Numbers
12 orders
450 families
75,500 species
Orders:
Amblypygi - Tailless Whipscorpions
Palpigradi - Microscorpions
Pseudoscorpiones - Pseudoscorpions
Ricinulei - Hooded Tickspiders
Schizomida - Short-tailed Whipscorpions
Solifugae - Windscorpions
Subclass
Acari - Mites and Ticks
Identification
One or two body parts
Four pairs of legs (usually - although immature ticks, for example, can have three pairs of legs)
No antennae
Internet References
Common Names of Arachnids 2003 Fifth Edition - The American Arachnological Society; Committee on Common Names of Arachnids