Discrete color morphs of ants, in the case reported here morphs with whitish versus blackish gaster pigmentation, may be misinterpreted as different species. This became clear in a morphometric study including 91 worker individuals of Tapinoma jandai sp. nov., Tapinoma melanocephalum (Fabricius, 1793) and Tapinoma pygmaeum (Dufour, 1857). The three species are clearly separable by a principal component analysis of 17 morphological characters under exclusion of pigmentation characters. This result is confirmed by a clear separation in a linear discriminant analysis with the classification of any specimen being confirmed with posterior probabilities of p > 0.9994. The study showed that just the holotype series of Tapinoma pithecorum Seifert, 2022 is a rare color variant of Tapinoma pygmaeum and consequently the former taxon is a junior synonym of the latter. In contrast, all remaining samples that formed the basis of the description of Tapinoma pithecorum are recognized as an undescribed species which is introduced here as Tapinoma jandai sp. nov. A consideration of 14 taxa of minute species showing similarities to the new species revealed that none of these is suspected of representing a senior synonym of T. jandai sp. nov. and that Tapinoma indicum Forel, 1895 should be transferred to the genus Ravavy Fisher, 2009. Based on examples in Formica, Camponotus, Cardiocondyla and the two cases reported here, it is argued that ant classification by simple color patterns is under increased risk of misclassification.

Ant taxonomy, color dimorphism, ghost ant, numeric morphology-based alpha-taxonomy

The recent description of Tapinoma pithecorum by Seifert, 2022 that was supposed by Seifert (2022) to be a sister species of the notorious pest ant Tapinoma melanocephalum (Fabricius, 1793), prompted a phylogenomic investigation of the genus Tapinoma Foerster, 1850 with special attention to the relationship of T. pithecorum and T. melanocephalum (Perez-Flores et al. 2025 accepted). The genetic data confirmed the separate identity of these two species with the last common ancestor dating back 13 million years. The original description of Tapinoma pithecorum was based on samples from a greenhouse in Hannover Zoo (holotype series) and of outdoor populations from Pakistan, India, Malaysia, the Fiji and Christmas Islands as well as the south of the Arabian Peninsula (Seifert 2022). The holotype series clustered in a principal component analysis together with the outdoor populations, separating it from Tapinoma indicum Forel, 1895 and T. melanocephalum (Seifert 2022). However, when making this analysis in 2021, I was not aware of the greenhouse occurrence of Tapinoma pygmaeum (Dufour, 1857) and its color dimorphism. This tiny Dolichoderine ant was known until recently only as a native faunal element of the Iberian region, the South of France and Italy. Recent findings in greenhouses of Switzerland and Belgium as well as in a domestic garden in England (Seifert et al. 2024) and in urban outdoor habitats near Orleans /France expanded our knowledge on the ecological and geographic range of this species considerably.

These alerting findings prompted a critical reinvestigation of the Tapinoma pithecorum material and an extension of sample size under inclusion of T. pygmaeum. The unpleasant result was that just the holotype series of Tapinoma pithecorum from the greenhouse (but not any other sample I had named T. pithecorum) belonged to T. pygmaeum when the evaluation excludes gaster pigmentation. The holotype series represents a probably rare case of Tapinoma pygmaeum nest population having a whitish gaster being distinctly lighter than the blackish brown head and mesosoma. On the other hand, in structurally and morphometrically clear Tapinoma melanocephalum which usually show in dried condition dark heads and mesosomas and whitish gasters, homogenously dark samples occur rarely. I suppose that alternate gaster pigmentation in these related species is directed by a simple mutation and cannot be used as a leading taxonomic character. Since we need a taxonomically available name for the outdoor populations named by Seifert (2022) as T. pithecorum and those populations identified genetically by Perez-Flores et al. 2025) as this species, this paper will describe a new species Tapinoma jandai sp. nov. and clarify its separation from T. pygmaeum.

The backbone of the investigations presented here is Numeric Morphology-Based Alpha-Taxonomy (NUMOBAT, Seifert 2009). Numerically evaluated by direct stereomicroscopic investigation were 25 samples with 51 workers of Tapinoma melanocephalum, 8 samples with 17 workers of T. jandai sp. nov. and 11 samples with 30 workers of Tapinoma pygmaeum. For details see Suppl. material 1.

Details of equipment, measuring, data analyses and classification procedures are given in Seifert (2022) and Seifert et al. (2024). Principal component analysis (PCA), linear discriminant analysis (LDA), and nested analyses of variance ANOVA tests were performed with the software package SPSS 15.0. In order to give a reader an impression which characters were numerically investigated, character descriptions from these sources are repeated here. Users intending to run their own analyses should study these primary sources. Measuring tiny structures such as basal funiculus segments requires high-resolution stereomicroscopy and careful spatial adjustment. The system used here was a Leica M165C high-performance stereomicroscope equipped with a 2.0× planapochromatic objective (resolution 1050 lines/mm which allowed the use of instrumental magnifications of up to 360×). Sixteen primary measurements were taken in workers (CL, CW, dAN, EL, ExCly, ExClyW, ExOcc, Fu2L, Fu2W, Fu3L, MGr, ML, MW, nExCly, PoOc and SL).

CL Maximum cephalic length measured between points A and B; A is the posteromedian margin point of head capsule; B is an imagined median point situated at the same transversal level as the most anterior points of clypeus left and right of clypeal excision. Bilateral asymmetries are averaged.

CS Cephalic size with CS=(CL +CW)/2.

CW Maximum cephalic width.

dAN Minimum distance of the inner (centripetal) margins of antennal socket rings which is best measurable in dorsofrontal view.

EL Eye length; maximum diameter of the compound eye over all structurally defined ommatidiae; bilateral mean.

ExCly Maximum depth of anteromedian clypeal excision as it appears in frontodorsal view and with median line of head positioned perpendicular in the visual field; bilateral asymmetries are averaged.

ExClyL/W Ratio ExCly /ExClyW.

ExClyW Width of clypeal excision at the level of the base centers of the two most apical and largest setae.

ExOcc Depth of excavation of posterior vertex with head in full face view. Procedure: Using high magnification with low depth of focus, adjust the head to the measuring position defined for CL. Focus both posterior corners of vertex until they form a sharp contour, adjust them to equal horizontal level within the visual field and superimpose the corners with the horizontal line of the cross-scale. Change the focal level until the median part of posterior vertex forms a sharp contour. Read the depth.

Full face view Dorsal aspect of head adjusted for measuring both maximum CL and CW. Use of high magnifications with low depth of focus improve the correct spatial adjustment.

Fu2L Median length of second funiculus segment in dorsal view. Dorsal view is given when the swiveling plane of 1^st funiculus segment is positioned in the visual plane. Take care to really measure median length as the segment’s sides have unequal length and to recognize the real distal margin of the segment. The latter has a very thin cuticle, frequently producing a narrow, shining ribbon that seems to be, by optical impression, demarcated from the rest of the segment. The median line of the segment is visualized by centre of the patch reflecting the coaxial light.

Fu2W Maximum width of second funiculus segment in dorsal view. Procedure: use the same spatial adjustment of the ant as in measuring Fu2L but rotate it in the visual plane by 90°. The use of transmitted-light is important to visualize the real cuticular surface which is often obscured by pubescence.

Fu3L Median length of third funiculus segment in dorsal view measured under the same conditions as given for Fu2L.

IFu2 Index Fu2L / Fu2W.

ML Mesosoma length from the caudalmost point of lateral metapleuron to rear margin of anterior pronotal fringe (in workers). In workers, if anterior measuring point is concealed, keep the orientation of measuring line, choose a higher magnification, measure from the caudalmost point of lateral metapleuron to the level of promesonotal border and multiply by 1.415. This factor is an average estimated from several species.

MGr Depth of metanotal groove / depression in lateral view; the upper reference line extends between the highest points of mesonotum and propodeum perpendicular to which depth measuring is performed.

MW Maximum pronotal width.

nExCly Bilateral sum of pubescence hairs and smaller setae protruding just visibly at magnifications > 200× across margin of clypeal excision. The two large anteriormost setae are not counted.

PoOc Postocular distance taken in the same spatial adjustment as in measuring CL: distance from the transversal level of posterior eye margin to median point of hind margin of head; bilateral asymmetries are averaged.

SL Scape length excluding articulatory condyle. Transmitted light is useful to visualize the real distal end of scape that may be obscured by pubescence.

I wish to thank Christophe Galkowski, Milan Janda, John Longino, Wouter Dekoninck, Raymond Delarze, Beatrice Viussoz and Brian Fisher for sending collection material. The equipment for the imaging was co-financed by tax money on the basis of the state budget passed by the Sächsischer Landtag according to the Antragsnummer 100590787 of the Sächsische Aufbaubank issued 3 August 2021.