External Anatomy
Spiders, unlike insects, have only two tagmata instead of three: a fused head and thorax (called a cephalothorax or prosoma) and an abdomen (called the opisthosoma). The exception to this rule are the assassin spiders, whose cephalothorax is divided into two parts by an elongated "neck". Except for a few species of very primitive spiders (family Liphistiidae, also called segmented spiders), the abdomen is not externally segmented. The abdomen and cephalothorax are connected with a thin waist called the pedicle or the pregenital somite, a trait that allows the spider to move the abdomen in all directions. This waist is actually the last segment (somite) of the cephalothorax and is lost in most other members of the Arachnida (in scorpions it is only detectable in the embryos). Unlike insects, spiders have an endoskeleton in addition to their exoskeleton.
Read more about this topic: Spider Anatomy
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