Taken primarily from Kozloff, 1987, 1996 p. 330 (Copyright 1987, 1996, University of Washington Press. Used in this web page by permission of University of Washington Press)
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| Lower taxonomic level | Main Page | Alphabetic Index | Systematic Index | Glossary |
| 1a | Adults parasitic on other Crustacea (Cirripedia, Mysidacea, Decapoda): females generally grotesque, often asymmetrical, with most appendages reduced or absent; males small, symmetrical (a male is usually associated with each female) | Suborder Epicaridea |
| 1b | Adults free-living, parasitic on fishes, or commensal with other isopods; females and males recognizable as normal isopods, even if specialized for clinging to fishes | 2 |
| 2a | Uropods ventral to the pleotelson (photo) or decidedly lateral to the pleotelson | 3 |
| 2b | Uropods terminal, close to the midline | 5 |
| 3a | Uropods ventral, forming a valvelike covering over the pleopods (photo) | Suborder Valvifera |
| 3b | Uropods lateral, flattened, and, together with the pleotelson, forming a caudal fan | 4 |
| 4a | Body at least 7 times as long as wide, nearly cylindrical; exopodites of uropods arching over the pleotelson; pereopod 1 larger than the others and subchelate (pereopods 2 and 3 may also be subchelate) | Suborder Anthuridea |
| 4b | Body usually not more than 5 times as long as wide; exopodites of uropods not arching over the pleotelson; pereopod 1 neither especially enlarged nor subchelate (except in the genus Tecticeps, in which it is subchelate) | Suborder Flabellifera |
| 5a | Body slender, at least 7 times as long as wide; pleon with 2 free pleonites plus the pleotelson, these 3 units being nearly equal; pereopod 1 subchelate: generally under 2 mm long; living between sand grains | Suborder Microcerberidea |
| 5b | Body usually robust, much less than 7 times as long as wide; pleon not consisting of 3 nearly equal units (if there are 3, the pleotelson is much larger than the free pleonites); pereopod 1 generally not subchelate (but sometimes subchelate); mostly more than 2 mm long (but some species may be smaller); in various habitats | 6 |
| 6a | Aquatic; pleon generally with not more than 2 free pleonites (sometimes 3) plus the pleotelson; both antenna 1 and antenna 2 well developed | Suborder Asellota |
| 6b | Essentially terrestrial, although certain species are restricted to moist situations near the high tide line; pleon generally with 5 free pleonites plus the pleotelson; antenna 1 rudimentary, with only 2 or 3 articles | Suborder Oniscoidea |
Suborder
Epicaridea
Suborder Flabellifera
Suborder Valvifera
Suborder Oniscoidea