The taxon-group of Acari has most number of valid species (almost 55,000) in Arachnida, however Acari was disappeared in textbooks of invertebrate nowadays. Acariformes and Parasitiformes sensu lato are used as separated taxon-groups instead of Acari. Although two-name system, tick (sucking blood) and mite (others) were common for meaning Acari in some countries, Japanese use one word “dani” for Acari. French use three categories, tick, mite and ciron (ceron). The “ciron” means cheese mites (and some small bugs) and was used as the symbol of minimums in some French literature (e.g. Fables by Jean de la Fontaine in 1668, Pensées by Blaise Pascal in 1670). Acarine species are recorded almost 2,000 in Japan, and 1% (almost 20 species) of them are harmful as common sucking blood ticks. Acari have diverse eating habits, while other members of Arachnida are only predators. The diversity of Acarine eating habits may have maintained species diversity of them. Oribatida as a decomposer has various physical appearance. The much morphological diversity is a strategy to defend against predators. The oribatid mite have not only morphological defense but also chemical defense as chemical secretion from opisthonotal glands and physical defense as jumping. These various defense strategies are also helpful in maintaining diverse species.