The Hydracarina, commonly called 'water mites', includes more than 5000 named species worldwide. In Australia there are 413 described species in 89 genera, representing 22 families (Harvey 1998). Larval water mites are 6-legged and are extremely heteromorphic relative to nymphal and adult mites. This key is designed for identification adult mites (which can be recognised by the presence of a genital opening); however, nymphs may be identifiable for some taxa. Males can be differentiated from females by the presence of a complicated group of internal sclerites, the 'ejaculatory apparatus', associated with the genital opening.

Water mites are parasitic on insects as larvae and are predatory as deutonymphs and adults. A few taxa are parasitic on bivalves or crayfish in their postlarval stages (Walter & Proctor 1999).