Desoria violacea (formerly Isotoma violacea) is a common species with a distinctive dark violet-blue colouration. The long setae on the abdomen are relatively long, about the same length as (or slightly longer than) the fifth abdominal segment (Fig. 1). There are four teeth on the mucro (Fig. 2).
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.
It is possible that 'violacea' as understood here is a complex of two species, one that occurs in mountains and another more southern species. Desoria hiemalis is similar to Desoria violacea but the setae on the abdomen are much longer (2.0 to 2.3 times as long as abd5 according to Potapov (2001)). None of the specimens of Desoria violacea in the NHM have setae this long. Further taxonomic study is needed on fresh material to clarify the situation.