One of our smallest woodlice, reaching 2.5mm, and often confused with juveniles of other species. T. pygmaeus is usually pale pinkish-yellowish in colour, but characteristically the head appears almost white, contrasting with the body pigments. As other Trichoniscus spp. the eye is composed of three black ommatidia and it has a smooth body surface. All pigments are retained in alcohol.
In juvenile T. pusillus s.l. the head is speckled with pigment to match the body colour. Trichoniscoides spp. and juvenile Androniscus dentiger have the body covered in tubercles and eyes comprising a single ommatidium.
Distribution and Habitat
Although common throughout Britain and Ireland, it is under-recorded in many areas. It occurs wherever there are suitable friable soil conditions, including semi-natural grasslands, woodlands and coastal (supralittoral) habitats, and synanthropic sites such as churchyards, disused quarries and railway cuttings.
Searching the underside of large stones or dead wood partially embedded into soil is the simplest way to find this species. It is typically associated with other soil-dwelling trichoniscids, including Androniscus dentiger, Haplophthalmus spp. and Trichoniscoides spp.
This summary is based on the detailed account in Gregory (2009).