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Suborder Oniscidea - Woodlice

Pillbug - Armadillidium vulgare Alloniscus perconvexus Woodlouse IMG_7732 - Haplophthalmus danicus Another Elk Grove Woodlouse - Porcellio laevis Isopoda - Unknown, 1a - Cylisticus convexus Isopod Common Pillbug - Armadillidium vulgare? - Armadillidium vulgare Unknown Synocheta
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
Explanation of Names
Oniscidea Latreille 1802
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 Maryland spp. in (3)
key to NA families in (4)
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.
Primary decomposers of leaf litter and secondary decomposers of each other's dung.(5)
Remarks
Pillbugs/sowbugs breathe through gills. This is why they need moisture, but will drown if submerged
Oniscidea contains almost all known terrestrial isopods. The order Phreatoicidea, which mainly occurs in areas formerly part of Gondwana, also has a few semiterrestrial species (especially in Phreatoicopsidae).
Water conducting channels run around the body and beneath the surface of each body segment. Urine is recycled through these channels an additional water from the environment is taken up as needed by capillary action. Ammonia from the urine evaporates and oxygen is readily absorbed at the posterior end of each animal by its pelopods, which are remnants from whenn these were aquatic. Beneath the outer surface of the pleopods are gills that are bathed by the recycled wataer that constantly circulates around their bodies.(5)
Some woodlice also have oxygen-absorbing surfaces on their pleopods that look and act like tiny lungs. These take oxygen directly from the air.(5)
Internet References
Taxon profile & key to littoral spp. in CA (Brusca et al. 2001)
info on color morphs (mainly of captive species but also native ones)(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.American Isopod and Myriapod Group
5.The World Beneath our Feet: A Guide to Life in the Soil
James B. Nardi. 2003. Oxford University Press.