any of certain small, terrestrial crustaceans of the order Isopoda, especially members of the genus Oniscus. Like the related pill bug, it is sometimes called the wood louse. O. asellus, which grows to a length of 18 mm (0.7 inch), is widely distributed in Europe and has also been introduced into North America. The oval, gray body, which is rather flattened and arched, is covered with broad, armourlike plates. Two elbowed antennae extend about half the length of the body, and there are seven pairs of limbs. This species is found under stones, in moist leaf litter, and in cellars.