Acanthomyrmex

Eguchi, Bui and Yamane (2011) - Acanthomyrmex species nest in dead twigs and wood fragments, and under or between stones. Colonies of Acanthomyrmex glabfemoralis contain a single dealate normal queen, or instead contain single or multiple dwarf queens, while colonies of A. humilis contain ergatoid queens only (Eguchi et al. 2008). Acanthomyrmex glabfemoralis and Acanthomyrmex humilis gather seeds (Eguchi et al. 2004, Eguchi pers. observ.).

Identification
Eguchi, Bui and Yamane (2011) -Workers of Vietnamese species have the following features (see also Moffett 1986; Eguchi et al 2008): Worker dimorphic; frontal lobe reduced, in full-face view only partially concealing the torulus; frontal carina present, usually conspicuous; antennal scrobe present, usually conspicuous; anteromedian margin of clypeus weakly convex with a shallow median emargination in major; anteromedian margin of clypeus always armed with 2 to several processes in minor; median clypeal seta reduced or absent in major, but always present in minor; lateral portions of clypeus not forming a conspicuous ridge in front of torulus in major, but raised into a ridge in front of torulus in minor; mandible in major massive, edentate or armed with a few indistinct teeth on masticatory margin; mandible in minor broadly triangular, and armed with 5–10 small to tiny teeth in addition to relatively conspicuous apical and preapical teeth; antenna 12-segmented, with 3-segmented club; eye moderately developed; mesosoma short and robust especially in major; promesonotal suture absent dorsally; metanotal groove obsolete or shallow dorsally; propodeum armed with a pair of long spines; propodeal lobe well developed, angulately produced posterodorsad; petiole pedunculate anteriorly and with distinct node; postpetiole much shorter than petiole; suture between first gastral tergite and sternite basally in the form of a rounded M-shape; postpetiole articulated at base of the M.

The minor worker of Acanthomyrmex is similar to the worker of Pristomyrmex, but in the latter the antenna is 11-segmented, the masticatory margin of the mandible is almost vertical to the basal margin and 3-, 4- or 5-toothed, and the suture between first gastral tergite and sternite is not in the form of a rounded M-shape.

Biology
Acanthomyrmex species nest in cavities in dead twigs and wood fragments, under or between stones and in litter on the forest floor, and their colony size is usually small (Moffett 1985, Terayama et al. 1998, Eguchi et al. 2004, Yamada et al. 2018). Colonies are often aggregated in patches. Because reproduction by ergatoid queens is dominant in some species of Acanthomyrmex (Terayama et al. 1998, Yamada et al. 2018), such aggregation of colonies may be caused by the low dispersal ability of the ergatoid founder. Species in this genus predate on fig seeds, and soldiers (e.g. Acanthomyrmex careoscrobis) are specialised to crack such seeds (Moffett 1985).

Yamada et al. (2018) - Acanthomyrmex workers have ovaries but lack spermatheca, and exhibit a distinct dimorphism, i.e., major and minor worker that are morphologically different. Majors are considered to serve in colony defense, seed milling and food storage (Moffett 1985; Buschinger & Maschwitz 1998; Terayama et al. 1998; Gobin & Ito 2000, 2003; Eguchi et al. 2008). All female castes were shown to lay trophic eggs in Acanthomyrmex ferox and nutrient redistribution via trophic eggs among colony members was confirmed (Gobin & Ito 2000).

Queens in the genus Acanthomyrmex have so far been described for seven species: two species (A. ferox and Acanthomyrmex thailandensis are characterized by the presence of alate queens, four species (Acanthomyrmex humilis, Acanthomyrmex padanensis, Acanthomyrmex minus, and Acanthomyrmex sulawesiensis) are characterized by the complete replacement of alate queens with ergatoid queens (see Moffett 1986 and the original descriptions); and a single species (Acanthomyrmex glabfemoralis) exhibits intraspecific polymorphism of queen phenotype (Eguchi et al. 2008). Eguchi et al. (2008) reported that queenright colonies of A. glabfemoralis at four sites in northern Vietnam (Chua Yen Tu (Quang Ninh Province), Ky Thuong Nature Reserve (Quang Ninh), Cuc Phuong National Park (Ninh Binh) and Pu Mat National Park (Nghe An)) had a single dealate queen, whereas those at Van Ban (Lao Cai) had a single or multiple ergatoid queens. Both queen types were found in separate colonies at Tay Yen Tu Nature Reserve (Bac Giang), though intracolonial polymorphism (i.e., production of both queen types in a single colony) has so far not been documented in this species (Eguchi et al., 2008).

Moffett (1986) mentioned remarkable “variations” of the minor of Acanthomyrmex basispinosus and Acanthomyrmex mindanao. These “variations” are likely misrecognition of ergatoid queens, because the abnormal “minor” individuals show morphological traits similar to those of ergatoid queens in A. humilis, A. padanensis, A. minus and A. sulawesiensis (Yamada et al. unpublished). According to our morphological examinations of specimens obtained through colony sampling (Yamada et al. unpublished), Acanthomyrmex concavus and Acanthomyrmex dusun also have ergatoid queens instead of alate queens (images of an ergatoid A. concavus queen given at AntWeb: https://www.antweb.org/specimen/CASENT0101061), while Acanthomyrmex crassispinus in Taiwan exclusively had ergatoid queens, with similar morphological traits to those of A. glabfemoralis in Vietnam. These observations suggest that reproduction by ergatoid queens is more prevalent in the genus Acanthomyrmex than previously recognized.

Castes
Dimorphic worker polymorphism, i.e, two distinctive worker castes, minors and majors, with no intermediates. Queen polymorphism, winged and ergatoid queens.

Worker Morphology
These characters are collated in the table "WorkerMorphology". View table.

Species Uncertain

 * Acanthomyrmex sp.1:
 * Acanthomyrmex sp.2:
 * Acanthomyrmex sp.3:
 * Acanthomyrmex:

Nomenclature

 *  ACANTHOMYRMEX [Myrmicinae: Myrmecinini]
 * Acanthomyrmex Emery, 1893a: cclxxvi. Type-species: Acanthomyrmex luciolae, by subsequent designation of Bingham, 1903: 191.
 * [Acanthomyrmex also described as new by Emery, 1893f: 244].

Description
Moffett (1986) - Dimorphic myrmicine ants. Worker castes with trunks short and stout, convex dorsally in profile except for prominent propodeal spines (and usually with cylindrical humeral spines in minor workers); antennae 12-merous, with scape having a thin, plate-like flange encircling its base. Major caste with head huge, articulating with trunk ventrally so that the vertex of the head projects back over the trunk. Minor workers with a prominent medial hair on anterior border of clypeus.

Minor Worker Small to moderate sized ants (total length 2.7 to 5.0 mm), usually with little size variation within a series. Head large (ca. 50% wider than trunk), wide (CI at least slightly greater than 100); head width 0.70 to 1.23 mm. Cephalic sculpture typically densely foveate, occasionally alveolate or virtually smooth. Eyes moderately large (length of eye in full-face view 17 to 25% of head length), oval, and strongly convex; located mediolaterally, with the distance between anterior margin of eye and mandible bases representing 24 to 34% of head length. Antennal fossae far apart, at anterior margin of head. Feeble scrobe usually present dorsad to eyes for all or at least the basal part of the scape (bent downwards apically in some species to accommodate part of the funiculus); the frontal carinae extend back to form the dorsalmost borders of the scrobes. Antennae 12-merous, moderately thick, with a three-segmented funicular club; club about as long as remainder of funiculus excluding basal funicular segment. There is a prominent flange above the basal radical, forming a thin, disc-shaped "base" to the scape.

Clypeus produced back between bases of antennae; posterolateral borders of clypeus raised to form trenchant ridges which border the antennal fossae in front. Clypeus usually with lateral foveae or rugae; anterior margin with several small, well-separated projecting lobes and with a prominent medial hair. Mandibles massive (MI 69 to 92), with a wide, subtriangular blade which curves ventrad toward apex, with the margin having four to ten tiny teeth separated by wide gaps in addition to two relatively prominent apical teeth; dorsal mandibular surfaces feebly punctate and with a very feeble rugulose microsculpture. Maxillary palpi with four segments; labial palps with three (Acanthomyrmex basispinosus and Acanthomyrmex mindanao examined).

Trunk very short and robust, evenly convex in profile, with anterior margin of propodeum not at all impressed dorsally; trunk diminishing in width posteriorly. Pronotum fused with mesonotum; promesonotal suture obsolete. Propodeum invariably bispinose; pronotum generally armed with dorsolateral spines, which are located anterodorsally to the dorsalmost point reached by the groove between the pronotum and mesothorax laterally. Sculpture foveate on pronotum and dorsally on trunk; areolate-rugose on pleura of remainder of trunk, and smooth on declivity of propodeum. Petiole pedunculated in front; node usually with dorsolateral denticles or spines. Postpetiole variable, low and rounded dorsally or with a prominent node; usually rugose. Gaster oval, less wide than head, and longer than deep. Legs moderately long, with hind femora 85 to 148% as long as head; middle and hind tibiae with a single basal spur. Long, erect to suberect pilosity generally dense on head, sparser on trunk, and variable on legs, petiole, postpetiole and gaster; on head hairs mostly arise from foveae.

Major Worker Head enormous, appearing greater in volume than remainder of body, and of a length about twice that of trunk or longer. Foramen located ventrally far from posterior margin of head, with the portion of the head behind the foramen extending back over the trunk; trunk partially hidden within a cavity beneath head posteriorly. Cephalic sculpture foveate (at least on vertex), but with foveae generally small, shallow and sparse relative to those of minor workers. Ocelli lacking; compound eyes similar to those of minor worker, but slightly more anterior on head, and of a length 9 to 14% of that of the head. Antennae similar in size to those of minor workers, and thus much smaller relative to head size; scrobes as described for minors but more prominent, with a shallow recess for basal portion of funiculus typically also present dorsad to eyes. Clypeus smooth, without small projecting lobes along anterior margin, and usually lacking pilosity. Mandibles massive, similar to minors but smaller relative to head size (MI ca. 50 to 60), typically with dorsal masticatory margin worn; with a ventral process shaped like a massive, blunt tooth beneath mandible basally (occasionally present as small denticle in minors). Trunk, waist, and gaster similar to minors, but pronotal spines lacking, and pronotum smooth anteriorly.

Queen Previously undescribed. Head smaller than in major caste, and attached to trunk nearer to posterior margin of head; not projecting back over trunk. Mandibles, antennae and compound eyes similar to worker castes, and (relative to head length) intermediate in length between those of majors and minors; ocelli present. Clypeus as in majors. Mandibles each with a ventral process like those of majors. Trunk very short and stout, quadrate in lateral view; in dorsal view rounded, almost as wide as long. Pronotal spines lacking; propodeal spines present. Scutellum dorsolaterally with short, thick spines projecting posteriorly above propodeal spines; posterior margin of scutellum directly above that of propodeum. Known only for Acanthomyrmex ferox; for further details see ferox description.

Male Previously undescribed. Head broad; eyes similar to those of queen but much larger relative to head size (length about 40% of that of head). Antennal scrobes absent. Antennae 13-merous. Scapes thick, not reaching posterior margin of head, and virtually lacking a basal flange. First and seventh funicular segments very short, about as long as broad, with the segments between them each about a third the length of scape; third and fifth segments somewhat compressed (others cylindrical), the fifth curved; segments beyond the sixth longer, and progressively lengthening distally (terminal segment about 80% of scape length). Mandibles as long relative to head length as in queen or major, but much less massive, only very slightly curved ventrad apically; the six to eight teeth along masticatory margin less reduced than in female castes, and without large gaps between them; ventral mandibular process lacking.

Trunk similar to that of queen, but somewhat longer; mesonotum with an impressed Mayrian furrow; scutellum higher; propodeum lacking spines, pinched laterally below spiracles, and extending out posteriorly beyond back margin of scutellum. Postpetiole with node very low and rounded; postpetiole low, smooth, and more elongate than in females; legs long and slender. Parameres curving sharply ventrad midway along length, with narrow, rounded tips having very short pilosity. Digitus heavily sclerotized, curving strongly ventrad near base, and rounded distally; cuspidal lobes very low and inconspicuous. Aedeagus subrectangular, ventral margin serrate, with tiny, very sharp recurved teeth. Wings as in Pristomyrmex, but with a complete discoidal cell (m-cu vein present).

Male known only for Acanthomyrmex ferox; for further details see description of that species.

Larva Known for two species. The larvae of A. ferox are considerably different from those of Acanthomyrmex notabilis (see Wheeler and Wheeler, 1977, 1983, and in preparation). The differences are strong enough to suggest separation at the generic or even tribal level (Wheeler and Wheeler, personal communication). The alternative is a rapid evolution of the larvae within the genus Acanthomyrmex.