Arthropod exoskeleton

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The head of an ant: Chitin Type

Arthropods are covered with a tough or resistant exoskeleton, which may be mineralised or constructed of a tough polymer such as chitin, and is often biomineralized with materials such as calcium carbonate. This external skeleton is moulted as the organism grows.

Microscopic structure[edit]

A: Cuticle and epidermis; B: Epicuticle detail . 1: Epicuticle; 1a: Cement layer; 1b: Wax layer; 1c: Outer epicuticle; 1d: Inner epicuticle. 2: Exocuticle; 3: Endocuticle; 2+3: Procuticle; 4: Epidermis; 5: Basement membrane; 6: epidermic cell; 6a: Pore canal; 7: Glandular cell; 8: Trichogen cell; 9: Tormogen cell; 10: Nerve; 11: Sensilia; 12: Hair; 13: Gland opening.

A typical arthropod exoskeleton is a multi-layered structure with four functional regions: epicuticle, procuticle, epidermis and basement membrane.[1] Of these, the epicuticle is a multi-layered external barrier that, especially in terrestrial arthropods, acts as a barrier against desiccation. The strength of the exoskeleton is provided by the underlying procuticle, which is in turn secreted by the epidermis. Arthropod cuticle is a biological composite material, consisting of two main portions: fibrous chains of alpha-chitin within a matrix of silk-like and globular proteins, of which the most well-known is the rubbery protein called resilin. The relative abundance of these two main components varies from approximately 50/50 to 80/20 chitin protein, with softer parts of the exoskeleton having a higher proportion of chitin. Although the cuticle is relatively soft when first secreted, it soon hardens in a poorly understood process that involves sclerotization and/or tanning mediated by hydrophobic chemicals called phenolics. Different types of interaction between the proteins and chitin leads to varying mechanical properties of the exoskeleton.
In addition to the chitino-proteinaceous composite of the cuticle, many crustaceans, some myriapods and the extinct trilobites further impregnate the cuticle with mineral salts, above all calcium carbonate, which can make up to 40% of the cuticle. This can lead to great mechanical strength.

Mechanical properties[edit]

The two layers of the cuticle have different properties. The outer, sclerosed layer is very strong under compressive forces, but much weaker under tension.[2] When it fails, it does so by cracking.[2] The inner layer is not sclerosed, and is thus much softer; it is able to resist tensile forces but is liable to failure under compression.[2]

This combination is especially effective in resisting predation, as predators tend to exert compression on the outer layer, and tension on the inner.[2]

The degree of scleritisation affects how the cuticle responds to deformation. Below a certain point - and this point will be higher the more scleritised the cuticle is - deformation is elastic and the original shape is returned to after the stress is removed. Above this point, plastic (non-reversible) deformation occurs until finally the cuticle cracks.[2]

Segmentation[edit]

The arthropod exoskeleton is typically divided into different functional units to allow flexibility in an often otherwise rigid structure. For example, the head is a fused capsule; and the trunk is often divided into a series of articulating sclerites called tergites. In addition, the characteristic limbs of arthropods need to be jointed. The internal surface of the exoskeleton is often elaborated into a set of specialised structures called apodemes that allow the attachment of muscles. Such endoskeletal components of the arthropod skeleton can be highly complex, as in crabs and lobsters.

Ecdysis[edit]

Time series photos of a Tibicen Dog Day Cicada moulting in Ohio USA.

The relative rigidity of the exoskeleton 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 moulting or ecdysis, which is under the control of a hormone called ecdysone. Moulting is a complex process that is invariably dangerous for the arthropod involved. Before the old exoskeleton is shed, the cuticle separates from the epidermis through a process called apolysis. New cuticle is excreted by the underlying epidermis, and mineral salts are usually withdrawn from the old cuticle for re-use. 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. Newly molted arthropods typically appear pale or white, and darken as the cuticle hardens.

References[edit]