Haplophthalmus mengii (Zaddach, 1844)

The pygmy woodlice of family Trichoniscidae (and Styloniscidae) are readily distinguished from other woodlice by the shape of the antennal fagellum which comprises a tapered cone (of indistinct segments) bearing a terminal bristle.

The genus Haplophthalmus is readily recognised by having distinct longitudinal dorsal ridges on the body which has a continuous body outline (all other trichoniscid woodlice have a stepped body outline) and has eyes composed of a single black ocellus.  

This 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 and both, in addition to distinct longitudinal dorsal ridges, have a distinct pair of dorsal projections on the third pleonite (these are at most feebly developed in H. danicus). 

The 7th pereiopod of male H. mengii bears a distinct spine on the carpus and short club-shaped spines on the propodus (see image right).