Tetramorium simillimum is a small pale colored ant. This species has a monomorphic worker caste with 12-segmented antennae, three-segmented antennal club, antennal scrobes, short antennal scapes that do not surpass the posterior margin of the head, a gradually sloped mesosoma, and small propodeal spines. Like all myrmicines, T. simillimum has two waist segments and a gaster armed with a stinger. Tetramorium simillimum is believed to be native to Africa and is now widely distributed across tropical regions. The species can achieve dense populations in disturbed habitats and is likely to adversely affect native biodiversity.
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 but weakly impressed with the frontal carinae usually becoming indistinct at its posterior apex. Antennal insertion surrounded by a raised sharp-edged ridge. Posterior margin of head weakly concave medially. 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. Anterior margin of clypeus lacking median notch. 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 spines. 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 sparsely distributed; short and thick. Specimen vary between uniformly yellowish brown to yellowish brown with a distinctly darker brown to black gaster.
Tetramorium simillimum, together with T. caldarium, look very much like Wasmannia auropunctata in general size and shape. The Tetramorium species can be separated on closer examination by the 3-segmented (versus 2-segmented) antennal club, the much shorter propodeal teeth, and the difference in petiole shape. All Tetramorium can be separated from other genera with introduced or commonly intercepted species by the raised sharp-edged ridge surrounding the antennal insertions.
Tetramorium simillimum 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 sparse, short and thick (versus moderately to very abundant, long and thin for T. bicarinatum, T. nr. caespitum, T. caldarium, T. lanuginosum, T. simillimum and T. tsushimae; versus wave-shaped for T. insolens and T. pacificum); (3) cephalic dorsum with short lateral rugae intersecting longer longitudinal rugae (versus primarily subparallel, non-intersecting longitudinal rugae for T. nr. caespitum and T. tsushimae); and (4) propodeum armed with small triangular teeth (versus long and robust spines in T. bicarinatum, T. insolens and T. pacificum). It is very difficult to separate T. caldarium from T. simillimum. The antennal scrobe of T. simillimum is more impressed, and the frontal carina remains distinct even at its posterior apex. It also appears that T. simillimum may have a more broad petiolar peduncle and node in profile, and a more flat posterior head margin, but these characters require additional analysis.