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Worker small, monomorphic. Head subrectangular, with rounded posterior corners, rather convex lateral borders, and convex, moderately large eyes at the middle of the sides. Ocelli absent. Mandibles triangular, their apical margins with numerous unequal teeth. Maxillary palpi 3-jointed; labial palpi 2-jointed. Clypeus convex; its anterior border entire or feebly notched in the middle; its posterior portion extending back between the frontal carinae; its sides not greatly narrowed and without a trenchant ridge in front of the antennal fovea. Frontal carinae short, rather far apart, diverging behind, not prolonged as borders of scrobe-like depressions. Antennae long and slender, 12-jointed, with a 3-jointed club, which is shorter than the remainder of the funiculus, terminal joint somewhat enlarged, as long as the two preceding joints together. Thorax rather long and slender, distinctly constricted in the mesoepinotal region, with very long straight epinotal spines, but without metasternal spines. Pronotum on each side above with a bluntly angular elevation, the inferior border broadly rounded. Peduncle of petiole long and slender, the node compressed anteroposteriorly, very slightly squamiform. Postpetiole small, scarcely broader than the petiole, constricted behind. Gaster ovate, rather small. Legs long and slender; middle and hind tibiae without spurs.
Female similar to the worker, but larger. Thorax not broader than the head including the eyes; pronotum not covered by the anterior portion of the mesonotum, which is short and convex. Epinotum sloping, with stout spines. Abdomen shaped much as in the worker. Fore wings with a single cubital, a discoidal and a closed radial cell.
Male nearly as large as the female. Head small, with prominent eyes and ocelli. Mandibles well developed, with several teeth. Antennae 11-jointed, the second funicular joint representing three fused joints. Mesonotum without distinct Mayrian furrows. Petiolar node very low. Cerci distinct; hypopygium with a bluntly rounded point; external genital valves short and stout, obtusely pointed. Legs very slender. Wings as in the female.
I include in this genus also Mayr's M. africanaHNS., which is hardly more than a subspecies of aculeataHNS. Emery placed both of these forms in TetramoriumHNS. Their habitus is certainly that of certain forms of MacromischaHNS, as Mayr observed, but Emery was right in excluding them from that Neotropical genus. Both species are confined to the rain forests of West Africa (Map 27) and do not, nest in the ground like the species of TetramoriumHNS but build loose carton nests between leaves or on their under surfaces. Mayr claimed that the male aculeataHNS has 11-jointed antennae, but Emery, after examination of six specimens, maintained that these appendages are 10-jointed and that Mayr's specimens must have been abnormal. There are four males in the Congo collection from two different localities and all of them have 11-jointed antennae. Emery probably overlooked the third funicular joint, which is rather rigidly articulated with the second joint so that the suture can be distinctly seen only in a favorable light. The number of joints in the male antennae, the shape of the clypeus in the worker and female, the absence of spurs on the middle and hind tibiae, the long slender legs and antennae, the absence of the Mayrian furrows in the male, and the reduced number of palpal joints are all characters which seem to me to justify a new generic name. The peculiar habits, too, are important in this connection, although alone they would hardly justify a change in Emery's allocation of the species, since in a well-marked genus like MyrmicariaHNS we have seen that some of the smaller species build carton nests on leaves whereas the larger species nest in the ground. The genus TetramoriumHNS certainly becomes more homogeneous by the removal of the two Mayrian species.