Woodlouse

Woodlouse
Oniscus asellus
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Subphylum:	Crustacea
Class:	Malacostraca
Order:	Isopoda
Suborder:	Oniscidea Latreille, 1802
Infraorders and Families
Infaorder/Section Diplocheta Ligiidae Infraorder Holoverticata Section: Tylida Tylidae Section: Microcheta Mesoniscidae Section: Synocheta Schoebliidae Styloniscidae Titaniidae Trichoniscidae Turanoniscidae Section: Crinocheta Bathytropidae Berytoniscidae Detonidae Halophilosciidae Olibrinidae Oniscidae Philosciidae Platyarthridae Pudeoniscidae Rhyscotidae Scyphacidae Speleoniscidae Sphaeroniscidae Stenoniscidae Tendosphaeridae Actaeciidae Armadillidae Armadillidiidae Atlantidiidae Balloniscidae Cylisticidae Eubelidae Periscyphicidae Porcellionidae Trachelipodidae Dubioniscidae Hekelidae Irmaosidae Pseudarmadillidae Scleropactidae

A woodlouse (known by many common names: see below; 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.^[1]

Ecology

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.^[2]

The woodlouse has a shell-like exoskeleton, which it must progressively shed as it grows. The 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, which 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 woodlouse are able to roll into a ball-like form when threatened by predators, leaving only their armoured back exposed. This ability 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.^[3]

Aquatic isopods

Ligia exotica, a semi-terrestrial woodlouse

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).^[4]

Common names

Common names for woodlice vary throughout the English-speaking world. A number of common names make reference to the fact that some species of woodlice can roll up into a ball. Other names compare the woodlouse to a pig.

Names include: "armadillo bug",^[5] "carpenter" (Newfoundland),^[6] "cheeselog" (Reading, Berkshire),^[7] "cheesy bug" (North-West Kent),^[8] "doodlebug" (also used for the larva of an antlion),^[9] "pill bug" (usually applied only to the genus Armadillidium),^[5] "potato bug",^[10] "roly-poly",^[10] "sow bug",^[3] "roll up bug",^[11] "chuggypig" or "chucky pig",^[12] "slater",^[13] "gramersow" (Cornwall),^[14] "butcher boy" or "butchy boy" (Australia),^[15] and "wood bug" (British Columbia, Canada).^[16]

Pillbugs and pill millipedes

Comparison of the pill bug Armadillidium vulgare and the pill millipede Glomeris marginata

Pillbugs (woodlice of the family Armadillidiidae) can be confused with pill millipedes. Although they are distantly related to one another, the two are taxonomically distinct.

Both of these groups of terrestrial segmented arthropods are about the same size. They live in very similar habitats, and they can both roll up into a ball. Pill millipedes and pillbugs appear superficially similar to the naked eye. This is an example of convergent evolution.

Pill millipedes can be distinguished from woodlice on the basis of having two pairs of legs per body segment instead of one pair like all isopods. Pill millipedes also have thirteen body segments, whereas the woodlouse has eleven. In addition, pill millipedes are smoother, and resemble normal millipedes in overall colouring and the shape of the segments.

British Isles

There are over 40 native or naturalised species of woodlouse in the British Isles, ranging in colour and in size (3–30 mm) of which only five are common: Oniscus asellus (the common shiny woodlouse), Porcellio scaber (the common rough woodlouse), Philoscia muscorum (the common striped woodlouse), Trichoniscus pusillus (the common pygmy woodlouse), and Armadillidium vulgare (the common pill bug).

References

Further reading

• Christian Schmidt & Andreas Leistikow (2004). "Catalogue of the terrestrial Isopoda (Crustacea: Isopoda: Oniscidea)". Steenstrupia 28 (1): 1–118. http://www.zmuc.dk/commonweb/JOURNALS/PDF/Vol28-1/Schmidt&Leistikow.pdf.  (all genera published until end of 2001)