Pill woodlouse  (Armadillidium vulgare)

Common and widespread (1).

Woodlice are not insects, but are crustaceans; more closely related to crabs and shrimps than insects. The body is divided into three main regions, the head, the thorax (known in woodlice as the 'pereion'), and the abdomen ('pleon') (2). The pill woodlouse is so called because it is able to roll into a ball when threatened; it is often confused with the pill millipede (Glomeris marginata) for this reason (3). This woodlouse is typically slate grey in colour, but red or patchy forms may arise (2).

This species is very common in the south-east of England, is found in parts of western and northern England and becomes rare in Scotland (2).

Occurs only on calcareous soils, except in coastal areas (2), and is able to withstand much drier conditions than most other woodlice (3). It shows a distinct preference for chalky or limestone sites with stony turf (2).

Woodlice feed on dead organic matter, which they detect by means of taste and smell (2). During the breeding season, reproductive females develop a 'brood pouch', which consists of overlapping leaf-like structures known as 'oostegites', that form a 'false floor' below the body. The fertilised eggs pass into this fluid-filled chamber, and the young crawl out of the brood pouch when they are fully developed.

Woodlice undergo a series of moults before reaching maturity, growing at each stage; the stages between these moults are known as 'stadia', and are generally similar in structure and appearance. Mature woodlice continue to moult. Prior to moulting, the calcium contained in the old cuticle is removed and stored as conspicuous white blotches, these blotches disappear after moulting as the calcium is used to reinforce the new cuticle (2). The rear part of the body moults a few days before the front half, and occasionally woodlice may be seen with half a pinkish body and half a 'usual' grey body for this reason (4). The discarded cuticle is frequently eaten by the newly moulted woodlouse (2).

Not currently threatened.

Conservation action has not been targeted at this common species.

For more on invertebrates and their conservation see Buglife, the Invertebrate Conservation Trust at:
http://www.buglife.org.uk/

• Abdomen: in arthropods (crustaceans, insects and arachnids) the abdomen is the hind region of the body, which is usually segmented to a degree. In crustacea (e.g. crabs) the limbs attach to the abdomen; in insects the limbs are attached to the thorax (the part of the body nearest to the head) and not the abdomen. In vertebrates the abdomen is the part of the body that contains the internal organs (except the heart and lungs).
• Calcareous: containing free calcium carbonate, chalky.
• Thorax: part of the body located near the head in animals. In insects, the three segments between the head and the abdomen, each of which has a pair of legs.

1. National Biodiversity Network Species Dictionary (March 2003): http://www.nhm.ac.uk/nbn/
2. Sutton, S. L. (1972) Invertebrate types: woodlice. Ginn & Company Ltd., London.
3. Nichols, D., Cooke, J. & Whiteley, D. (1971) The Oxford Book of Invertebrates. Oxford University Press, Oxford.
4. Woodlice online (March 2003): http://www.geocities.com/CapeCanaveral/Hangar/7649/wliceod.htm