Note: information below is also available under General Information, which can be accessed while running the Key.

BEST WAY TO USE THE KEY

For best results in keying specimens, the authors strongly recommend that users select both Retain Uncertainties and Allow Misinterpretations from the "Key" menu by placing a checkmark beside each option. 

Users will need a dissecting microscope for examination of all taxa, and a compound microscope for many of the mite groups.  This key allows identification of all families of arachnids (and subfamilies for water mites) that are known from Australian inland waters.  Water mite families and subfamilies are identified in a subkey associated with the taxon Hydracarina.

ABOUT THE TAXA

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). 

PREPARATION OF SPECIMENS FOR IDENTIFICATION

Identification of arachnids other than spiders often requires special preparation. 

(a) Spiders, non-hydracarine mites, and the larvae of water mites: specimens from these groups may be preserved in 70-80% ethanol.  Non-hydracarine mites should be cleared in lactic acid or Nesbitt's Fluid, and then placed in glycerol in a cavity slide for examination under a compound microscope.  Permanent mounts can be made with Hoyer's Fluid or polyvinyl alcohol preparations (Krantz 1978). 

(b) Nymphal and adult Hydracarina: these mites often make poor specimens if they are stored in ethanol for longer than a few days. Formalin is an even worse presevative as it makes the body of the mite rigid and difficult to clear.  Ideally, water mites should be killed, preserved and stored in Koenike's Fluid (10% glacial acetic acid, 50% glycerol, 40% water), also known as GAW (Harvey 1998).  This preservative keeps mites soft and flexible, and allows them to retain some of their original colour patterns.  Water mites that have been briefly exposed to ethanol usually make good specimens if they are rapidly transferred to Koenike's Fluid and left for several days to soften. 

Although many families and subfamilies of water mites can be identified from uncleared specimens, most require clearing.  Water mites are cleared in a solution of 10% KOH, and are returned to Koenike's Fluid when the body contents have become transparent.  If examination under a compound microscope is necessary, the body contents of the mite should be expelled through a small hole torn in soft membranes, and the deflated body of the mite arranged in a glycerine-filled cavity slide so that all appendages are visible (this may require dissection).  High quality permanent mounts of water mites are made in glycerine jelly (Cook 1974). 

USEFUL REFERENCES FOR AQUATIC MITES

Cook, D.R. 1976. Water mite genera and subgenera. Mem. Amer. Ent. Inst. 21: 1-860.

Cook, D.R. 1986. Water mites from Australia.  Mem. Amer. Ent. Inst. 40: 1-568.

Harvey, M.S. 1998. The Australian water mites: a guide to families and genera. CSIRO Publishing, Collingwood, Victoria.

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

Walter, D.E. and H.C. Proctor. 1999. Mites: ecology, evolution and behaviour.  University of New South Wales Press, Sydney, New South Wales.

ABOUT THE AUTHORS

Spider section:

composed by Brendan Mckie with helpful comments from Robert Raven (Queensland Museum).

Mite section:

Mark Harvey is Curator of Arachnids at the Western Australian Museum, Perth, WA.  Heather Proctor is a lecturer in the School of Environmental Studies, Griffith University, Brisbane, QLD.  David Walter is a senior lecturer in the Department of Zoology and Entomology, University of Queensland, Brisbane, QLD.