Common woodlouse  (Oniscus asellus)

Common and widespread throughout Britain (2).

The common woodlouse is one of the commonest and widely spread of the British woodlice (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 common woodlouse is typically grey with irregular light patches, but yellow and orange forms may occur near to the sea (2). The surface of the body is dotted with raised blotches; adults usually have a glossy body, but in contrast juveniles often have a rough body texture (2). There are currently two recognised subspecies of the common woodlouse, Oniscus asellus asellus and O. asellus occidentalis, which differ in the details of their appearance and ecology (3).

Ubiquitous throughout Britain, the common woodlouse (subspecies Oniscus asellus asellus) is one of the most widespread and common terrestrial arthropods in western Europe (3). The subspecies O. asellus occidentalis is found mainly in the south-west of Britain and western France (3).

Occurs in moist places in many habitats, and is frequently found under bark and amongst leaf litter in gardens and woodlands (1). This species avoids dry habitats, and unlike many woodlice, it can tolerate acid soils (2).

Woodlice feed on dead organic matter, which they detect by means of taste and smell (2). The common woodlouse is gregarious, and typically spends the day concealed beneath stones, logs and other objects. When threatened, this species defends itself by clamping down onto the surface; the feet can grip the substrate very tightly, and this woodlouse is able to cling on tenaciously (2).

Mating tends to take place at night, and is very rarely observed for this reason. When a male finds a receptive female, he climbs onto her back and drums her with his front legs whilst 'licking' her head with his mouthparts. He moves to one side of the female, bending his body beneath hers, and transfers sperm to one of the female's genital openings. He then moves to the other side and transfers sperm to the remaining genital opening (2). During the breeding season, reproductive females develop a 'brood pouch', which consists of overlapping leaf-like structures known as 'oostegites', which 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. They 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 threatened at present.

No conservation action has been targeted at this 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).
• Subspecies: a different race of a species, which is geographically separated from other populations of that species.
• 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 (Jan 2003): http://www.nhm.ac.uk/nbn/
2. Sutton, S. L. (1972) Invertebrate types: woodlice. Ginn & Company Ltd., London.
3. Bilton, D. T., Goode, D. & Mallet, J. (1999) Genetic differentiation and natural hybridisation between two morphological forms of the common woodlouse, Oniscus asellus Linnaeus 1758. Heredity, 82: 462-469.
4. Woodlice online (March 2003): http://www.geocities.com/CapeCanaveral/Hangar/7649/wliceod.htm