Acanthomyrmex humilis

This species nests in cavities of rotting twigs and wood fragments.

Identification
Eguchi, Bui and Yamane (2008) - This small-sized species should be included in the Acanthomyrmex notabilis group defined by Moffett (1986). It may be misidentified as Acanthomyrmex careoscrobis, Acanthomyrmex concavus, Acanthomyrmex foveolatus or Acanthomyrmex mindanao if one carelessly uses Moffett's keys, and it is rather similar to Acanthomyrmex minus and Acanthomyrmex padanensis. It is, however, characterized among the known Acanthomyrmex species by the unique shape of petiole in the major, minor and ergatoid mentioned in the description: anterior pedicel of petiole very long; petiolar node very low, with the apex at most very weakly emarginated medially in posterior view; postnodal face in lateral view long and gently sloping.

Distribution based on Regional Taxon Lists
Oriental Region: Vietnam.

Biology
Colonies of Acanthomyrmex glabfemoralis contain a single dealate normal queen, or instead contain single or multiple dwarf queens, while colonies of Acanthomyrmex humilis contain ergatoid queens only (Eguchi et al. 2008). A. glabfemoralis and A. humilis gather seeds (Eguchi et al. 2004, Eguchi pers. observ.).

Nomenclature

 * . Acanthomyrmex humilis Eguchi, Bui & Yamane, 2008: 238, figs. 22-30 (s.w.ergatoid q.) VIETNAM.
 * Status as species: Yamada, Ito, et al. 2018: 10.{nomenplus}}

Worker
Holotype major. HL 1.73 mm; HW 1.55 mm; SL 0.85 mm; HFL 0.84 mm; CI 90; SI 55; FI 54. Major (1 paratype and 2 nontypes). HL 1.68-1.88 mm; HW 1.50-1.69 mm; SL 0.81-0.86 mm; HFL 0.82-0.88 mm; CI 90-91; SI 50-54; FI 52-55.

Minor (1 paratype and 2 nontypes). HL 0.83-0.85 mm; HW 0.88-0.91 mm; SL 0.80-0.82 mm; HFL 0.80-0.83 mm; CI 104-107; SI 88-92; FI 88-94.

Major Body reddish brown to dark reddish brown; antennae, legs, basal and apical part of gaster light brown to yellowish brown. Body sculpture as in Figs. 22-24. Head in full-face view roundly convex posteriorly, at most with inconspicuous posteromedian concavity; dorsal and lateral faces of cranium sparsely with standing hairs; frontal lobe poorly developed, only partly concealing antennal socket; frontal carina conspicuous; antennal scrobe deep and conspicuous, posteriorly bent anteroventrad to form an inconspicuous “funicular scrobe” above compound eye; median part of clypeus produced anteriad, with median emargination on anterior margin; median and lateral setae of anterior clypeal margin short and thin, or absent; antenna 12-segmented with 3-segmented club; antennal scape when laid backward reaching or extending a little beyond midlength of head, basally with weak leading edge; outer face of mandible shagreened or dimly rugose; masticatory margin of mandible almost edentate (teeth having been worn down to nothing, or inconspicuous apical and preapical teeth often recognizable). Mesosoma in lateral view short and stout; promesonotal suture absent dorsally; pronotal spine absent; metanotal groove present as inconspicuous broad impression just anteriorly to base of propodeal spine; metapleural lobe well developed, angularly projecting posterodorsad; propodeal spine in lateral view down-curved, without standing hairs (but with a few very short decumbent to appressed hairs). Dorsum of each femur with a few standing hairs. Petiole in lateral view with very long anterior pedicel; petiolar node in lateral view low but with relatively angulate apex, in posterior view narrowly raised with truncate apex which is often emarginate weakly (as in the holotype); "post-nodal" face behind apex of petiolar node till articulation with postpetiole long and gently sloping in lateral view, with low step near articulation (Fig. 22); subpetiolar process vestigial; postpetiole in lateral view weakly raised dorsally, (a little) shorter than high and (a little) shorter than broad (excluding helcium). First gastral tergite sparsely with erect hairs.

Minor Body dark reddish brown; mandibles, antennae, legs, basal and apical part of gaster light brown to yellowish brown. Body sculpture as in Figs. 25-27. Head with standing hairs, in full-face view with its posterior margin broadly concave; frontal lobe poorly developed, only partly concealing antennal socket; frontal carina conspicuous; antennal scrobe shallow but conspicuous, posteriorly not bent anteroventrad; anterior clypeal margin armed with several teeth, with conspicuous median and lateral setae; posteromedian part of clypeus broadly inserted between frontal lobes; eye in full-face view strongly convex, located around or a little behind midlength of head; antenna 12-segmented with 3-segmented club; antennal scape when laid backward reaching or extending a little beyond posterolateral corner of head; masticatory margin of mandible with relatively small apical and preapical teeth followed by small denticles that are relatively broadly separated. Mesosoma in lateral view relatively stout; its dorsum sparsely with standing hairs; promesonotal dome in lateral view convex moderately; promesonotal suture absent dorsally; pronotal spine relatively short; metanotal groove absent; metapleural lobe well developed, angularly projecting posterodorsad; propodeal spine long, in lateral view slender and weakly sinuate, without standing hairs but with a few very short appressed hairs. Dorsum of each femur with scattering standing hairs. Petiole in lateral view with very long anterior pedicel; petiolar node in lateral view low but with relatively angulate apex, in posterior view narrowly raised, with round or truncate apex; "post-nodal" face behind apex of petiolar node till articulation with postpetiole very long and gently sloping in lateral view (Fig. 25); subpetiolar process vestigial to absent; postpetiole in lateral view weakly raised dorsally, almost as long as high, and almost as long as broad (excluding helcium). First gastral tergite sparsely with standing hairs.

Queen
Ergatoid (1 paratype and 1 nontype). HL 1.10-1.11 mm; HW 1.17-1.19 mm; SL 0.83-0.84 mm; HFL 0.85-0.86 mm; CI 106-109; SI 70-71; FI 71-73.

Ergatoid Body reddish brown; mandibles, antennae, legs, basal and apical part of gaster light brown to yellowish brown. Body sculpture as in Figs. 28-30. Head with standing hairs, in full-face view weakly concave posteromedially; frontal lobe poorly developed, only partly concealing antennal socket; frontal carina conspicuous; antennal scrobe conspicuous, posteriorly bent anteroventrad to form an inconspicuous “funicular scrobe” above compound eye; anterior clypeal margin armed medially with four projections, with a conspicuous median and lateral setae; compound eyes located a little in front of midlength of head; ocelli absent; antenna 12-segmented with 3-segmented club; antennal scape when laid backward not reaching posterolateral corner of head; masticatory margin of mandible with relatively small apical and preapical teeth followed by small denticles (teeth often having been worn down to nothing). Mesosoma more strongly shortened than that of the minor; its dorsum sparsely with standing hairs; promesonotal suture absent dorsally; pronotal spine much reduced to a dent; metanotal groove present as inconspicuous broad impression just anteriorly to base of propodeal spines; metapleural lobe well developed, angularly projecting posterodorsad; propodeal spine in lateral view very weakly sinuate, without standing hairs but with a few very short decumbent to appressed hairs. Dorsum of each femur with a few standing hairs. Petiole in lateral view with a very long anterior pedicel; petiolar node in lateral view low but with relatively angulate apex, in posterior view narrowly raised, with truncate or very weakly concave apex; "post-nodal" face behind apex of petiolar node till articulation with postpetiole relatively long and gently sloping in lateral view, with low step near articulation (Fig. 28); subpetiolar process vestigial to absent; postpetiole in lateral view relatively highly raised dorsally, much shorter than high and shorter than broad (excluding helcium). First gastral tergite with standing hairs among very short appressed hairs.

Type Material
Type material. Holotype (major): Vietnam: Dong Nai: S. Cat Tien N.P. (Crocodile Lake Trail, < 160 m alt.), 17.X. 2004, leg. K. Eguchi, (colony: Eg04-VN-691). Paratypes: 3 majors, 7 minors, 1 ergatoid queen from the same colony to which the holotype belonged.

References based on Global Ant Biodiversity Informatics

 * Huong N. T. T., P. V. Sang, and B. T. Viet. 2015. A preliminary study on diversity of ants (Hymenoptera: Formicidae) at Hon Ba Nature Reserve. Environmental Scientific Conference 7: 614-620.