Common Pill Woodlouse

• It is the best-studied terrestrial isopod in the world: even the details of its mitochondrial DNA are known, which is atypically large (20-24 kb against the usual 15-17).
• It breathes through pseudotracheae, lungs folded into the abdomen that meet 94% of its oxygen needs and that allowed it to colonise dry land.
• The female broods her eggs in a pouch on her underside, and the young emerge from it as though she were giving birth to live offspring.
• It is a land crustacean, not an insect: it still has modified gills and that is why it depends on damp.
• It rolls into a perfect ball when disturbed, a behaviour called conglobation that protects the soft underside and reduces water loss.
• It harbours bacteria of the genus Wolbachia that manipulate its reproduction: they feminise genetic males, which develop as functional females. Three different feminising strains are known.
• That manipulation goes beyond sex: the infection also alters the composition of its microbiota, which runs to more than 200 bacterial genera.
• It is an important decomposer of leaf litter and returns nutrients to the soil.

Skeleton type

Invertebrate - articulated calcareous exoskeleton (crustacean)

Habitat & environment

Damp soils rich in organic matter: parks, gardens, allotments and field margins. It tolerates sunny places better than other woodlice and shelters from the sun under stones or timber

Size at birth

At birth the young lack one of the seven pairs of legs, which grows later

Sexual maturity

They do not breed until a year old

Gestation / incubation

Incubation lasts between 3 and 9 weeks inside the female's pouch

Litter / clutch size

A female can lay more than 200 eggs; a Uruguayan study in a maize crop found an average fecundity of 37 eggs and an average fertility of 28 embryos per clutch

Sources consulted: Animal Diversity Web (Universidad de Michigan); Souty-Grosset et al. 1994; Boletín de la Sociedad Zoológica del Uruguay (biología reproductiva); Real Decreto 630/2013, Catálogo Español de Especies Exóticas Invasoras; Universitat Autònoma de Barcelona (B.E.E.S.)

Best time: All year round, under stones, flowerpots and timber in damp corners of parks and gardens