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Worker: Length 1.7-2 mm. - Head, excluding mandibles, 1 1/5 times as long as wide, occipital margin convex except for broad and distinct medial impression, corners somewhat angulate; sides slightly convex, impressed at eyes; anterior clypeal margin convex, posterior margins raised in form of compressed and reflected teeth. Frontal lobes raised in form of sub-vertical carinae. Eyes very convex, situated half-way between occipital corners and mandibular insertions. Mandibles with 6 distinct teeth along cutting edge and a much larger apical tooth.
Pronotum with distinct though small anterior dorso-lateral tubercles forming the end of slight lateral carinae. Mesonotum with an anterior low, massive lobe on each side, descending carinae on each side terminating in small acute teeth, and bituberculate carinae on declivous surface just above mesoepinotal impression, of which the posterior tubercle is the larger. Basal surface of epinotum margined with carinae terminating in acute and sub-vertical spines. Peduncle and node of petiole of about equal length; in profile node with almost right-angled corners and a minute tubercle on median dorsal surface. Petiole, about 1 1/2 times longer than postpetiole, the latter with rounded dorsal and ventral surfaces. Postpetiole from above sub-pyriform with deeply excised posterior margin, distinctly wider than long. First gastric segment with parallel anterior and posterior margins, about as wide as long.
Sub-lucid, finely shagreened, mandibles finely striate, basal margin of first gastric segment longitudinally striate. - Hairs of head numerous, narrow-squamate; of thorax sparse, confined to projections, narrow-squamate; of pedicel and gaster sparse, /squamate; of antennal scapes land legs more abundant, narrow-squamate. - Pale ferruginous, cutting margins of mandibles infuscated.
Female (dealate): Length 2.5 mm. - Similar to the worker, with the usual sexual differences, but the sculpturing is much rougher, the pilosity more abundant, and the color brown with a grayish bloom, except for ferruginous appendages. Sides of the thorax coarsely rugulose, posterior of pronotal dorso-lateral spines large, epinotal spines large and acute, postpetiole from above 1.6 times broader than long, sides convex, diverging posteriorly.
Described from one female and four workers taken by myself July 24, 1936, by the Oronoque River near its outlet with the New River of the Courantyne basin, British Guiana. The ants had formed in the sandy soil a crater 80 mm. in diameter, terminating in a turret 18 mm. high and 35 mm. in diameter in the form of a funnel opening. This led by a tunnel 8 mm. in diameter to the nest chamber 110 mm. below the surface. The fungus garden was about 15 mm. in diameter, sessile and without bromatia. 460 cm. from this nest was a young colony of Atta sexdensHNS.
This species is near M. foreli MannHNS from which it differs in distinctly smaller size, in having in the worker bituberculate carinae just above mesoepinotal impression and anterior pronotal tubercles, and in the female having pronotal tubercles and differently shaped scutellum.