Spiders are chelicerates and therefore arthropods.[6] As arthropods they have: segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins; heads that are composed of several segments that fuse during the development of the embryo.[5] Being chelicerates, their bodies consist of two tagmata, sets of segments that serve similar functions: the foremost one, called the cephalothorax or prosoma, is a complete fusion of the segments that in an insect would form two separate tagmata, the head and thorax; the rear tagma is called the abdomen or opisthosoma.[6] In spiders the cephalothorax and abdomen are connected by a small cylindrical section, the pedicel.[7]
The pattern of segment fusion that forms chelicerates' heads is unique
among arthropods, and what would normally be the first head segment
disappears at an early stage of development, so that chelicerates lack
the antennae typical of most arthropods. In fact chelicerates' only appendages ahead of the mouth are a pair of chelicerae, and they lack anything that would function directly as "jaws".[5][8] The first appendages behind the mouth are called pedipalps, and serve different functions within different groups of chelicerates.[6]
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