What subclass of arachnids includes ticks and mites?

The subclass of arachnids that includes ticks and mites is Acari. Members of this group are often called acarines, and they belong to the class Arachnida alongside spiders, scorpions, harvestmen, and other relatives.

Acari is a remarkably diverse group. Its members include free-living soil mites, parasitic mites, dust mites, chiggers, and ticks. Ticks are commonly treated as part of Acari even though they form the distinct order Ixodida; mites are spread across many other orders and lineages. The two major divisions traditionally discussed within Acari are Acariformes and Parasitiformes.

A common mistake is choosing Ixodida, which refers specifically to ticks rather than both ticks and mites. Parasitiformes is also too narrow: it includes ticks and some mite lineages, but not all mites. Arachnida is the broader class containing Acari, not the subclass itself. Acari is also sometimes written as Acarina in older sources.