Wood - Louse, a name commonly applied to certain terrestrial Crustacea of the order Isopoda (see Malacostraca), which are found in damp places, under stones or dead leaves, or among decaying wood. They form the tribe Oniscoidea and are distinguished from all other Isopoda by their habit of living on land and breathing air, and by a number of structural characters, such as the small size of the antennules and the absence of the mandibular pulp. As in most Isopods, the body is flattened, and consists of a head, seven thoracic segments which are always free, and six abdominal segments which may be free or fused. The "telson" is not separated from the last abdominal segment. The head bears a pair of sessile compound eyes as well as the minute antennules and the longer antennae. Each of the seven thoracic segments carries a pair of walking legs. The appendages of the abdomen (with the exception of the last pair) are flat membranous plates and serve as organs of respiration. In many cases their outer branches have small cavities opening to the outside by slit-like apertures, and giving rise internally to a system of ramifying tubules filled with air. From their similarity to the air tubes or tracheae of insects and other air-breathing Arthropods these tubules are known as "pseudotracheae." The female wood-louse carries her eggs, after they are extruded from the body, in a pouch or "marsupium" which covers the under surface of the thorax and is formed by overlapping plates attached to the bases of the first five pairs of legs. The young, on leaving this pouch, are like miniature adults except that they are without the last pair of legs. Like all Arthropoda, they cast their skin frequently during growth. As a rule the skin of the hinder half of the body is moulted some days before that of the front half, so that individuals in process of moulting have a very peculiar appearance.
Some twenty-four species of wood-lice occur in the British Islands. Some, like the very common slaty-blue Porcellio scaber, are practically cosmopolitan in their distribution, having been transported, probably by the unconscious agency of man, to nearly all parts of the globe. Equally common is the brown, yellow-spotted Oniscus asellus. Armadillidium vulgare belongs to a group which have the power of rolling themselves up into a ball when touched and resembles the millipede Glomeris. It was formerly employed in popular medicine as a ready-made pill. The largest British species is Ligia oceanica, which frequents the sea-shore, just above highwater mark. In many points of structure, for instance in the long, many-jointed antennae, it is intermediate, as it is in habits, between the truly terrestrial forms and their marine allies. Finally, one of the most interesting species is the little, blind, and colourless Platyarthrus hof'mannseggi, which lives as a guest or commensal in the nests of ants. (W. T. CA.)
