Several "varieties of Isotoma viridis" have been described over the years, all with three teeth on the mucro and conspicuous macro-setae. The longer macro-setae (especially on abdomen IV-VI) are ciliated, unlike most isotomids. (Peter Shaw wasted several days of his PhD in York trying to key this species as a member of the entomobryidae, using the 1982 Fjellberg key which suggests that large bodies and ciliated macrochaetae generally suggest Entomobryid. As the key goes on to explain, there's one exception to this rule, which is this common genus). The separation of Isotoma from the closely related genus Desoria is the possession of manubrial teeth (see below). Three of these forms, Isotoma anglicana, Isotoma caerulea and Isotoma riparia are now recognised as species in their own right. Isotoma riparia only occurs in very wet habitats and has a distinctive stripe down the middle of the body.
Isotoma viridis and Isotoma anglicana can be reliably separated (using slide-mounted specimens) by examining the manubrial 'teeth'. Isotoma viridis has a single pair of 'teeth' on the thickened apical edge of the manubrium (Fig. 2) whereas Isotoma anglicana has two pairs of manubrial teeth. Isotoma anglicana is more bluish-grey/brown in colour than Isotoma viridis which tends to be more greenish (Figs. 1 and 3) but this is not entirely reliable.
Many of the literature records for Isotoma viridis must refer to Isotoma anglicana which is undoubtedly widespread and common. The two species are sometimes found mixed together in the same sample.
The third species in this complex (as classification stands at present) is Isotoma caerulea, (caerulea meaning bluish - sadly colour seems not to be much help in this group.) It is distinguished by having a larger macroseta in the outside of the 2nd pair up the manubrium, while Isotoma anglicana has the inner of these 2 macrosetae larger than the outer. Neither Steve Hopkin nor Peter Shaw created a distribution map for this species as it is probably hugely overlooked, if it is valid at all (It is not listed in Popatov's 2001 monograph of the isotomidae). We have 1 collection from the midlands (collected by Michelle Fountaine, ID by Arne Fjellberg) and a couple of records from near Reading by Robert Norledge (ID from Hopkin's key by R. Norledge and confirmed by Peter Shaw). It is possible, if not likely, that a detailed genetic survey of UK Isotomas would identify multiple clades that correspond poorly to the existing morpho-classifications.