A small arachnid with a body in two segments and usually covered by a carapace. Most have four pairs of legs, but some have fewer, and their mouthparts are adapted for sucking, sawing, biting, or stinging. Many lack eyes. There are more than 48,000 species, comprising the subclass Acarina. They are extremely diverse, occupying almost every terrestrial and aquatic habitat. Many are parasites of vertebrates and invertebrates.