Phylogeny and Classification of Spiders

Spiders are one of the eleven orders of the class Arachnida, which also includes groups such as harvestmen (Opiliones), ticks and mites (Acari), scorpions (Scorpiones), false scorpions (Pseudoscorpiones), windscorpions (Solifugae), and vinegaroons (Uropygi). All arachnid orders occur in North America. Arachnida today comprises approximately 640 families, 9000 genera, and 93,000 described species, but the current estimate is that untold hundreds of thousands of new mites, substantially fewer spiders, and several thousand species in the remaining orders, are still undescribed (Adis & Harvey 2000, reviewed in Coddington & Colwell 2001, Coddington et ol. 2004). Acari (ticks and mites) are by far the most diverse, Araneae (spiders) second, and the remaining taxa orders of magnitude less diverse. Discounting secondarily freshwater and marine mites, and a few semi-aquatic or intertidal forms, all extant arachnid taxa are terrestrial. Arachnida evidently originated in a marine habitat (Dunlop & Selden 1998, Dunlop & Webster 1999), invaded land independently of other terrestrial arthropod groups such as myriapods, crustaceans, and hexapods (Labandeira 1999), and solved the problems of terrestrialization (desiccation, respiration, nitrogenous waste removal without loss of excess water, and reproduction) in different ways. Although the phylogeny of Arachnida is still controversial (Coddington etcd. 2004), specialists agree that the closest relative of Araneae is a group of orders collectively known as Pedipalpi: Amblypygi, Schizomida, and Uropygi (Shultz 1990).