Any of certain terrestrial crustaceans of the order Isopoda, especially members of the genus Oniscus. Native to Europe, they have been introduced into North America. Sow bugs grow to 0.7 in. (18 mm) long. The oval, gray body is rather arched and is covered with broad, armourlike plates. Two elbowed antennae extend about half the length of the body, and there are seven pairs of limbs. The sow bug lives in moist places, especially under stones, in moist leaf litter, and in cellars. See alsopill bug.