Hexapods are arthropods, i. e., animals with segmented bodies. Segments are organized in three distinct functional units, or tagmata: the head, thorax, and abdomen (Chapman 1998, Weidner 1982). The head is a capsule formed by the fusion of several segments (the exact number is controversial, Weidner 1982). It features mouthparts for feeding as well as a pair of antennae (absent in Protura) and other sensory organs.
The most distinctive feature of the hexapods is the reduction of walking appendages to six, with three body segments consolidating to form the thorax, which provides much of the locomotory ability of the animals (Kristensen 1981, 1991). This is in contrast to other arthropods, most of which have more than three pairs of legs. The hexapod abdomen, primitively with 11-segments plus a postsegmental telson, is specialized for digestion, excretion, and reproduction. It generally lacks legs, but many apterygote (wingless) hexapods and some pterygote insects feature a variety of abdominal appendages, including a pair of cerci on the terminal segment (absent in Collembola and Protura), which function as sense organs (Chapman 1998, Weidner 1982).
The hexapod central nervous system consists of the brain, which is located in the head and a nerve cord composed of a series of ganglia extending ventrally along the longitudinal axis of the body (ventral nerve cord, Chapman 1998, Niven et al. 2008). In the basic hexapod body plan, there was most likely one ganglion associated with each body segment, but modern hexapods display varying degrees of ganglionic fusion (Chapman 1998, Nation 2002). The central nervous system controls muscles, glands, and other organs, and it receives input from a diverse array of sensory systems.
Hexapods possess many different kinds of sensory receptors that monitor both the external and internal environment. A great variety of mechano- and chemosensory systems have been described across different groups of hexapods (Chapman 1998, Nation 2002, Weidner 1982); however, visual perception appears to be important only in insects, many of which feature highly specialized compound eyes (Horridge 1975); while the mostly soil- and litter-dwelling Collembola, Protura, and Diplura entirely lack eyes, although some Collembola have ocelli (Gillot 2005).
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