This is my personal attempt to consolidate multiple discussions addressing the current situation concerning Crematogaster scutellaris/schmidti along the (Northern) Adriatic coast and adjacent areas. These discussions are ongoing across several observations, and my aim is to clarify the situation.

First, for those unfamiliar with the subject, a general overview: C. scutellaris and C. schmidti are two sister species, and as recent findings suggest, likely even sibling species. Both are conspicuous and abundant Crematogaster s. str. species, easily recognized by their prominent reddish head and contrasting black gaster pigmentation, which sometimes extends anteriorly through part or the entire thorax.

C. scutellaris (or at least the current concept of this species) is largely considered a Western Mediterranean species, prevalent in the Maghreb and the Iberian Peninsula. Eastward, its distribution ranges across Southern France, along the Southern Italian Alps and the Julian Alps into the Balkans. Its Eastern range was believed to end somewhere in Eastern Croatia or possibly Montenegro, with dubious records reaching as far as Northwestern Greece.

Conversely, C. schmidti is distributed in the Eastern Mediterranean, with its westernmost records in the Venetian Plain. Eastwards, it extends along the Black Sea into West Asia, reaching as far as Central Asia.

Based on this, the sympatric zone of both species should extend from Venetia, across the Northwestern Balkans, to Eastern Croatia/Southeastern Bosnia and Herzegovina, or possibly even Montenegro.

However, correctly identifying both species is problematic due to reduced interspecific differences in sympatric populations compared to allopatric ones, suggesting regional hybridization. This means that specimens of C. scutellaris from the Western end of its distribution are easily distinguishable from those of C. schmidti from the Eastern end, but in the sympatric zone, both species appear more similar and are harder to distinguish.

The only way to reliably differentiate the two species, regardless of a specimen's origin, is through morphometric clustering or the simpler discriminant function, which is based on a slightly larger postpetiole width, as well as the larger distance of the frontal carinae and the proportional spine width relative to head width in C. schmidti (from Seifert, 2018):

36.958*CW – 46.715*FR – 11.769*SPTI – 52.405*PpW – 0.723 =
<0 in C. schmidti
>0 in C. scutellaris

(0% error only with nest series)

However, another character often used in the sympatric zone, and more applicable for identification based on photos, is pigmentation. Simply put, the scutellaris-like pigmentation pattern is characterized by a red head and a mostly blackish thorax, petiole, postpetiole, and gaster. The schmidti-like pigmentation pattern features a red head, thorax, and petiole/postpetiole, contrasting with the black gaster. More precisely, the brightness relations between dorsal head and dorsal postpetiole in nest means are as follows (from Seifert, 2018):

schmidti-like pigmentation pattern: 0.66-1.15 (this basically means that the dorsal postpetiole is about 66-115% as bright as the dorsal head)
scutellaris-like pigmentation pattern: 0.46-0.75

t's important to note that this character has only been used in the transgression zone. Western Mediterranean C. scutellaris are generally more color-variable than Eastern C. scutellaris and can occasionally also show a superficially schmidti-like pigmentation.

However, concerning the new issue: Recent efforts by Dr. Bernhard Seifert showed that some C. schmidti specimens from the sympatric zone can also exhibit a scutellaris-like pigmentation pattern. This suggests the introgression of one or possibly multiple pigmentation genes from C. scutellaris into C. schmidti. However, this introgression appears to be unidirectional, as there are no known records of C. scutellaris specimens from the transgression zone showing a schmidti-like pigmentation.

This yet unpublished research was recently presented by Bračko (2023), who concluded that C. scutellaris is entirely absent from Slovenia, attributing all records to C. schmidti. However, based on my personal correspondence with Dr. Seifert, I believe this might be a misunderstanding or a result of Bračko's own research.

Further research, possibly also in the context of Batesian mimicry by species of the Camponotus lateralis-complex, will need to resolve the currently somewhat confusing situation.

What does this mean for pigmentation-based identifications on iNaturalist? Since the introgression appears to be unidirectional, I see no problem in continuing to identify specimens with a schmidti-like pigmentation as C. schmidti. For specimens with a scutellaris-like pigmentation in the transgression zone, my personal suggestion would be to identify them all as Crematogaster s. str. (except for the rare cases where morphometric measurements can or have been made) because, based on pictures alone, it's not possible to differentiate between true C. scutellaris and C. schmidti with a scutellaris-like pigmentation pattern. It might also be worth to collect observations of specimens with a scutellaris-like pigmentation in the transgression zone in a project or observation field, to more easily reevaluate them in case of a revision.

Lastly, I'd like to remind that all of this concerns only the present concept of C. scutellaris and C. schmidti in the transgression zone and does not include described (C. gordani, etc.) or potential undescribed cryptic species from the scutellaris/schmidti-complex.

Please feel free to leave any comments, questions, or suggestions you may have!

iNat observations where this situation is also mentioned or discussed:
https://www.inaturalist.org/observations/193292763
https://www.inaturalist.org/observations/194531976
https://www.inaturalist.org/observations/188496835
https://www.inaturalist.org/observations/131626535

References:
Arcos, J.; García, F. Hormigas de la península Ibérica e islas Baleares. Barcelona, 490 pp.
Borowiec, L.; Salata, S. 2012. Ants of Greece – checklist, comments and new faunistic data (Hymenoptera: Formicidae). Genus (Wroclaw) 23 (4):461-563
Bračko, G. 2023. Atlas of the ants of Slovenia. Ljubljana: Biotechnical Faculty, 251 pp.
Karaman, M. G. 2010. New data of the genus Crematogaster Lund, 1831 (Hymenoptera: Formicidae) in Crna Gora (Montenegro), Part I: Crematogaster scutellaris group of species. Türkiye Entomoloji Dergisi 34:419-434
Seifert, B. 2018. The ants of Central and North Europe. Boxberg: Lutra, 407 pp.