Contrary to what many people believe, spiders are not insects - they are air-breathing arthropods. Most spiders are venomous, but not many are harmful to humans. Many people are afraid of spiders. The fear of spiders is called arachnophobia.
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 groups of organisms. Spiders are found worldwide on every continent except for Antarctica, and have become established in nearly every habitat with the exception of air and sea colonization. As of 2008, approximately 40,000 spider species, and 109 families have been recorded by taxonomists; however, there has been confusion 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.
Contributions by Patrick0Moran, Lexor, and Richman.