All members of the genus Arrhopalites have only one ocellus on each side of the head. Arrhopalites sericus is a scarce species. There are only two specimens in the NHML slide collection that can be identified with certainty as Arrhopalites sericus. They were both collected from Coombe Martin in Devon by M.E. Bacchus in October 1955.
Arrhopalites sericus is morphologically similar to Arrhopalites pygmaeus. The fourth antennal segment (ant4) is clearly subdivided into five subsegments, there is no large spine on the ventral side of the dens near the mucro, and the empodium of the foot bears a long filament (Fig. 1). In Arrhopalites sericus, the tip of the female sub-anal appendage (SAA) is feathered (Fig. 2) whereas in Arrhopalites pygmaeus, the SAA is simple and is not expanded at the tip.
Arrhopalites bifidus is very similar but has SAA split at the tip instead of feather-like. There is just one, unconfirmed report of A. bifidus by Gough from NW Scotland, so this species cannot be confirmed on the UK list.