Desoria tigrina (formerly Isotoma tigrina) is widespread and common. Indeed it is much more common than the map would suggest since all literature records for 'Isotoma olivacea' are probably Desoria tigrina. The mucro has four teeth but DOES NOT possess a lateral seta (see Potapov (2001) for a full description). None of the specimens labelled 'Isotoma olivacea' in the NHML slide collection have a mucro with a lateral seta and all key out to Desoria tigrina. The confusion has arisen because the descriptions of 'Isotoma olivacea' in several earlier identification keys, including Gisin (1960), were for Desoria tigrina.
     The long setae on the abdomen are relatively short, only about 0.5 times as long as the fifth abdominal segment (Fig. 1). In life, Desoria tigrina is light to dark grey or brown (never white or dark blue or violet). Most records are from habitats rich in organic matter. Under unfavourable conditions, six prominent spines may develop on abd5 (Fig. 2).

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Fig.1: Dorsal side of Desoria tigrina collected from Ruislip in February 1951 by M.E. Bacchus. The setae on the fifth abdominal segment (abd5) are only about 0.5 times as long as abd5.

Fig.2: Posterior end of the abdomen of the 'ecomorphic' form of Desoria tigrina ('Spinisotoma') collected from Lightwater, Surey in May 1951 (collector unknown). Under unfavourable conditions, six prominent spines (*) develop on abd5. man, manubrium.