<P> As a rule, 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 . </P> <P> 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 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 New cuticle is excreted by the underlying epidermis, and mineral salts are usually withdrawn from the old cuticle for re-use .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 . A newly molted arthropod typically is pale in colour; in that state it is said to be teneral or a callow . It generally darkens or otherwise gains colour as its exoskeleton hardens . </P> <P> 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 metamorphosis in which young animals may be totally different from older phases, such as the nauplius larvae of crustaceans, the nymphs of say, the Odonata, or the larvae of Endopterygota, such as maggots of flies . Such larval stages commonly have ecological and life cycle roles totally different from those of the mature animals . Secondly, often a major injury in one phase, such as the loss of a leg from an insect nymph, or a claw from a young crab, can be repaired after one or two stages of ecdysis . Similarly, delicate parts that need periodic replacement, such as the outer surfaces of the eye lenses of spiders, or the urticating hairs of caterpillars, can be shed, making way for new structures . </P>

What is the function of an arthropod exoskeleton