A small and inconspicuous bug which occurs under stones on wet gravel at the margins of fast streams in upland Britain. Adults overwinter and mate in May, giving rise to a new generation of adults from July onwards. According to Southwood & Leston (1959) this bug is highly evolved in that the male has complex genital structures, the VI and VII abdominal segments being fragmented to form a series of asymmetrical accessory claspers. The recording scheme has only one record of this species, but NBN Gateway shows record for Wales, northern England and northern Scotland. There are 19 records up till 1989 and 40 from 1990 onwards.
C. alienum is the largest of the three species in the Dipsocoridae and Ceratocombidae (1.8 – 2.4 mm) with forewings that extend beyond the tip of the abdomen (macropterous) that usually appear split in the middle (costal fracture). The rostrum does not extend beyond the thorax, the colour is red-brown, the pronotum tapers uniformly inwards from the base and the male has the accessory claspers referred to above.