Species of Tetracanthella are characterised by possession of four prominent anal spines (Fig. 1). Tetracanthella wahlgreni is the most common member of the genus in UK/Eire where it is confined to bogs and mountains and can be extremely abundant (e.g. Cragg (1961) found Tetracanthella wahlgreni at densities of more than 60,000 per square metre at Moor House National Nature Reserve). It is easy to identify by the absence of a mucro (Fig. 2), its larger size (up to 1.7 mm) and longer empodial appendage (between 0.5 to 0.7 times the length of the claw).
     Tetracanthella arctica Cassagnau, 1959 is very similar to Tetracanthella wahlgreni. However, Tetracanthella arctica only has 2+2, 2+2, 3+3 macrosetae on the dorsal side of abdominal segments 1, 2 and 3 respectively whereas in Tetracanthella wahlgreni the setae are 2+2, 3+3, 3+3. All the 'Tetracanthella wahlgreni' I have checked in the NHML collection have 3+3 macrosetae on abd2 but Tetracanthella arctica should be looked for in collections of Tetracanthella from coastal sites in Ireland and Scotland.
     The map includes records for Tetracanthella kendalli Bagnall, 1939 which Deharveng (1987) sunk as a junior synonym of Tetracanthella wahlgreni.

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Fig. 1 (above): Anal spines of Tetracanthella wahlgreni collected from Ross & Cromarty, Scotland in July 1970 by M.E. Bacchus.

Fig. 2 (above): Furca of Tetracanthella wahlgreni collected from the summit of Tryfaen, Wales (3300 feet) in September 1933 by W.M. Davies. Note the absence of a mucro. d, dens; m, manubrium.