Diagnosis of worker among Antkey species. Antenna 12-segmented. Antennal scape length less than 1.5x head length. Eyes medium to large (greater than 5 facets). Antennal sockets and posterior clypeal margin separated by a distance less than the minimum width of antennal scape. Dorsum of mesosoma with metanotal groove, but never with a deep and broad concavity; lacking erect hairs. Propodeum and petiolar node both lacking a pair of short teeth. Propodeum with dorsal surface distinctly shorter than posterior face; lacking posteriorly projecting protrusion. Waist 1-segmented (may be hidden by gaster). Petiolar node appearing flattened. Gaster armed with ventral slit; with four plates on its dorsal surface, and with the fifth plate on the ventral surface. Distinct constriction not visible between abdominal segments 3+4. Hairs not long thick and produced in pairs. Bicolored with darker brown head and paler brownish yellow body, gaster and appendages.
Tapinoma melanocephalum can be distinguished from Brachymyrmex species, with which it bears superficial resemblance, by the 12-segmented (versus 9-segmented) antenna, and by the presence of a ventral slit (versus acidopore) on the gaster tip. It can be differentiated from Technomyrmex species by the absence (versus presence) or erect hairs on the mesosoma, and by the gaster which has 4 plates on the dorsal surface and 6 on the ventral surface (versus 5 plates on both surfaces). It is most easily differentiated from Tapinoma sessile by the bicolored with dark head and light body, gaster and appendages (versus uniform brown color).
Brachymyrmex species, Technomyrmex species, Tapinoma sessile