Almost all adult arachnids have eight legs, and arachnids may be easily distinguished from insects by this fact, since insects have six legs. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception. The first pair, the chelicerae, serve in feeding and defense. The next pair of appendages, the pedipalps have been adapted for feeding, locomotion, and/or reproductive functions. In Solifugae, the palps are quite leg-like, so that these animals appear to have ten legs. The larvae of mites and Ricinulei have only six legs; the fourth pair appears when they moult into nymphs. However, there are also adult mites with six, or even four legs
Source Wikipedia http://en.wikipedia.org/wiki/Arachnid accesed on 30/10/13
Source Wikipedia http://en.wikipedia.org/wiki/Arachnid accesed on 30/10/13