A study of numeric morphology-based alpha-taxonomy (NUMOBAT) considering the speciesFormicaexsectaNylander, 1846 andF.fennicaSeifert, 2000 was performed in 166 nest samples with 485 worker individuals originating from 117 localities of the Palaearctic west of 59°E. The presence of intraspecific pilosity dimorphism is shown forF.exsecta. The setae-reduced phenotype, termed the Rubens morph, shows a frequency of about 25%, and the more abundant setae-rich phenotype, termed the Normal morph, one of 75%. The frequency of nests containing workers of both phenotypes is 15.5% in 58 samples from Denmark, Sweden, and Finland. Applying the DIMORPH test of Seifert (2016) on this territory, it is demonstrated that the association of Rubens and Normal phenotypes within the same nest cannot be interpreted as parabiosis of independent species (p=0.017) or as temporary (p=0.0004) and permanent (p=0.0001) socially parasitic association, whereas genetically mediated intraspecific dimorphism is most likely (p=0.659, allpdata according to Fisher’s exact test). The Rubens morph ofF.exsectais phenotypically most similar toF.fennicabut is safely separable by four different forms of exploratory data analyses using nest centroids (NC) as input data: NC-Ward, NC-part.hclust, NC-part.kmeans, and NC-NMDS-k-means. Data on zoogeography and the narrow climate niche indicate thatF.fennicais unlikely to occur in Norway.