Halpophthalmus is a difficult genus and only male specimens of H.montivagus can be distinguished from H. mengii.
Both are small species, up to 4mm in length, bearing distinct longitudinal dorsal ridges. Both also have a distinct pair of dorsal projections on the third pleonite (which are at most feebly developed in H.danicus).
The 7th pereiopod of male H. montivagus lacks the prominent spine on the carpus (present in H. mengii) and has long tapered spines on the propodus.
Distribution and Habitat
This woodlouse has a scattered distribution across the chalk and limestone of southern England, but due to confusion with H. mengii, it is probably under-recorded. It seems to be restricted to sites, typically ancient deciduous woodland, where friable soils have developed over limestone or chalk.
Typically, it is found clinging to the underside of deeply embedded stones and dead wood, or among friable humus-rich soil and limestone rubble, often in damper spots. It is typically associated with other soil dwelling trichoniscid woodlice such as H. danicus, Trichoniscus pygmaeus and Trichoniscoides albidus.
This summary is based on the detailed account in Gregory (2009).