Velvet mites

The Trombidioidea and Erythraeoidea are superfamilies of mites in the Suborder Prostigmata in the Order Acariformes. They include a number of families of terrestrial mites in the Cohort Parasitengona, the group to which water mites (Hydracarina) also belong. No trombidioids or erythraeoids are truly subaquatic, but many species live near water and may venture out on the surface of ponds, bogs and marshes. They are often termed 'velvet mites' because many species possess a dense pelage of velvety hairs. However, some are more poorly furred and may be mistaken for water mites. Conversely, some water mites may be mistaken for trombidioids; the Australian genus Austrotrombella was originally placed in the trombidioid family Trombellidae, but was moved to the water mite family Hydryphantidae by Harvey (1996).

How does one differentiate a non-velvety parasitengone from a water mite' Most trombidiods have a distinctive 'thumb-claw complex' in which the palpal tarsus is inserted near the middle or the base of the tibia; water mites that have similar grasping palps always have the tarsus inserted distally. Water mites also possess unique structures termed 'glandularia'. A glandularium is a combination of a trigger hair and a gland opening, often combined in a sclerite. No other parasitengone group possesses glandularia.

References:

Harvey, M.S. 1996. A revised placement for Austrotrombella Southcott (Acarina: Hydryphantidae). Trans. Roy. Soc. S. Aust. 120: 37-40.

Krantz, G.W. 1978. A Manual of Acarology. 2nd edition. Oregon State University Book Stores, Corvallis, Oregon.