A class in the Chelicerata that contains over 100,000 species, and includes the mites (Acari) and spiders. Almost all species are terrestrial, with 2 pairs of anterior appendages: the inner chelicerae, and the outer palpi. The body is divided into the anterior prosoma (with usually 4 pairs of legs) and the posterior abdomen. Most arachnids are predators. In many species there is no copulation, the sperm being delivered to the female within a spermatophore.