Appearance''Armadillidium vulgare'' may reach a length of 18 millimetres , and is capable of rolling into a ball when disturbed; this ability, along with its general appearance, gives it the name ''pill-bug'' and also creates the potential for confusion with pill millipedes such as ''Glomeris marginata''. It can be distinguished from ''Armadillidium nasatum'' and ''Armadillidium depressum'', the only other British species in the genus, by the gap that ''A. nasatum'' and ''A. depressum'' leave when rolling into a ball; ''A. vulgare'' does not leave such a gap.
DistributionThe native distribution of ''A. vulgare'' ranges across Europe, especially in the Mediterranean Basin. In the United Kingdom, ''A. vulgare'' is very common in southern and eastern England, but is more confined to coastal areas in the north. Similarly, in Ireland, ''A. vulgare'' is common in the south and east, but rarer in the north and west.
''A. vulgare'' has also been introduced to many locations in North America, where it may reach population densities of up to 10,000 individuals per square metre. It is now one of the most abundant invertebrate species in California coastal grassland habitats. It has also been introduced, to a lesser extent, to sites across the world.
Habitat''Armadillidium vulgare'' is able to withstand drier conditions than many other woodlouse species, and is restricted to calcareous soils or coastal areas. It feeds chiefly on decaying plant matter, but also grazes lichens and algae from tree bark and walls.
It is able to regulate its temperature through its behaviour, preferring bright sunshine when temperatures are low, but remaining in shadow when temperatures are high; temperatures below −2 °C or above 36 °C are lethal to it. ''A. vulgare'' is less susceptible to cold during the night, and may enter a state of dormancy during the winter in order to survive temperatures that would otherwise be lethal.
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