The Acariformes are the most diverse of the two superorders of mites. There are over 32,000 described species in 351 families, and a total estimates of 440,000 to 929,000.
Another group often mentioned is the Actinedida. But in treatments like the present one this is split up between the Sarcoptiformes (and formerly the separate Endostigmata) and Trombidiformes (which contains the bulk of the "Actinedida"), because it appears to be a massively paraphyletic "wastebin taxon", uniting all Acariformes that are not "typical" Oribatida and Astigmata. The Trombidiformes present their own problems. The small group Sphaerolichida rather uncontroversially appears to be the most ancient lineage among them. However, the Prostigmata are variously subdivided into the Anystina and Eleutherengona, and Eupodina. The delimitation and interrelationships of these groups are entirely unclear; while most analyses find one of the latter two but not the other to be a subgroup of the Anystina, neither of these mutually contradicting hypotheses is very robust; possibly this is a simple error because phylogenetic software usually fails in handling non-dichotomous phylogenies. Consequently it may be best for the time being to consider each of the three main prostigmatan lineages to be equally distinct from the other two.
The Trombidiformes are most noted for the economic damage caused by many plant parasite species. All of the most important plant pests among the Acari are Trombidiformans, such as spider mites and eriophyid mites. Many species are also predators, fungivores and animal parasites. Some of the most conspicuous species of free-living mites are the relatively large and bright red velvet mites (see photo), that belong to the family Trombidiidae.
Oribatid mites and to a much lesser extent others are a source of alkaloids in poison frogs (namely small species like the Strawberry Poison-dart Frog Oophaga pumilio). Such frogs raised without these orbatids in their diet do not develop the strong poisons associated with them in the wild.