A woodlouse (plural woodlice) is an isopod crustacean with a rigid, segmented, long exoskeleton and fourteen jointed limbs. Mostly they feed on dead plant material, and they are usually active at night. Woodlice form the suborder Oniscidea within the order Isopoda, with over 5,000 known species. Woodlice in the genus Armadillidium and in the family Armadillidae can roll up into an almost perfect sphere as a defensive mechanism, hence some of the such as pill bug or roly-poly. Most woodlice, however, cannot do this.