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** Spider **
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For other uses, see Spider (disambiguation).
Temporal range: Pennsylvanian - Holocene, 319â0 Ma
An assortment of different spiders.
Scientific classification e
/See Spider taxonomy/.
109 families, c. 40,000 species
*Spiders* (order *Araneae*) are air-breathing arthropods that have eight
legs and chelicerae with fangs that inject venom. They are the largest
order of arachnids and rank seventh in total species diversity among all
other orders of organisms.^ Spiders are found worldwide on every
continent except for Antarctica, and have become established in nearly
every habitat with the exceptions of air and sea colonization. As of
September 2015, at least 45,709 spider species,^ and 114 families have
been recorded by taxonomists.^ However, there has been dissension within
the scientific community as to how all these families should be classified,
as evidenced by the over 20 different classifications that have been
proposed since 1900.^
Anatomically, spiders differ from other arthropods in that the usual body
segments are fused into two tagmata, the cephalothorax and abdomen, and
joined by a small, cylindrical pedicel. Unlike insects, spiders do not have
antennae. In all except the most primitive group, the Mesothelae, spiders
have the most centralized nervous systems of all arthropods, as all their
ganglia are fused into one mass in the cephalothorax. Unlike most
arthropods, spiders have no extensor muscles in their limbs and instead
extend them by hydraulic pressure.
Their abdomens bear appendages that have been modified into
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