Acanthomyrmex padanensis

Colonies nest in downed twigs on the forest floor. Two of three Acanthomyrmex padanensis colonies examined were polygynous, in which two or more ergatoid queens had mated and laid eggs. Colonies were small, with less than 20 minor workers and a few majors. The ergatoid queens and majors of A. padanensis had two ovarioles per ovary, while minors had only one. (Terayama et al. 1998)

Identification
Terayama et al. (1998) - This species mostly resembles Acanthomyrmex dusun from Sarawak with clearly demarcated funicular scrobe in major worker. However, it is easily distinguished from the latter by the clypeus with straight anterior margin and lacking a median projecting lobe, the straight propodeal spines, and convex dorsal margin of petioiar node in posterior view.

Distribution based on Regional Taxon Lists
Indo-Australian Region: Indonesia.

Castes
As in other species of this genus, workers and soldiers are strikingly different, with the latter having heads twice as long as that of workers. Queens in A. padanensis are ergatoid, with a general shape more similar to workers (Terayama et al. 1998). Only ergatoid queens have a spermatheca. The ergatoid queens of A. padanensis have fewer ovarioles than the winged queens of A. ferox.

Two of three colonies were polygynous, with two or more mated ergatoid queens laying eggs. There were less than 20 workers and a few soldiers per colony

Nomenclature

 * . Acanthomyrmex padanensis Terayama, Ito & Gobin, 1998: 257, figs. 1-6, 19-21 (s.w.q.) INDONESIA (Sumatra).
 * Status as species: Yamada, Ito, et al. 2018: 10.{nomenplus}}

Worker
Major. (holotype). HL 2.30 mm; HW 2.15 mm; SL 1.08 mm; CI 93; SI 50; WL 1.05 mm; PL 0.58 mm; PH 0.40 mm; DPW 0.28 mm; PPL 0.28 mm; PPH 0.45 mm; PPW 0.35 mm; TL ca. 6.0 mm.

Head slightly longer than wide, with parallel sides and strongly concave posterior margin in full face view; cephalic median furrow well developed, extending below to the frontal area; frons and vertex subopaque with shallow punctures sparsely, and without hairs. Mandible massive, subopaque, and with two blunt teeth near basal portion of masticatory margin. Anterior margin of clypeus almost straight, without a projecting median lobe. Antennal scape slightly longer than the funiculus, with more than 15 long erect hairs; 2nd segment 1.3 X as long as wide; 3rd to 9th segments each wider than long. Antennal scrobe developed, with a clearly demarcated funicular scrobe. Eye 0.23 mm in diameter.

Alitrunk short, smooth and shining; dorsum with several relatively long erect hairs. Propodeal spines long and straight, 0.35 mm long, and converging to the tip; propodeal declivitous surface microreticulate. Petiole long, 1.45 X as long as high, largely smooth; a pair of posterodorsal hairs present; lateral petiolar hairs present; node with weakly convex anterior and posterior margins and obtusely angulate dorsum in profile; in posterior view, node reverse V-shaped as in Fig. 19. Postpetiole 0.62 X as long as high, with 3 pairs of erect hairs; anterior and posterior margins straight; dorsal margin convex and highest at anterior 1/3 in profile; in dorsal view, postpetiole 3.0 X as wide as long.

Gaster subopaque, with erect hairs sparsely.

Head including mandibles castaneous; alitrunk, petiole, and postpetiole reddish brown; gaster dark brown; antennae and legs yellowish brown.

Minor. HL 0.95 mm; HW 1.15 mm; SL 1.00 mm; CI 121; SI 87; WL 1.08 mm; PL 0.50 mm; PH 0.33 mm; DPW 0.17 mm; PPL 0.21 mm; PPH 0.28 mm; PPW 0.28 mm; TL 3.8 mm.

Head wider than long, with convex posterior margin in full face view; frons and vertex coarsely punctate; punctures 0.07-0.10 mm in diameter. Mandible subopaque and impunctate. Clypeus with 2 longitudinal rugae; median clypeal lobes feebly developed; clypeal index 73. Eye 0.25 mm in diameter.

Pronotum coarsely punctate; punctures 0.07-0.09 mm in diameter; pronotal tooth 0.33 mm long; mesonotal dorsum straight in profile; lateral surfaces of mesonotum and propodeum with strong carinae. Propodeal spine long, 0.45 mm long.

Petiole 1.5 X as long as wide, with obtusely angulate dorsal margin of node in profile; a pair of posterodorsal hairs present; lateral petiolar hairs present; in posterior view, node reverse U-shaped as in Fig. 20. Postpetiole 0.75 X as long as high, with 2 pairs of dorsal and a pair of lateral hairs, highest in anterior portion in profile; in dorsal view, post petiole 0.58 X as long as wide.

Gaster shining, with erect hairs moderately.

Body yellowish brown excepting the gaster dark brown.

Queen
Ergatoid. HL 1.25 mm; HW 1.00 mm; SL 0.93 mm; CI 80; SI 93; WL 1.08 mm; PL 0.50 mm; PH 0.38 mm; DPW 0.25 mm; PPL 0.20 mm; PPH 0.40 mm; PPW 0.38 mm; TL 4.2 mm.

General shape as in minor worker excepting the shorter pro notal teeth (0.10 mm long), and much higher postpetiole (PPH 0.40 mm).

Type Material
Holotype. Major worker, Ulu Gadut, Padang, Sumatra, Indonesia, 28.111.1997, F. Ito leg.

Paratypes. Six minor workers, 2 ergatoid queens, same colony as holotype; 2 major workers, 9 minor workers, 4 ergatoid queens, same locality, 30.111.1997, F. Ito leg.; 3 major workers, 10 minor workers, 1 ergatoid queen, same data.

Etymology
The specific epithet refers to the type locality.

References based on Global Ant Biodiversity Informatics

 * Terayama M., F. Ito, and B Gobin. 1998. Three new species of the genus Acanthomyrmex Emery (Hymenoptera: Formicidae) from Indonesia, with notes on the reproductive caste and colony composition. Entomological Science 1: 257-264.