A woodlouse, also known as a pill bug (Armadillidium only), armadillo bug or sow bug is a terrestrial crustacean with a rigid, segmented, calcareous exoskeleton and jointed limbs. They need moisture and so are usually found in damp, dark places, such as under rocks and logs. They are usually nocturnal and are detritovores, feeding only on dead plant matter.
They belong to the order Isopoda, sub order Oniscidea and there are over 3,000 known species.
In Britian there are 37 native or naturalized species ranging in coloour and in size (3 - 30 mm) of which only five are common - Oniscus asellus (Common shiny woodlouse), Porcellio scaber (Common rough woodlouse), Philoscia muscorum (Common striped woodlouse), Trichoniscus pusillus (Common pygmy woodlouse) and Armadillidium vulgare (Common pill bug).