Other Common Names
cressbug, pillbug, sowbug, woodlouse, rock slater, roly-poly
Numbers
>10,200 spp. worldwide
(1) (but probably much more, many checklists seem to underestimate the number of recorded species), >1200 in our area; 11 suborders total, 7 represented in North America
Size
2 to >300* mm (*Bathynomus, a deep sea genus)
Identification
Key to suborders of our fauna
1. Parasitic species, often grossly modified in adaptation for a parasitic lifestyle →
Cymothodea (in part)
Freeliving species, usually resembling a typical isopod → 2
2. Uropods set at the tip of the telson → 3
Uropods set at the sides of or folded under the telson → 4
3. Pleopod 3 fused into 1 piece; ♀ pleopod 2 absent; 9 segments visible between the head and telson (7 pereonites + 2 pleonites), the last two broad and reaching the margin of the body; cave, phreatic and interstitial habitats → Microcerberidea [in our fauna only very elongate, worm-like forms]
Pleopod 3 with seperate branches; ♀ pleopod 2 present; 7-9 segments visible between the head and telson (7 pereonites + 0-2 pleonites); almost all potential aquatic habitats including freshwater and intersitial →
Asellota
4. First pair of antennae almost completely reduced, appearing as 2 nearly microscopic structures between the main pair of antennae; fully terrestrial air-breathing isopods with a water-conducting system around the bases of the legs and pleopodal lungs, sometimes in aquatic habitats →
Oniscidea
First pair of antennae normal or reduced but never to the point of invisibility; aquatic species, sometimes in the intertidal zone but not air-breathing → 5
5. Uropods neatly folded under the telson, covering the pleopods in a protective case, not normally visible in dorsal view →
Valvifera
Uropods set to the side of the telson to form a tail fan with the telson, occasionally folded under but never encasing the pleopods in a protective cover, usually visible in a dorsal view → 6
6. Uropods reduced peg-shaped; minute woodborers or sea grass stemborers → Limnoriidea
Uropods larger and flattened; habits various, if wood-boring then much larger and pilling→ 7
7. Pleonites fused into 1 segment (8 free segments between the head and telson); telson domed with ventrolateral ridges making a branchial chamber; body under 6x as long as wide and arch- or pancake-shaped →
Sphaeromatida
3-5 free pleonites, or if all pleonites fused into 1 segment then body longer than 6x as long as wide or longer and subcylindrical; telson flat →
Cymothodea (in part)
Habitat
from deep sea to terrestrial habitats
See Also
Amphipods are usually laterally compressed (although some aquatic isopods can be too) and have 3 sets of uropods
Insects only have six legs; the isopod group with the least number of legs (Gnathiidae) has 10 legs and most isopods have 14 legs
Myriapods have numerous leg pairs and lack the strong tagmosis (formation of distinct "zones" of modified body segments) that occurs in isopods
