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Isopoda are malocostracan crustaceans with a 7 segmented thorax, 5 or fewer visible abdominal segments, and no carapace. The antennules are biramous, the antennae have the expodite (outer branch) scale-like. The thoracic appendages are uniramous and leg-like and in some families the first pair are enlarged and chelate. The thoracic appendages lack gills, but gill structures commonly are present on the abdomen. In overall structure Isopoda are very similar to Amphipoda. The main distinguishing features are (i) when gills occur in Isopoda they are on the abdomen but in Amphipoda on the thorax, and (ii) Isopoda have a single pair of backward-facing appendages (uropods) on the terminal abdominal segment, Amphipoda have 3 pairs, though the third is reduced in one family. The uropods of Isopoda are biramous (two-branched) and the basal piece (peduncle) may be short and squat or long and cylindrical. The overall first impression can be of two biramous leg-like appendages or of four backward-facing structures either like legs, like flattened or fan-shaped paddles, or as widely separated triangular small points. Twelve isopod aquatic families are recognised by Horwitz et al., 1995. Five of these (Cirolanidae, Sphaeromatidae, Janiridae, Oniscidae and Styloniscidae) are flat-bodied forms of a generalised 'pill-bug' appearance. The flat isopods are not readily mistaken for anything else and identification to family is relatively straightforward. Two other families (Anthuridae, Microparasellidae) are elongate but because they have a reduced number of abdominal segments they are quite readily distinguished. All seven of these families mainly comprise marine or terrestrial species and although they can occur in freshwater, none are very common in aquatic samples. The remaining five families (Phreatoicidae, Hypsimetopidae, Phreatoicopsidae, Amphisopidae, Mesamphisopidae) comprise the superfamily Phreaticoidea or suborder Phreatoicidea. These are elongate isopods with the full complement of five visible abdominal segments, and they are the most commonly met isopods of Australian inland waters. Phreatoicids occur throughout the south of the continent (also overseas in South Africa, India, South America and New Zealand) but not in the northern half of mainland Australia. They are particularly common in Tasmania where they can be found in almost any body of water. Phreaticoid isopods are approximately cylindrical, or else in life are curled ventrally as in a typical Amphipod. Some species are moderately flattened from side to side. The division into families is not well established. We follow the most recent conventionally-published system, that of Horwitz et al. (1995), but note that in the phylogeny given by Wilson (1999) all but the Phreatoicidae are merged into a single family, the Amphisopodidae. References: Horwitz, P., Knott, B. and Williams, W.D. (1995) A Preliminary Key to the Malacostracan Families (Crustacea) found in Australian Inland Waters . Co-operative Research Centre for Freshwater Ecology Identification Guide No. 4., Albury, NSW. Williams, W.D. (1980) Australian Freshwater Life: The Invertebrates of Australian Inland Waters. The Macmillan Company of Australia, Melbourne.
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