Moult of the arthropods

The Arthropod S (crustacean insects, , Arachnida S etc…) are characterized by an external skeleton (Exosquelette) inextensible, the cuticule or carapace. The moults allows these animals, by changing to them cuticule periodically, to grow in the face (driven growth) or to acquire new bodies, to even change form (driven metamorphosis). Thus, at much of insects, one or two particular moults allows the metamorphosis larval stages at the adult stage.

The old carapace, become too small, that the animal gives up, is called the Exuvie. One calls more particularly exuviation the rejection of the old carapace. The term ecdysis (begun again of the Greek), used in English language to indicate the moults of the arthropods, is translated better into French by exuviation , because it corresponds strictly at the time of the rejection of the cuticule, whereas one can consider that the moults ( grinds English , molt in American) includes also preparatory stages with the exuviation (known as pre-exuviales) and stages which make him continuation (known as post-exuviales). The various stages of the moult will be below detailed.

An arthropod is more vulnerable during all the moult, not only during the exuviation, where it does not have any more the possibility of fleeing and where the risks of wound are frequent, but also in the pre stages and post-exuviales where the old woman cuticule softens and where the news is not yet sufficiently hardened.

Various stages of the moult

Schematically, the various stages of the moults (in the broad sense) are the following ones:

1. The first visible demonstration of the moult is the apolyse ( apolysis in English), which corresponds to the separation of the skin (compound of a unicellular layer) and old cuticule. This separation, which occurs several days before the exuviation, is often perceptible only with methods of microscopy, but it is accompanied sometimes in certain arthropods by a change by brightness by the cuticule and by behavioral changes (stop of the food for example).

2. The apolyse is followed by the secretion of a liquid between the skin and the cuticule. This liquid, also called liquid of moult or liquid exuvial contains enzymes which cause to digest the internal layers (and the least hard) of the cuticule. The components resulting from this digestion are recovered by the organization and are stored to be re-used later on in the news cuticule. This digestion continues until the exuviation and causes to make the animal softer, but also more vulnerable.

3. Cuticular separation is often followed of rehandlings of the skin, in particular of the more or less many Mitose S, which make it possible the skin to grow and to possibly change before the synthesis of the news cuticule. These rehandlings are thus more numerous at the time of a moult of metamorphosis than at the time of a moult of growth.

4. Then the synthesis begins from the news cuticule, made up of several successive layers made up of Chitine and proteins. It is thus about a pre-exuviale cuticular synthesis, which can extend over several days. From this moment there, the arthropod has two cuticules, incomplete, one under the other, separated by the liquid exuvial. The muscular fasteners, always anchored on old the cuticule, are gradually rebuilt on the news.

5. Then, when the news cuticule is sufficiently thick, it is the exuviation (the moult with the restricted direction)! The rejection of the old woman cuticule is carried out thanks to a stereotyped behavior of the animal, which carries out rhythmic movements and which inflates its body to the maximum, often by swallowing water or air (according to whether it carries out an aquatic life or terrestrial). The old woman cuticule tears according to lines of Déhiscence, i.e. lines of less resistance, which were almost completely digested by the liquid exuvial. The movements of the animal enable him to be extracted from its old carapace and to extend the news to the maximum.

6. After the exuviation, a resumption of secretion cuticular, known as is carried out post-exuviale, which consists of a deposit of new chitino-proteinic layers.

7. The cuticular synthesis post-exuviale is accompanied by a hardening by the cuticule or sclerification ( sclerotisation in English). This hardening is carried out various manners: in shellfish primarily by incorporation of limestone, in the insects by new connections between the chitino-proteinic fibers which end in a tanning of proteins. At the end of this stage, which can take several days in the large arthropods, the animal thus recovers finally its physical capacities.

Moults and development of the aptérygotes

The aptérygotes or Apterygota are Hexapodes considered as being primitive insects, which do not have wings and do not undergo a metamorphosis: it is said that they are ametabolic . The animal which leaves egg is a youthful (one cannot speak about larva since there is no metamorphosis). Its morphology is similar to the adult from which it differs only by the size and sexual maturity. The development consists only of moults of growth, of which the number is not defined. When the animal reaches sexual maturity, there remains able to moult and can reproduce, according to the species, after each moult or all the two moults.

Moults and development of the ptérygotes

The insects ptérygotes or Pterygota typically have wings, which they acquire by undergoing a more or less important metamorphosis according to the groups. The egg gives rise to a larva, which undergoes several moults of growth (defining several larval stages) before undergoing one or two moults of metamorphosis. The animal after the metamorphosis, is called Imago: it is the adult insect, at the final stage of its development, because it does not undergo any more moults.

One distinguishes more particularly:

the Hétérométabole S, which carry out a metamorphosis " incomplète" , in the sense that one gradually sees appearing the outlines of the wings during the larval stages. Their metamorphosis is generally carried out during an only moult safe for the éphéméroptère S which have a stage intermediate, called Subimago, between the larva and the imago. In the hétérométaboles, one still distinguishes the Paurométabole S, whose larvae and adults live in the same medium and the Hémimétabole S, which change medium with the adult moult.
the Holométabole S, which carry out a metamorphosis " complète" , in the sense that the wing outlines appear only with the metamorphosis. This one is always carried out during two successive moults, thus revealing always an intermediate stage between the larva and the adult, stage which one calls Chrysalide (primarily at the Lépidoptères), nymph (at the Lépidoptères and the Coléoptères) or Pupe (at the Diptères and the Hyménoptères).

Many species of ptérygotes moult a fixed number of times, which differ according to the species, but it is not a general rule, because of many insects are able to adapt their number of moults according to the conditions of their environment.

Attention, some traps of terminology:

certain insects ptérygotes, can nevertheless have lost their wings during their development or of their evolution: in this case, one says that they are apterous (and not aptérygotes!). For example, the adult ants, while being ptérygotes, are generally apterous: the winged ants, which are in fact the reproductive adults, very quickly lose their wings after their exit of the nest of birth.
the term of " nymph" in English a significance different of French nymph has and corresponds rather to a " larve" having wing outlines
the " term; pupa" in English indifferently gathers what one indicates by nymph or pupe.

Moults and development of shellfish

The Crustacé S moult an indefinite number of times. They can moult several times after having reached the adult state (moults of growth). They can nevertheless undergo moults of metamorphosis, but contrary to the insects, which carry out them at the post-embryonic end of the development, the shellfish metamorphose themselves with the whole beginning of their development, during the moults which follow their blossoming. For example, the Homard leaves its egg in the form of a Larve which planktonique, is pulled by the currents. It generally undergoes three driven larval , during which it grows bigger, but does not change a form basically, nor of behavior. Then it undergoes then a moult of metamorphosis , which will modify it clearly and will be accompanied by a change of lifestyle, since it joined the ocean floor and carries out a life Benthique. At this time there, the animal is a very small lobster, called youthful . It will have to still carry out several driven growth before becoming an adult able to reproduce, then it will be able to still undergo several moults after having reached its sexual maturity. Therefore, contrary to the case of the insects, the adult state is not depend on the metamorphosis in shellfish.

It is very frequently said, in shellfish, that an animal is in pre-moult when it is in pre-exuviale period (between the apolyse and the exuviation) and in post-moult , after the exuviation. The variable interval (sometimes very long in Shellfish) where the animal is not in period of moult is called intermue (but there can be confusion between this period of not-moult and the period between two exuviations.)

Moults and development of the other arthropods

The spiders generally moult a fixed number of times, variable according to the species. At some of them, of the moults continue in an adult state, in particular at the species living several years.

Endocrine control of the moult

The moult of the arthropods is started by a Hormone Stéroïde, called Ecdysone or hormone of moult , often secreted by a specialized gland, the Glande of moult (called prothoracic gland in the insects or body Y in shellfish). More precisely, the gland of moult secretes the ecdysone, which is transformed by other bodies into the active hormone, the 20-hydroxy-ecdysone. This hormone controls primarily the pre-exuviale phase of the moult: the hormonal concentrations of 20-hydroxy-ecdysone in the hemolymph increase with the apolyse and reach their maximum at the time of initiation of the news cuticule, then fall with the exuviation. Other hormones, in particular of the peptide neurohormones, more particularly control the exuviation and the phases post-exuviales.

In the insects, the moults of metamorphosis occur when the ecdysone is secreted in absence of an other hormone, the youthful Hormone, worked out by the body confine to bed . On the other hand, the larval moults occur when the ecdysone is secreted in the presence of youthful hormone.

See too

Ecdysone
Imago
the moult at the squamates

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