The Arachnida belongs to the Phylum Arthropoda, Subphylum Chelicerata, and is typically placed at the level of Class. Arachnids are characterised by having 4 pairs of legs as adults and lacking antennae. Their mouthparts are chelicerae; however, only some arachnids retain the ancestral chelate (pincer-like) form of the chelicerae, while other groups have hook-like or styletiform mouthparts.

There are two major groups of arachnids that have aquatic representatives: the spiders (Order Araneae ) and the mites (Subclass Acari ). Species from four families of spiders may be encountered in Australian fresh waters. Despite being able to survive for some time under water, none of these spiders is truly subaquatic.

The vast majority of aquatic arachnids are mites. Larval mites are 6-legged and are often morphologically very different from the 8-legged nymphal and adult mites. This key is designed for identification of post-larval stages; however, it may work for larvae of some groups of mites. Representatives of four major suborders of mites occurs in fresh water: Mesostigmata , Oribatida , Astigmata and Prostigmata (Walter & Proctor 1999). The greatest radiation of freshwater mites has been in the prostigmatan group Hydracarina (also called Hydrachnellae, Hydrachnidia, Hydrachnida). 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).