Almost all extant arachnids are terrestrial , living mainly on land. However, some inhabit freshwater environments and, with the exception of the pelagic zone , marine environments as well. They comprise over 100,000 named species .
Arachnid , (class Arachnida), any member of the arthropod group that includes spider s, daddy longlegs , scorpion s, and (in the subclass Acari ) the mite s and tick s, as well as lesser-known subgroups. Only a few species are of economic importance—for example, the mites and ticks, which transmit diseases to humans, other animals, and plants.