Spiders and their Kin Spiders and Their Kin Arachnids belong to a class of animals which are grouped under the phylum Arthropoda. Other kin that are closely related to the spiders are centipedes and milipedes which are included on this page. Spiders, the common name for about 34,000 species of arthropod animals having eight walking legs, anterior appendages bearing fangs and poison glands, and specialized reproductive organs on the second appendages of the male. They commonly make extensive use of silk that they spin. Like other arachnid species, spiders are terrestrial, although a few have adapted to freshwater life by trapping air bubbles underwater and carrying the bubbles with them.
Spiders are numerous and occur worldwide.
The body structure of a spider is similar to that of other arachnids in being divided into an anterior cephalothorax, or prosoma, and a posterior abdomen, or opisthosoma. The two parts are separated by a narrow stalk, or pedicel, which gives the animal a flexibility that facilitates its use of silk. The cephalothorax ordinarily bears four pairs of simple eyes that tend to be larger in hunting spiders and smaller in spinners of elaborate webs. Each of the first pair of appendages, or chelicerae, bears a fang with an opening from a poison gland at the tip. The next two appendages are pedipalps, rather leglike but generally modified into a kind of feeler.