Armadillidium depressum Brandt, 1833
GB IUCN status: Least Concern
Our seven Armadillidium species and Eluma caelata (Family Armadillidiidae) are readily recognised in the field by their truncated 'square' uropods that end flush with the body and their ability to roll into a protective sphere (similar truncated uropods are also seen in the non-native Armadillidae species).
This large pill-woodlouse (to 20 mm body length) is typically slate grey, sometimes flecked with yellow, and thus reminiscent of the common A. vulgare. Armadillidium depressum differs in the pleon being more curved outwards, giving a 'splayed' or 'flattened' appearance and in the scutellum being raised as a broad plate visible between the antennae. The male 1st pleopods are diagnostic.
Small immatures look very different from the adults, typically being brownish grey and flecked with an ornate yellow pattern (see images).
It has a distinct south-western bias, but outlying populations, probably a result of recent introduction, occur sporadically elsewhere as far north as south-west Scotland (BMIG Newsletter 41, 2020, pg 5-6). In Ireland there is a handful of record, mainly form the south east.
Armadillidium depressum is heavily synanthropic and typically occurs on dry limestone walls or loosely mortared walls in towns, villages and farmyards, or on weathered rock faces in quarries and railway cuttings.
It can be found under loose stones, especially capping stones on walls. It may occur in large numbers inside old houses. It also occurs under rocks and stones in railway cuttings and disused quarries where limestone is exposed. It can be much easier to find at night (torch-light surveys) when it is active on the surface. It has also been recorded from the trunks of Beech trees (Alexander, 2011). Typically, it is associated with A. vulgare, Porcellio scaber and more rarely P. spinicornis.
This summary is based on the detailed account in Gregory (2009).