Diagnosis of worker among Antkey species. Worker caste monomorphic. Head shape roughly subrectangular. Antenna 12-segmented. Antennal club 3-segmented. Antennal scapes not conspicuously short; easily extended beyond eye level; do not extend beyond posterior margin of head. Antennal scrobe present. Antennal insertion surrounded by a raised sharp-edged ridge. Posterolateral corners of head unarmed, without spines. Eyes medium to large (greater than 5 facets); distinctly less than half head length. Frontal lobes do not obscure face outline between mandible and eye; relatively far apart so that the posteromedian portion of the clypeus, where it projects between the frontal lobes, is much broader than one of the lobes. Mandibles triangular. Cephalic dorsum with short lateral rugae intersecting longer longitudinal rugae. Mesosoma with erect hairs. Pronotal spines absent. Propodeum armed with small but distinct triangular teeth. Slope of mesosoma gradual. Waist 2-segmented. Petiole with a square-shaped node; pedunculate; lacking large subpetiolar process postpetiole attached to lower surface of gaster. Postpetiole not swollen; in dorsal view not distinctly broader than long or distinctly wider than petiole. Erect hairs moderately distributed, long and thin. Color uniformly dark brown.
Tetramorium nr. caespitum is distinguished from its fellow congeners that are introduced in the United States by the following combination of characters: (1) petiolar node square-shaped (versus evenly rounded for T. lanuginosum and T. tonganum; versus wave-shaped for T. insolens and T. pacificum); (2) erect hairs long and thin (versus short and thick for T. caldarium and T. simillimum); (3) cephalic dorsum with primarily subparallel, non-intersecting longitudinal rugae (versus short lateral rugae intersecting longer longitudinal rugae for T. bicarinatum); and (4) propodeum armed with small triangular teeth (versus long and robust spines in T. bicarinatum, T. insolens and T. pacificum). It is difficult to separate T. nr. caespitum from T. tsushimae, but the former is monogynous (versus polygynous). Additional morphometric characters for separating the two are given in Steiner et al. (2006), and an online cyber-identification engine for identifying members of the T. caespitum complex is available at the following portal <http://homepage.boku.ac.at/h505t3/DiscTet>.