A woodlouse (known by many common names;, (plural woodlice) is a crustacean with a rigid, segmented, long exoskeleton and fourteen jointed limbs. Woodlice form the suborder Oniscidea within the order Isopoda, with over 3,000 known species.
Woodlice in the genus Armadillidium can roll up into an almost perfect sphere as a defensive mechanism, hence some of the common names such as pill bug or roly-poly. Most woodlice, however, cannot do this
Porcellio scaber (left) and Oniscus asellus (centre) living on fallen wood Woodlice need moisture because they breathe through gills, called pseudotrachea, and so are usually found in damp, dark places, such as under rocks and logs. They are usually nocturnal and are detritivores, feeding mostly on dead plant matter, although they have been known to feed on cultivated plants, such as ripening strawberries and tender seedlings. Woodlice then recycle the nutrients back into the soil. In artificial environments such as greenhouses where it can be very moist, woodlice may become abundant and damage young plants.
The woodlouse has a shell-like exoskeleton, which it must progressively shed as it grows.
he moult takes place in two stages; the back half is lost first, followed two or three days later by the front. This method of moulting is different from that of most arthropods, who shed their cuticle in a single process.
A female woodlouse will keep fertilised eggs in a marsupium on the underside of her body until they hatch into small, white offspring. The mother then appears to "give birth" to her offspring.
Some species of woodlice are able to roll into a ball-like form when threatened by predators, leaving only their armoured back exposed. This ability, or dominant behavior, explains many of the woodlouse`s common names.
Metabolic rate is temperature dependent in woodlice. In contrast to mammals and birds, invertebrates are not "self heating": the external environmental temperature relates directly to their rate of respiration. They are not generally regarded as a serious household pest as they do not spread disease and do not damage wood or structures; however, their presence can indicate dampness problems.
Woodlice are eaten by a wide range of insectivores, but the only animals known to prey exclusively on woodlice are spiders of the genus Dysdera, such as the woodlouse spider Dysdera crocata.
Although woodlice are terrestrial crustaceans, several forms have returned to water. Although most of these are amphibious, some have become aquatic, such as the sea slater (Ligia oceanica), which belongs to family Ligiidae.
Other examples include some Haloniscus species from Australia (family Scyphacidae), and in the northern hemisphere several species of Trichoniscidae and Thailandoniscus annae (family Styloniscidae). Species for which aquatic life is assumed include Typhlotricholigoides aquaticus (Mexico) and Cantabroniscus primitivus (Spain).