*ephippiata (Brian, 1945)
*grisescens (Schäffer, 1896)
Desoria tigrina (formerly Isotoma tigrina) is widespread and common, with a tendency towards human-impacted soils or habitats rich in organic matter. In life, Desoria tigrina is light to dark grey or brown (never white, dark blue or violet). Additionally, all UK 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 abdomen 5 are taxonomically important, and unfortunately it looks as though an error has slipped through into the Hopkin key. To distinguish between Desoria violacea and tigrina in addition to colour pattern, find the ratio of the longest seta on abd. 5 to the length of the segment. For Desoria tigrina this ratio is <0.7, while Desoria violacea has longer seta with a ratio around 1.1 (Popatov 2001). In the 2007 AIDGAP key this key character is inverted, so that Desoria tigrina is shown as having the longer setae. I think that most IDs have been done by colour so this is probably not a major problem, but worth being aware of if keying in this genus.
Under unfavourable conditions, six prominent spines may develop on abd5 (Fig. 2) - a remarkable habit found in several species in the genera Desoria and Isotoma, known as ecomorphosis. The defunct genus Spinisotoma was created to house new species of isotomid with anal spines, which turned out (Stach, 1961) to be ecomorphic specimens of previously described species.