Haplophthalmus is a difficult genus and only male specimens of H.mengii can be distinguished from H. montivagus.
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). Characteristically, Haplophthalmus species have a continuous body outline (all other trichoniscid woodlice have a stepped body outline).
The 7th pereiopod of male H. mengii bears a distinct spine on the carpus and short club-shaped spines on the propodus.
Distribution and Habitat
There are verified records for H. mengii from across the full breath of Britain and Ireland. It inhabits a wide range of semi-natural and synanthropic habitats, both inland and on the coast.
It is typically found clinging to the underside of deeply embedded stones and dead wood. It also occurs among friable humus-rich soil, well-rotted compost and, on the coast, within damp peaty soil that accumulates below shingle or boulders.
This summary is based on the detailed account in Gregory (2009).