The Monomorium carbonarium species group in the Nearctic and Europe (Hymenoptera: Formicidae)

A taxonomic revision of the independently living species of the Monomorium carbonarium species group, considering 14 described and two newly described taxa, is provided. Taxonomic decisions were based on Numeric Morphology-Based Alpha-Taxonomy considering 22 morphometric characters in 129 samples with 411 workers and 78 gynes. A diagnosis of the group and a key for the workers of the ten recognized independent species is given. Discrete gyne dimorphism – that is, development of volant winged gynes and wingless gynes – is more widely distributed in the group than previously known. This lead in the past, in connection with insufficiency for species separation in workers, to erroneous description of intraspecific gyne morphs as different species by DuBois (1986). Monomorium minimum (Buckley 1867), M. minimum emersoni Gregg 1945 and M. trageri DuBois 1986 are recognized as junior synonyms of M. carbonarium (Smith 1858) whereas M. wheelerorum DuBois 1986 and M. minimum subsp. cyaneum Wheeler 1914 fall under junior synonymy of M. ergatogyna Wheeler 1904. Monomorium peninsulatum Gregg 1945 is confirmed as junior synonym of M. viridum Brown 1943. Monomorium gallicum n.sp. from W Europe and M. lorenzoi n.sp. from S California are described as new. All species can be clustered in the worker caste by multivariate analyses. Species delimitation of gynes is insufficiently solved due to small sample size, polymorphism and unavailable gynes in two species. Within the eight species confirmed for the New World and having their origin there, only Monomorium carbonarium was introduced to Europe. Three species are found in Europe of which two, M. gallicum n.sp. and M. monomorium, are absent from the Nearctic. Supercoloniality, known from Europe in M. carbonarium and M. gallicum n.sp. and not reported so far from the Nearctic, is supposed to be a new development that took place within the last three decades. Supercolonies of the latter two species were observed in nine local populations introduced to urban environments in France, Germany and the Netherlands.