(303ISvir) Isotoma viridis (Bourlet, 1839)

*annulata Nicolet, 1847
*hibernica Carpenter, 1906
*palustris Tullberg, 1871
*plumbea Packard, 1873
*viatica (Nicolet, 1842)


The literature records shown in the map below include all localities for Isotoma 'viridis'.   This group is now split into four species: Isotoma viridis,  Isotoma anglicana, Isotoma caerulea and Isotoma riparia.  (All species have three teeth on the mucro).  Isotoma riparia only occurs in very wet habitats and has a distinctive stripe down the middle of the body - much like Isotomurus palustris, but without trichobothria.  Isotoma viridis  Isotoma anglicana and Isotoma caerulea can only be reliably separated by examining the manubrial 'teeth' and the chaetotaxy of the dens. Isotoma viridis has a single pair of 'teeth' on the thickened apical edge of the manubrium (Fig. 1) whereas Isotoma anglicana and Isotoma caerulea have two pairs of manubrial teeth.  The latter two are separated by the basal dorsal setae on the dens (3= anglicana, 2 = caerulea).  Isotoma viridis tends to be more greenish in coloutr than the others (and viridis means green)  (Fig. 2) but this is not entirely reliable.The literature records for these three species must be regarded as hopelessly mixed up.  Isotoma viridis and Isotoma anglicana are both undoubtedly widespread and common, sometimes found mixed together in the same sample.


Fig. 1: Single pair of manubrial 'teeth' (*) on the apical edge of the manubrium (man) of Isotoma viridis collected from Mitcham Common, Surrey in August 1977 by Brian Pitkin.

Fig. 2 : Isotoma viridis of 3 mm in length from a garden in Reading.

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