Mites and ticks are the most diverse arachnids and it has become commonplace to recognise an acariform and a parasitiform group; the latter including the predatory mites and ticks. All mites have a gnathosoma: a moveable unit at the front of the body which includes the mouth lips, chelicerae and pedipalps. Adult body size ranges from minute (a few hundred micrometres) up to about 40 mm in engorged ticks. They are found throughout the world in a wide range of terrestrial, and sometimes aquatic, habitats. As a group they show a highly diverse range of ecologies. There are approximately 55,000 described species. Fossil acariforms date from the Devonian: fossil parasitiforms from the Cretaceous.