Unlike insects, spiders have an endoskeleton in addition to their exoskeleton. The abdomen and cephalothorax are connected by a thin waist called the pedicel. The exception is the Liphistiidae, a basal family, which retains this more primitive character hence they are sometimes called segmented spiders. In the majority of spiders, the abdomen is not externally segmented. The exception to this rule are the assassin spiders in the family Archaeidae, whose cephalothorax is divided into two parts by an elongated 'neck'. Spiders, unlike insects, have only two main body parts ( tagmata) instead of three: a fused head and thorax (called a cephalothorax or prosoma) and an abdomen (also called an opisthosoma). The underside and head of a female spider Many species of spiders exhibit a great deal of sexual dimorphism. Male spiders have specialized pedipalps that are used to transfer sperm to the female during mating. Most spiders possess venom, which is injected into prey (or defensively, when the spider feels threatened) through the fangs of the chelicerae. All spiders are capable of producing silk of various types, which many species use to build webs to ensnare prey. Spiders also have several adaptations that distinguish them from other arachnids. These characteristics include bodies divided into two tagmata (sections or segments), eight jointed legs, no wings or antennae, the presence of chelicerae and pedipalps, simple eyes, and an exoskeleton, which is periodically shed. The anatomy of spiders includes many characteristics shared with other arachnids.

The epigastric plates and furrow are visible, as well as the hourglass marking on the underside of the abdomen, which is a characteristic feature of widow spiders. The ventral side of a brown widow spider.

Iron spider eyes