Taxonomic history
DuBois, 1986 PDF: 84 (m.).Raised to species: DuBois, 1981b PDF: 35.Senior synonym of Monomorium minimum emersoni: DuBois, 1986 PDF: 82.It is not easy to separate M. cyaneum from M. ebeninum and M. depressum . In the syntypes of M. cyaneum examined ( LACM ) the mesopleuron is smooth, whereas the DuBois (1986) diagnosis and keys indicate punctate mesopleura for the species. There are Mexican workers ( LACM ) with punctate mesopleura identified as M. cyaneum , but other material referable to this species (Mexico, LACM ) lacks this attribute. Perhaps there is variability in the character, but the limited material is insufficient to evaluate this possibility. In M. cyaneum the propodeum is more weakly angled than in the other two species and supposedly there are 8 or fewer hairs on the mesosoma (DuBois, 1986). The pilosity varies somewhat in samples referable to these three species. M. cyaneum and M. depressum share a petiole that is slightly curved in frontal view, as well as a petiolar node that is thicker in lateral view. M. ebeninum has a petiole that is flat or weakly concave in frontal view. Thus the problem is how to separate M. cyaneum and M. depressum . As pointed out above, the punctate sculpturing of the mesopleura does not seem to be universal in cyaneum (three syntypes examined, LACM , do not have appreciable sculpturing on the mesopleura). A characteristic of the syntypes and other specimens (from Hatillo, Mexico, LACM ) is the presence of a light metallic blue sheen on the entire body (from which the species name derives). A specimen labeled as cyaneum ( LACM ) by DuBois has mesopleural sculpturing but does not have the blue sheen.
