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Exoskeleton
An exoskeleton is a type of skeleton that is an external anatomical feature that supports and protects an animal's body, in contrast to the internal endoskeleton of, for example, a human. Whilst many, many other invertebrate animals (such as shelled mollusks) have exoskeletons in the sense of external hard parts, the characteristic is most associated with the arthropods (i.e. insects, spiders, myriapods and crustaceans). Exoskeletons contain rigid and resistant components that fulfill a set of functional roles including protection, excretion, sensing, support, feeding and (for terrestrial organisms) acting as a barrier against desiccation. Exoskeletons first appeared in the fossil record about 550 million years ago, and their evolution has been seen as a critical driving role in the Cambrian explosion of animals that took place subsequent to this time.
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 dessiccation. 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 70/30 protein/chitin, 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 dehydration 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 up to 40% of the cuticle. This can lead to great mechanical strength.
Cuticular Metals
Another very interesting fact about scorpions is that it has been noted that their exoskeleton has a focal distribution of metals. In the chelicera, sting, and tarsi there are high amounts of metals. It is a high concentration almost reaching 30% by weight. Source - Scorpion Biology and Research
Source
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