*binoculatus (Börner, 1901)
All members of the genus Arrhopalites have only one ocellus on each side of the head. Arrhopalites pygmaeus is a common species. Almost all the records are from caves.
The empodium of the foot has a long filament that reaches slightly beyond the tip of the claw (Figs. 1, 2). The fourth antennal segment (ant4) is divided into five clearly defined subsegments (Fig. 3). Both the inner and outer edges of the mucro are serrated (Fig. 4) and the female sub-anal appendage is simple (i.e. not expanded at the tip; Fig. 5).
In Arrhopalites pygmaeus, the first subsegment of ant4 is about the same length as ant3 (Fig. 3). There is one doubtful Irish record of Arrhopalites terricolus, distinguished by the first subsegment of ant4 being much shorter than ant3. However, the length of the first subsegment of ant4 is quite variable in the BMNH specimens of Arrhopalites pygmaeus, casting doubt on the A. terricola as a good species.