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).
Distribution and Habitat
Originally discovered on the Isles of Scilly, this native of eastern Australia is now well established in south-western England and occurs patchily along the western coasts of Britain as far north as the Scottish Western Isles. It appears to be continuing its spread in southern and western areas, and beginning to colonise eastern Britain (Gregory, 2016). It appears to be widely distributed on the Channel islands.
It typically occurs under stones and dead wood or among damp detritus and debris in gardens, damp scrub and woodland. It may be abundant when found. It is probably widely spread via plant nurseries and garden centres. Alexander (2010) reports its occurence from traps set adjacent to tree trunks 2 m above ground level in parkland in Cornwall.
Gregory, S.J. (2016) On the terrestrial landhopper Arcitalitrus dorrieni (Hunt, 1925) (Amphipoda: Talitridae): Identification and current distribution. Bulletin of the British Myriapod & Isopod Group 29: 2-13.