Naiadacarus arboricola Fashing, is reported from adults collected from water-filled treeholes and deutonymphs phoretic on Mallota species, Eristalis bardus and Somula decora (Diptera: Syrphidae) from Michigan, Illinois, and New York. Naiadacarus fashingi, new species, is described from deutonymphs phoretic on Brachypalpus oarus and seven other species of Syrphidae from Michigan, New York and New Jersey. Naiadacarus mydophilus, new species, is described from deutonymphs phoretic on Brachypalpus oarus from Michigan. Based upon known biologies of host insects and other mite species which utilize these hosts, it is hypothesized that N. lashingi and N. mydophilus may inhabit wet decaying wood andior moist treeholes rather than water-filled treeholes. Naiadacarus is hypothesized to be closely related to the genera Schwiebea, Histiogaster and Thyreophagus. The subfamily name Naiadacarinae is placed in synonymy with Rhizoglyphinae
