Monomorium monomorium, Bolton, 1987

Seifert, Bernhard, 2025, The Monomorium carbonarium species group in the Nearctic and Europe (Hymenoptera: Formicidae), Soil Organisms (Toronto, Ont.) 97 (1), pp. 55-84 : 78-79

Monomorium monomorium Bolton 1987

[type investigation]

Replacement name for Monomorium minutum Mayr 1855 [junior secondary homonym of Atta minuta Jerdon 1851 ]. This species has been described under the name Monomorium minutum from Italy. Mayr (1855: 453) gave three type localities “ Lombardie (Villa)... Insel Lido bei Venedig (Strobel)...im Kirchenstaate bei Imola (Pirazzoli)”. Investigated were 1 syntype worker labelled “Venedig Lido Coll. G. Mayr”, “ zu Zool. bot. Gesell Wien Bd. V. p. 273“, „Monomor. minutum det. G. Mayr“, “Type“; 1 syntype worker labelled “Lido, Strobl“, “minutum G. Mayr Type”, “TYPUS”, “ANTWEB CASENT 091600”; depository of both samples NHM Wien.

Comment. The text of the description of Atta minuta Jerdon 1851 is as follows: “Worker barely 1/12th of an inch long (= 2 mm B.S.), head oblong, eyes minute, advanced; thorax narrow; abdominal pedicles long, narrow, the first much more raised than the second; antennae gradually thickening, of a rufous color with the abdomen somewhat darker of fuscous. Female about 1/3 of an inch long (= 8 mm!, B.S.), similar in form to the worker, abdomen larger proportionally, and head small- er...makes a temporary nest in various situations, in an empty box, between the back of a book and its leaves, even among the loose pages of a book, in an empty shell...Nothing is used in its construction...” This original description does not give any drawing of the ants and makes genus or species identification questionable. The conclusion of Bolton (1987) that Atta minuta Jerdon belongs to the genus Monomorium and that it is a junior synonym of Monomorium pharaonis (Linnaeus 1758) is speculative. The reported total length of the gyne of 8 mm speaks against this. Dry mounted gynes of M. pharaonis have a total length of 4 to 5 mm and even living, physogastric gynes will never be longer than 6 mm. Anyway, I do not perform here a name reversal by depositing Atta minuta under Incertae Sedis and maintain the current use.

All material examined. Numeric phenotypical data were taken in 11 samples with 29 workers. The material originated from France (4 samples), Greece (1), Italy (1), Netherlands (1) and Spain (4). For details see supplementary information SI1, SI2 .

Geographic range. The unclear species separation of the tiny blackish Monomorium in the past led to controversial interpretations if this species is of Europe- an origin or introduced here (see Gómez et al. 2024). I have strong doubts that M. monomorium can be considered as a true tramp species with cosmopolitan distribution since the separation from similar Orientalic or Polynesian-Australasian species was so far not clear. This refers in particular to species or populations related to or conspecific with Monomorium liliuokalanii Forel 1899 (see statements in the diagnosis of the M. carbonarium group above). As long as no counterevidence is presented, I consider Monomorium monomorium as truly West Palaearctic-Mediterranean and as a strongly under-recorded species. All available samples are from elevations below 150 m a.s.l. in Spain, S France, Italy and Greece. The finding in the Netherlands is an introduction with plant material.

Diagnosis: --Worker ( Tab. 2, Figs. 8–10, key). Smallest species of the M. carbonarium group, CS 407 ± 17 µm. Head rather short, CL/CW 1.179 ± 0.028. Clypeal spines reduced to short, blunt, often two-cusped dents ( Fig. 10); as result median clypeal excision extremely shallow, ExCly/CS 1.71 ± 0.44%, ExCly 7.0 ± 1.8 µm. Frons wide and frontal carinae only very slightly diverging frontad (FL/FR 1.043 ± 0.21, FL/CS 0.294 ± 0.008, FR/CS 0.282 ± 0.006). Eye small and preocular distance moderately large (EL/CS 0.183 ± 0.006, PrOc/CS 0.221 ±0.010). Scape and 2nd funiculus segment very short (SL/CS 0.742 ± 0.012, Fu2/CS 4.82 ± 0.39%, Fu3/CS 3.70 ± 0.29%). Mesosoma relatively short and moderately wide (ML/CS 1.188 ± 0.026, MW/CS 0.577 ± 0.011). Metanotal groove rather shallow, MGr/CS 3.72 ± 0.87%. Dorsal and caudal profile of propodeum forming a continuous convexity ( Fig. 9 ). Petiole moderately wide, long and high (PeW/CS 0.276 ± 0.017, PeL/CS 0.428 ±0.010, PeH/CS 0.362 ± 0.012). Postpetiole moderately wide and high (PpW/CS 0.313 ± 0.016, PpH/CS 0.282 ± 0.010). Dorsal surface of head glabrous with exception of longitudinal carinulae or rugulae on lateral clypeus. Contact area of mesopleuron and mesonotum with metapleuron and propodeum from ventral margin up to metanotal groove longitudinally carinulate or microrugulose. Head, mesosoma and gaster medium to dark brown.

--Gyne ( Tab. 5): The two available wingless gynes are similar to winged gynes by their high and rather long mesosoma. However, the mesosomal sclerites necessary for wing movements are fused and there are no remains of shed wings detectable. Hence these gynes are classified as wingless gynes which is also confirmed by a PCA ( Fig. 5 ). They are separable from other wingless gynes of the species group alone by the ratio SL/MH that is 0.658 and 0.706 in M. monomorium whereas it is> 0.760 in all known wingless gynes of the other species.

Taxonomic comments. If there is no unknown cryptic diversity, M. monomorium should be safely determinable considering the characters underlined above or given in the key.

Biology. (according to Emery 1916, Wagner et al. 2018, Gomez et al. 2024, M. Mei pers. comm., F. Rigato pers. comm., J. Reyez-Lopez pers. comm., A. Scupola pers. comm.): Nest populations are rather small and weakly polygynous. Supercolonies or occurrence as pest species have never been observed. The nests are easily overlooked due to small forager populations, minute size, rather slow movements and a tiny entrance hole of less than 1 mm width without any soil ejections surrounding it. It was observed to tend honeydew-producing insects on citrus trees. Among 11 sampling spots with habitat data, 6 were on alluvial, sandy soils close to sea and 3 on sandy lagoon soils at the Mediterranean Sea. The habitats are open or light stands of trees and shrub, often natural or semi-natural, but also a camping place, an urban park, an archeological site and a garden center in the Netherlands.

Figures

Figure 5. Principal component analysis separating winged gynes (upper cluster) and wingless gynes (lower cluster) of Monomorium carbonarium (black dots), M. ergatogyna (white rhombs), M. viridum (white squares), M. emarginatum (white triangles), M. gallicum n.sp. (black crosses), M. monomorium (white dots), M. ebeninum (black triangle) and M. compressum (black square). The gyne of M. compressum is dislodged because of extreme morphology.

Figure 9. Lateral aspect of Monomorium monomorium; Spain: Cordoba, 2013