Diagnosis of worker among Antkey species. Worker castes monomorphic. Head shape ovoid. Antenna 12-segmented. Antennal club 4-segmented or indistinct. Antennal insertions at least partly covered by frontal lobes. Antennal scapes not conspicuously short; easily extended beyond eye level. Antennal insertion not surrounded by a raised sharp-edged ridge. Antennal scrobe lacking. Posterolateral corners of head unarmed, without spines. Eyes medium to large (greater than 6 facets), but distinctly less than half head length. Frontal lobes do not obscure face outline between mandible and eye. Lobes 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. Frontal lobes short and broad, distinctly rounded anteriorly. Mandibles triangular. Carinae on cephalic dorsum strongly reticulated, composed of short intersecting longitudinal and transverse segments. Mesosoma with erect hairs. Pronotal spines absent. Propodeum armed with spines or teeth. Slope of mesosoma gradual. Waist 2-segmented. Petiole with a distinct and upright 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.
Among myrmicine genera with introduced and commonly intercepted species, Myrmica might be confused for Tetramorium or Pheidole. In addition to having a 4-segmented antennal club, Myrmica can be separated from Tetramorium by the lack of a raised ridge encircling the antennal insertion, and from Pheidole by the gradually sloping (versus humped) mesosoma. Among other genera with introduced and commonly intercepted species with antennal clubs that can be interpreted as either 4-segmented or indistinct, Myrmica species can readily be separated from the attines (Acromyrmex and Atta) by the lack of pronotal spines, and from Cephalotes by the lack of frontal lobes that obscure the lateral outlines of the face. Myrmica rubra can be separated from M. specioides by the carinae on cephalic dorsum which are strongly reticulated (versus mostly longitudinal, occasionally branching but rarely intersecting); and also by the frontal lobes which are short and broad, distinctly rounded anteriorly (versus narrow, distinctly longer than broad and tapering anteriorly).