An Introduction to the Invertebrates, Second Edition - tiera.ru

154 ARTHROPODA: GENERAL12.1 What defines an arthropod?The exoskeleton covers the whole body. The body segments areoften grouped into regions called ‘tagmata’ (singular ‘tagma’), forexample, head, thorax and abdomen. Most segments bear one pairof appendages: the most anterior segment may lack them, butprimitively probably each of the more posterior segments bore pairedappendages. In most groups, however, many appendages have beenlost, as in many parasites and in the posterior region of insects.In general, arthropods with highly specialised appendages tendto have fewer of them.The cuticle is moulted at intervals when the young animalincreases in size; the process is controlled by hormones. The brain,sense organs and nervous system are extremely well developed: rapidand precisely controlled movement is characteristic of arthropodsand complex behaviour has evolved. The main body cavity is a haemocoelcontaining blood, which enters the dorsally placed heart throughholes (ostia). The coelom may be represented only in the gonads; it issegmental in origin but becomes much reduced during development,when the walls of the coelomic sacs break down. There are nolocomotor cilia, but some sense organs are constructed from ciliarystructures. As will be explained, respiratory and excretory organsdiffer greatly between aquatic and land forms. Typically the sexes areseparate. The need for internal fertilisation is often associated withcomplex behaviour. Development is basically protostome (see Box 5.2)but much modified in most arthropods.12.2 What are the key features ofarthropod cuticle?The frequent statement that ‘to study cuticle is to study 90% of theproperties of an insect’ gives an emphasis that can be extended to allarthropods.12.2.1 The cuticleThe cuticle or exoskeleton is made of chitin and protein, without anycollagen. Chitin is a nitrogenous polysaccharide, related to mucus,from which perhaps it was derived. It is not in itself hard: microfibrilsof chitin are embedded in protein and the cuticle acquires strengthand rigidity when the protein molecule chains become cross-linked(‘tanned’) by phenols (quinones).12.2.2 CuticleislaiddownbyepidermalcellsThe epidermal cells that lay down the cuticle are spread over theanimal in a single-layered sheet. Each cell secretes its own overlyingcuticle, giving precise spatial control.