Acari (mites and ticks) is a subclass of arthropods closely related to spiders. Most of these creatures are tiny measuring 0.08–1mm in length, but some may reach lengths of 10-20mm.
Most mites and ticks have 8 legs, like their spider relatives, but some species have fewer. The head and thorax (cephalothorax in spiders) is broadly fused together with the abdomen, so they are similar to harvestmen (daddy-longlegs) in this way. Mites and ticks have well adapted mouthparts for feeding. Some have tubes for piercing and sucking, others have pincers for grasping and cutting, whilst others have combs for filter-feeding.
Many mites and ticks are parasites to man and other animals, and other invertebrates. Ticks mainly feed on the blood of mammals and birds, and can cause serious illness by spreading diseases, including causing serious conditions in humans and livestock. Plant mites feed on the juice of plants, whilst others feed on fungi.