Arthropod exoskeleton
Arthropods are covered with a tough, resilient integument, cuticle 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. The dorsal tergum, ventral sternum, and the lateral pleura form the hardened plates or sclerites of a typical body segment.
In either case, in contrast to the carapace of a tortoise or the cranium of a vertebrate, the exoskeleton has little ability to grow or change its form once it has matured. Except in special cases, whenever the animal needs to grow, it moults, shedding the old skin after growing a new skin from beneath.
Microscopic structure
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A typical arthropod exoskeleton is a multi-layered structure with four functional regions:
The cuticle is soft when first secreted, but it soon hardens as required, in a process of
The chitinous procuticle is formed of an outer exocuticle and the inner endocuticle, and between the exocuticle and endocuticle there may be another layer called mesocuticle which has distinctive staining properties.
In addition to the chitinous-proteinaceous composite of the cuticle, many
Mechanical properties
The two layers of the cuticle have different properties. The outer layer is where most of the thickening, biomineralization and sclerotisation takes place, and its material tends to be strong under compressive stresses, though weaker under tension.[5] When a rigid region fails under stress, it does so by cracking.[5] The inner layer is not as highly sclerotised, and is correspondingly softer but tougher; it resists tensile stresses but is liable to failure under compression.[5]
This combination is especially effective in resisting predation, as predators tend to exert compression on the outer layer, and tension on the inner.[5]
Its degree of sclerotisation or mineralisation determines how the cuticle responds to
Segmentation
Hardened plates in the exoskeleton are called sclerites. Sclerites may be simple protective armour, but also may form mechanical components of the
The arthropod exoskeleton is divided into different functional units, each comprising a series of grouped segments; such a group is called a tagma, and the tagmata are adapted to different functions in a given arthropod body. For example, tagmata of insects include the head, which is a fused capsule, the thorax as nearly a fixed capsule, and the abdomen usually divided into a series of articulating segments. Each segment has sclerites according to its requirements for external rigidity; for example, in the larva of some flies, there are none at all and the exoskeleton is effectively all membranous; the abdomen of an adult fly is covered with light sclerites connected by joints of membranous cuticle. In some beetles most of the joints are so tightly connected, that the body is practically in an armoured, rigid box. However, in most Arthropoda the bodily tagmata are so connected and jointed with flexible cuticle and muscles that they have at least some freedom of movement, and many such animals, such as the Chilopoda or the larvae of mosquitoes are very mobile indeed. In addition, the limbs of arthropods are jointed, so characteristically that the very name "Arthropoda" literally means "jointed legs" in reflection of the fact. The internal surface of the exoskeleton is often infolded, forming a set of structures called apodemes that serve for the attachment of muscles, and functionally amounting to endoskeletal components. They are highly complex in some groups, particularly in Crustacea.[citation needed]
Within
Chemical composition
Chemically, chitin is a long-chain
In its unmodified form, chitin is translucent, pliable, resilient and tough. In arthropods and other organisms however, it generally is a component of a complex matrix of materials. It practically always is associated with protein molecules that often are in a more or less sclerotised state, stiffened or hardened by cross-linking and by linkage to other molecules in the matrix. In some groups of animals, most conspicuously the Crustacea, the matrix is greatly enriched with, or even dominated by, hard minerals, usually calcite or similar carbonates that form much of the exoskeleton. In some organisms the mineral content may exceed 95%. The role of the chitin and proteins in such structures is more than just holding the crystals together; the crystal structure itself is so affected as to prevent the propagation of cracks under stress, leading to remarkable strength.[8] The process of formation of such mineral-rich matrices is called biomineralization.[9]
The difference between the unmodified and modified forms of chitinous arthropodan exoskeletons can be seen by comparing the body wall of say a bee larva, in which modification is minimal, to any armoured species of beetle, or the fangs of a spider. In both those examples there is heavy modification by sclerotisation. Again, contrasting strongly with both unmodified organic material such as largely pure chitin, and with sclerotised chitin and proteins, consider the integument of a heavily armoured crab, in which there is a very high degree of modification by biomineralization.
Moulting
The chemical and physical nature of the arthropod exoskeleton limits its ability to stretch or change shape as the animal grows. In some special cases, such as the abdomens of termite queens and honeypot ants means that continuous growth of arthropods is not possible. Therefore, growth is periodic and concentrated into a period of time when the exoskeleton is shed, called
After the old cuticle is shed, the arthropod typically pumps up its body (for example, by air or water intake) to allow the new cuticle to expand to a larger size: the process of hardening by dehydration of the cuticle then takes place. The new integument still is soft and usually is pale, and it is said to be teneral or callow. It then undergoes a hardening and pigmentation process that might take anything from several minutes to several days, depending on the nature of the animal and the circumstances.[10]: 16–20
Although the process of ecdysis is metabolically risky and expensive, it does have some advantages. For one thing it permits a complex development cycle of
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Honeybee larvae have flexible but delicate unsclerotised cuticles.
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This fully-grown robber crab has tough fabric forming its joints, delicate biomineralized cuticle over its sensory antennae, optic-quality over its eyes, and strong, calcite-reinforced chitin armouring its body and legs; its pincers can break into coconuts
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The fangs in spiders' chelicerae are so sclerotised as to be greatly hardened and darkened
See also
References
- ^ "NC State University". Archived from the original on 2008-09-06. Retrieved 2008-07-16.
- ISBN 978-3-11-016210-3. Retrieved 10 January 2013.
- ISBN 9781402062421.
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- ^ "external morphology of Insects" (PDF). Archived from the original (PDF) on 2011-07-19. Retrieved 2011-03-20.
- ^ "Insect Glossary". E-Fauna BC. Retrieved 21 February 2017.
- PMID 24681646.
- ISBN 0-8053-1957-3
- ISBN 978-0-595-22143-1.