cm (1.8–3.0 in). Its flight period is from May to September. "The silver mark on the underside of the hindwing is broken into two parts, a curved line and a dot, creating a ?-shaped mark that gives the species its common name."

Snakeskin may either refer to the skin of a live snake, the shed skin of a snake after molting, or to a type of leather that is made from the hide of a dead snake.

Insect physiology includes the physiology and biochemistry of insect organ systems.

<i>Peripatoides indigo</i> species of velvet worm

Peripatoides indigo is a velvet worm of the family Peripatopsidae. The Māori name for the velvet worm is ngaokeoke, from the Māori word 'ngaoki', to crawl.

Reptile scale Small rigid plate that grows out of a reptiles skin

Reptile skin is covered with scutes or scales which, along with many other characteristics, distinguish reptiles from animals of other classes. Scales are made of alpha and beta-keratin and are formed from the epidermis. They may be ossified or tubercular, as in the case of lizards, or modified elaborately, as in the case of snakes.

Sloughing in biology refers to the act of shedding or casting off dead tissue, such as cells of the endometrium, shed during menstruation, or the shedding of skin in amphibians.

Arthropod exoskeleton

Arthropods are covered with a tough, resilient integument or exoskeleton of chitin. Generally the exoskeleton will have thickened areas in which the chitin is reinforced or stiffened by materials such as minerals or hardened proteins. This happens in parts of the body where there is a need for rigidity or elasticity. Typically the mineral crystals, mainly calcium carbonate, are deposited among the chitin and protein molecules in a process called biomineralization. The crystals and fibres interpenetrate and reinforce each other, the minerals supplying the hardness and resistance to compression, while the chitin supplies the tensile strength. Biomineralization occurs mainly in crustaceans; in insects and Arachnids the main reinforcing materials are various proteins hardened by linking the fibres in processes called sclerotisation and the hardened proteins are called sclerotin. Four sclerites form a ring around each segment: a dorsal tergite, lateral sternites and a ventral pleurite.

Hysterocrates gigas is a member of the tarantula family, Theraphosidae found in Cameroon. It is known as the giant baboon spider, Cameroon red baboon spider, or red baboon tarantula.

Arthropod Phylum of invertebrates with jointed exoskeletons

An arthropod is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora.

Arthropod cuticle

The cuticle forms the major part of the integument of the Arthropoda. It includes most of the material of the exoskeleton of the insects, Crustacea, Arachnida, and Myriapoda.

Hemideina femorata, the Canterbury tree weta or in Māori, putangatanga, is a flightless nocturnal insect from the order Orthoptera and the genus Hemideina, it is endemic to Canterbury, New Zealand, on the South Island.


  1. John Ewer (2005). "How the ecdysozoan changed its coat". PLoS Biology . 3 (10): e349. doi:10.1371/journal.pbio.0030349. PMC   1250302 . PMID   16207077.
  2. 1 2 O. Erik Tetlie, Danita S. Brandt & Derek E. G. Briggs (2008). "Ecdysis in sea scorpions (Chelicerata: Eurypterida)". Palaeogeography, Palaeoclimatology, Palaeoecology . 265 (3–4): 182–194. Bibcode:2008PPP...265..182T. doi:10.1016/j.palaeo.2008.05.008.
  3. Russell Jurenka (2007). "Insect physiology". In Sybil P. Parker (ed.). McGraw-Hill Encyclopedia of Science & Technology. 9 (10th ed.). p. 323. ISBN   978-0-07-144143-8.
  4. Penny M. Hopkins (2001). "Limb regeneration in the fiddler crab, Uca pugilator: hormonal and growth factor control". American Zoologist . 41 (3): 389–398. doi: 10.1093/icb/41.3.389 .
  5. Liddell & Scott (1889). An Intermediate Greek-English Lexicon. Oxford: Clarendon Press.
  6. Erik Holm & Anna Sophia Dippenaar-Schoeman (2010). Goggo Guide: the Arthropods of Southern Africa. LAPA. ISBN   978-0-7993-4689-3.