Taken primarily from Kozloff, 1987, 1996 (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 | Flagellum of antenna 2 with more than 10 articles (but there may be as few as 6 articles in immature specimens) (picture); up to about 2.5 cm long; found in rock crevices (and sometimes on pilings) just above the high tide line | 2 |
| 1b | Flagellum of antenna 2 with not more than 4 articles; less than 2 cm long; not typically found in rock crevices (generally on uppermost portions of sancy beaches or truly terrestrial) | 3 |
| 2a | Basal article of uropod about as broad as long (picture) | Ligia (Ligia) pallasii |
| 2b | Basal article of uropod several times as long as broad (rarely found as far north as Oregon) | Ligia (Megaligia) occidentalis |
| 3a | Flagellum of antenna 2 with 3 or 4 articles; tracheal system absent; under debris on sandy beaches, near uppermost limits of intertidal zone | 5 |
| 3b | Flagellum of antenna 2 with 2 articles; tracheal system present in exopodites of pleopods 1 and 2; truly terrestrial, though occasionally found close to the uppermost limits of the intertidal zone | 4 |
| 4a | Capable of rolling up into a ball; antenna 2 not extending posteriorly beyond pereonite 2; pleotelson blunt | Armadillidium vulgare |
| 4b | Not capable of rolling up into a ball; antenna 2 not extending posteriorly beyond pereonite 2; pleotelson pointed | Porcellioscaber |
| 5a | Flagellum of antenna 2 with 3 articles | 6 |
| 5b | Flagellum of antenna 2 with 4 articles | 7 |
| 6a | Pleonites becoming gradually narrower than the pereonites; cephalon with large and tuberculatemedian and anterolateral lobes; antenna 2 not extending beyond pereonite 2 | Alloniscus perconvexus |
| 6b | Pleonites becoming abruptly narrower than the pereonites; cephalon rounded in front and with only small lateral lobes, if any; antenna 2 long, extending beyond pereonite 2 | Littorophiloscia richardsonae |
| 7a | Anterior margin of cephalon deeply 3-lobed, the median lobe forming a rostrum; basal article of uropod expanded, forming a large lobe lateral to the endopodite and nearly reaching the distal tip of the latter | Armadilloniscus holmesi |
| 7b | Anterior margin of cephalon 2-lobed, without a rostrumlikemedian lobe; basal article of uropod not expanded, the endopodite inserted upon it in a posterolateral position | Detonella papillicornis |
