The landhopper or woodhopper is an amphipod crustacean, and differs from isopods (i.e. woodice and waterlice) in having the body flattened from side to side (rather than top to bottom). It readily jumps into the air when disturbed (reminiscent of a very large flea - about 1 cm long!). It is the only amphipod that occurs inland on 'dry' (damp) ground and is characteristically dark (almost black) in colour.

A. dorrieni was once considered to be synomomous with A. sylvaticus, Haswell, 1879, but subsequently these have been shown to be distinct species. Unfortunately, the name A. sylvaticus has been used in the past and this has resulted in the confusion that two species of Arcitalitrus occur in Britain and Ireland.  In fact, only one, A. dorrieni, has been recorded (Gregory, 2016).

Confusion is most likely with the introduced semi-terrestial amphipod Cryptorchestia cavimana (Heller, 1865) which is less darkly pigmented and although able to penetrate far inland, rarely strays far from water margins. The two species are described and figured by Gregory (2016). Other British amphipods rarely occur far above the high tide mark and are often pale orange (most are intertidal).