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Suborder Oniscidea - Woodlice

woodlouse - Oniscus asellus Armadillidium vulgare Spwbug under bark on ground - Armadillidium vulgare Isopod - Porcellio scaber Woodlouse - Porcellio scaber perhaps Armadillidium vulgare - Armadillidium vulgare Freshwater Isopod Maybe ? - Porcellio Isopod ID request - Porcellionides
Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Crustacea (Crustaceans)
Class Malacostraca (Malacostracans)
Superorder Peracarida (Marsupial Crustaceans)
Order Isopoda (Isopods)
No Taxon (Scutocoxifera)
Suborder Oniscidea (Woodlice)
Other Common Names
Land Isopods
Numbers
~100 spp. in 37 genera of 15 families in our area, incl. 27 adventive spp. in 20 genera of 12 families(1), close to 4,000 spp. total(2)
Identification
The number of flagellomeres, the shape and size of the uropods+telson, the dorsal color pattern and the number of lungs (white patches under the abdomen) are often important in identifying woodlice.
"Pillbugs" are a vague group of woodlice: includes many unrelated taxa that just share the ability to roll into a ball. In our area, the only Pillbugs are in the families Armadillidae and Armadillidiidae, both which have the uropods flattened and about as long as the telson (the triangular- or hourglass-shaped terminal segment) and special grooves on the underside of the body that allow them to tuck their antennae inside the ball formed when rolling up. All other woodlice in North America have longer pointier uropods projecting past the hind margin of the body. Compare Porcellio scaber (a non-pilling woodlouse) and Armadillidium nasatum, a pillbug) here:

Armadillidae can be told from Armadillidiidae by the shape of the telson: in Armadillidae the telson is hourglass-shaped, while in Armadillidiidae the telson is rhomboidal. Geographic location also helps: Armadillidae is genrally restricted to southern regions (or greenhouses in northern regions) while Armadillidididae is common across most of the continent.
One other species in North America also pills up: Cylisticus convexus has an odd domed body adapted to curling up into a ball, but the ball is incomplete due to the long pointy uropods and the antennae are held outside of the ball.
key to 22 spp. found in MD in (3)
Range
worldwide
Habitat
wherever cool, dark, moist places are available to shelter woodlice from dryness and heat during the day
Food
Plant material, usually dead. If live plants are soft and moist enough on the outside, they will eat them and sometimes do damage.
Remarks
Pillbugs and sowbugs breathe through gills. This is why they need moisture, but will drown if submerged
See Also
Insects have 6 legs (Isopods have 14 in most species, rarely as few as 10)
Internet References
Taxon profile & key to littoral spp. in CA (Brusca et al. 2001)
so. CA spp. checklist(4)
Works Cited
1.Checklist of the terrestrial isopods of the New World (Crustacea, Isopoda, Oniscidea)
Leistikow A., Wägele J.W. 1999. Rev. Bras. Zool. 16: 1-72.
2.World catalog of terrestrial isopods (Isopoda: Oniscidea)
Schmalfuss H. 2003. Stuttgarter Beiträge zur Naturkunde, Serie A, 654: 341 pp.
3.A guide to the identification of the terrestrial Isopoda of Maryland, U.S.A. (Crustacea)
Shultz J.W. 2018. ZooKeys 801: 207-228.
4.Wright J. (yyyy) Southern California Oniscidea