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.

A typical arthropod exoskeleton is a multi-layered structure with four functional regions: epicuticle, procuticle, epidermis and basement membrane. 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 epithelial cells in the epidermis, which begins as a tough, flexible layer of chitin. 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 best-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.

The cuticle is soft when first secreted, but it soon hardens as required, in a process of sclerotization. The process is poorly understood, but it involves forms of tanning in which phenolic chemicals crosslink protein molecules or anchor them to surrounding molecules such as chitins. Part of the effect is to make the tanned material hydrophobic. By varying the types of interaction between the proteins and chitins, the insect metabolism produces regions of exoskeleton that differ in their wet and dry behaviour, their colour and their mechanical properties.

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. The tough and flexible endocuticle is a laminated structure of layers of interwoven fibrous chitin and protein molecules, while the exocuticle is the layer in which any major thickening, armouring and biomineralization occurs. Biomineralization with calcite is particularly common in Crustacea, whereas sclerotization particularly occurs in insects. The exocuticle is greatly reduced in many soft-bodied insects, especially in the larval stages such as caterpillars and the larvae of parasitoidal Hymenoptera.

In addition to the chitinous-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. The armoured product commonly has great mechanical strength.

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. When a rigid region fails under stress, it does so by cracking. 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.

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