Monomorium arboreum

Weber (1943) discovered this species nesting in humus about the base of a large fern which was growing epiphytically on a forest tree about 5 m above the ground. As indicated by the Kenyan samples noted below, which were taken from forest leaf litter, a subarboreal lifeway is not obligatory and it is more probably the case that arboreum is normally a litter-layer species which, in the case of Weber's sample, was taking advantage of a functional but unusual nest-site. (Bolton 1987)

Identification
Bolton (1987) - Weber described arboreum as a subspecies of Monomorium minutum, to which it is not truly related. M. arboreum belongs instead to the small complex of Afrotropical species including Monomorium schultzei, Monomorium firmum and their allies, and is a valid species in its own right. Its closest relative appears to be the Ethiopian Monomorium crawleyi but that species has a very broad metanotal groove which forms a distinctive shallow V-shaped trough in the alitrunk outline.

Distribution based on Regional Taxon Lists
Afrotropical Region: Kenya, Sudan, United Republic of Tanzania.

Nomenclature

 *  arboreum. Monomorium (Monomorium) minutum subsp. arboreum Weber, 1943c: 360 (w.q.) SUDAN. Raised to species: Bolton, 1987: 377.

Worker
Bolton (1987) - TL 2.1-2.3, HL 0.48-0.60, HW 0.39-0.46, CI 77-81, SL 0.38-0.48, SI 96-104, PW 0.25-0.30, AL 0.54-0.68 (15 measured).

Clypeal carinae narrow but sharply defined, moderately divergent anteriorly and not terminating in a pair of denticles at the anterior clypeal margin. Anterior margin and lateral margins of the prominent median portion of the clypeus meeting in a poorly defined obtuse angle. Maximum diameter of eye 0.20-0.24 x HW and with 6 ommatidia in the longest row. Outer circle of ommatidia enclosing more than one longitudinal ommatidial row. In full-face view the eyes with their posterior margins at or just behind the midlength of the sides and the scapes, when laid straight back from their insertions, just reaching the occipital margin. Sides of head almost parallel but feebly convergent posteriorly, rounding broadly and evenly into the relatively short occipital margin; the latter almost transverse, only with the feeblest median concavity in full-face view. Promesonotum evenly convex in profile, the highest point just in front of the midlength. Metanotal groove broad but only shallowly impressed and the propodeal dorsum behind the groove shallowly sloping posteriorly. Apex of promesonotal convexity on a much higher level than the propodeal dorsum, and the propodeal spiracle large and very conspicuous. Propodeal dorsum and declivity meeting in a rounded angle, the two surfaces with decidedly different slopes and not forming a single sloping surface. Subpetiolar process varying from a small lobe to a small blunt triangle. Petiole node in profile relatively low, subconical, with a narrowly rounded dorsum. Postpetiole smaller than petiole, its dorsal surface evenly broadly rounded in profile. All dorsal surfaces of head and body with standing hairs present, the promesonotal dorsum with 4-6, or very rarely 7 pairs present. Mandibles and dorsum of head unsculptured except for small pits from which hairs arise. Mesopleuron reticulate. Metanotal groove strongly and conspicuously cross-ribbed, the cross-ribs extending down the sides of the alitrunk; alitrunk otherwise unsculptured, smooth and shining. Petiole, postpetiole and gaster unsculptured. Colour dull yellow, the head dorsally and the first gastral tergite usually slightly darker in shade than the alitrunk and often with a brownish yellow tint.

Type Material
Bolton (1987) - Syntype workers, female, Sudan: Imatong Mts, 6200 ft (1890 m), 2.viii.1939, no. 1397 (N. A. Weber) [examined].