Micromyrma pygmaea Dufour 1857

Seifert, Bernhard, Kaufmann, Bernard & Fraysse, Lorenzo, 2024, A taxonomic revision of the Palaearctic species of the ant genus Tapinoma Mayr 1861 (Hymenoptera: Formicidae), Zootaxa 5435 (1), pp. 1-74 : 62

Micromyrma pygmaea Dufour 1857 [original description, zoogeography]

The species was described from a garden in Saint-Sever, SW France.The information provided by Dufour’s drawings and verbal description on petiole shape, number of funiculus segments and mandibular dentition make clear that it does not belong to the genus Plagiolepis but instead represents a tiny Tapinoma with very feeble clypeal excavation. Furthermore, outdoor nesting and the climatic situation of the type locality by the middle of the 19th century exclude considering introduced tropical Tapinoma species.

Material examined. Numeric phenotypical data were taken in 7 samples with 18 workers. They originated from Belgium (2 samples), France (1), Italy (1 sample), Spain (1) and Switzerland (2). For details see supplementary information SI1, SI2 .

Geographic range. Only 12 outdoor sites are documented so far from Spain, S France, south Switzerland and Italy in a rather small area between 1°E and 9°E, 41°N and 47°N and from close to sea level up to 1944 m .

Diagnosis:—Worker ( Tab. 7, Figs. 9–11 ): All shape ratios given below are primary ratios without RAV and all data are given as arithmetic mean ± standard deviation. Tiny size, CS 488 ± 23 µm. Head moderately elongated CL/CW 1.141 ± 0.024. Postocular distance very large, PoOc/CL 0.500 ± 0.007. Anteromedian clypeal excision absent or very shallow, ExCly/CS 0.55 ± 0.34 %, ExClyW 11.76 ± 2.07 %. Sum of pubescence hairs and small setae protruding at a few micron across margin of clypeal excision low, nExCly 0.97 ± 0.92. Posterior margin of head not or only very slightly excavated, ExOcc/CS 1.08 ± 0.49 %. Scape shorter than in any Palaearctic species, SL/CS 0.846 ± 0.019. Minimum distance of the inner margins of antennal socket rings relatively wide, dAN/CS 0.323 ± 0.009. Eye small, EL/CS 0.244 ± 0.006. Metanotal groove shallow, MGr/CS 1.57 ± 0.44 %. Mesosoma relatively narrow and shorter than in any small species, MW/CS 0.624 ± 0.023, ML/CS 1.157 ± 0.028. Second funiculus segment extremely short, shorter than in any Palaearctic species, Fu2L/CS 8.28 ± 0.39 %, IFu2 0.933 ± 0.057. Head, mesosoma and gaster dark brown. Antennae, femora and tibiae, metatarsae and tarsae medium brown to pale yellowish brown. Condition of pubescence without species-specific characters. Anteromedian clypeal margin with four conspicuous setae which are much longer than other setae on whole body.

—Male genital: See the drawing and description provided by Espadaler (1978).

Taxonomic comments. Tapinoma pygmaeum can not be confused with any Palaearctic species with outdoor occurrence. The very small size, the very large postocular index, the extremely short second funiculus segment and scape suggest proximity to species related to Tapinoma melanocephalum (Fabricius 1793) . There is indeed a high morphometric similarity to the recently described tropical Tapinoma pithecorum Seifert 2022 which occurrence in the temperate zone is restricted to heated greenhouses. Tapinoma pithecorum differs from T. pygmaeum by a whitish gaster pigmentation contrasting the blackish brown mesosoma, by a broader mesosoma and an even shorter 2 nd funiculus segment. Using absolute measurements given in mm, the two species can be separated by a discriminan 92.69*PoOc+344.0*ExCly+374.2*Fu2L-73.02*MW-16.08 with 14 workers of pithecorum showing values <0 and 18 workers of pygmaeum values> 0.

Biology. According to Espadaler & Garcia-Berthou (1997) and Espadaler (pers. comm. 2023), it is often found in habitats with certain degree of humidity as it occurred in a mixed forest along water courses, at a margin of a lake or frequently on mountain meadows. Occurrence in city parks (Ruiz-Heraz et al. 2011) and gardens shows that it does not avoid habitats with stronger anthropogenic disturbance. Altitudinal distribution and habitat type indicate that thermophily during the summer season is not very expressed and that nests are relatively frost resistant. In the region of its natural origin it is a comparatively rare and unspectacular ant but was certainly underrecorded in the past because collectors in the field were not alerted due to superficial similarity with tiny Plagiolepis species. Nests were found under stones, in rock crevices, in dead dry wood, in the stem of an old vine and of a Rubus plant and even in the pit of a peach still hanging on the tree. The nest habitats of two findings from the Canton of Vaud, Switzerland were an active bee hive and an unheated glasshouse of a private house. Lebas et al. (2016) reported French colonies to contain not more than 100 workers and to frequently translocate nest sites to have better access to food sources. Espadaler (pers. comm. 2023) estimated the population under a flat stone to consist of> 10 queens and likely 300– 400 without making digging attempts to enumerate the full population. In the altitudinal range of 1400 to 1500 m of the El Catllar mountain area / East Pyrenees, sexuals were observed by Espadaler 2 July 1977 and 18 July 2018. The introduced population in a glasshouse in Eeklo, Belgium is of particular interest as it was very polygynous and populous and foraged in the garden. We probably witness here the transition to supercoloniality in an introduction site of a species which is not known to be a pest species in the region of its natural origin. Considering global warming, nests of Tapinoma pygmaeum introduced to England and the Benelux countries should be well able to survive there the winters in outdoor habitats. Frost resistance is indicated by frequent occurrence in the El Catllar region between 1150 and 1944 m which corresponds to mean January temperatures between +1°C and -4°C.

Figures

FIGURE 9. Tapinoma pygmaeum, head in full face view, Ollon / Switzerland, 2017.05.21.

FIGURE 10. Tapinoma pygmaeum, lateral view, Eekloo / Belgium, 2022.02.04.

FIGURE 11. Tapinoma pygmaeum, anterior head in dorsofronal view, Ollon / Switzerland, 2017.05.21.