Opamyrma hungvuong, Yamane, S., Bui, T. V. & Eguchi, K., 2008

Yamane, S., Bui, T. V. & Eguchi, K., 2008, Opamyrma hungvuong, a new genus and species of ant related to Apomyrma (Hymenoptera: Formicidae: Amblyoponinae), Zootaxa 1767, pp. 55-63 : 57-62

Opamyrma hungvuong sp. n. (Figs. 1-12)

Type material. Holotype (worker): 21 Feb. 2000, Rao An, Son Kim II Commune (18°31'N; 105°27'E), Huong Son District, Ha Tinh Province, northern part of Central Vietnam, leg. T.V. Bui (IEBR). Paratype: 1 worker, same data as in the holotype ( KMNH).

Measurements and indices (holotype and paratype; those for paratype shown in parentheses). Head length (as measured from the anterior margin of clypeus to the posterior margin of head in full-face view) 0.73 mm (0.71); head width (maximum width of head in full-face view) 0.55 mm (0.55); cephalic index (head width/head length x 100) 75 (77); scape length (length of antennal scape excluding the basal condylar bulb) 0.38 mm (0.38); scape index (scape length/head width x 100) 69 (69); mesosomal length (as measured from the anterior margin of pronotum to the posterior margin of propodeum in profile) 1.12 mm (1.12); hind femur length (maximum length of hind femur) 0.50 mm (0.49); hind femur index (hind femur length/head width) 91 (89).

Worker description. Head long, almost rectangular, with slightly convex lateral margins and almost straight posterior margin in full-face view; in profile flattened dorsoventrally. Median part of clypeus with anterior margin weakly and broadly concave. Mandible slender, strongly curved at the apical end of trulleum (this can be clearly observed when the mandibles are opened); basal two-thirds almost parallel-sided in outer view (Fig. 5), with long but bluntly tapered apical tooth followed by a trapezoidal lobe (probably fusion of two preapical teeth: "mtl" in Fig. 3) and three inconspicuous teeth. Antennal scape (segment I) flattened dorsoventrally, narrowed toward base; segment II bead-like, in frontal view strongly narrowed at base ("as-II" in Fig. 2); segment III slightly longer than broad and narrowed basally; segments IV and V almost as long as broad; segments VI-XI broader than long; apical segment longer than broad and bluntly pointed at apex.

Pronotum longer than broad in dorsal view, with slightly convex dorsal face that merges into lateral face roundly; anterior slope short and steep. Remaining portion of mesosoma slightly narrower than pronotum and almost parallel-sided in dorsal view; nota and pleura roundly continuous; mesopleuron separated from metapleuron by a sulcus; lower portion of metapleuron defined posteriorly by a narrow furrow; propodeum with rather flat dorsum and steep posterior face.

Femur and tibia of fore leg broader than those of mid- and hind legs; relatively broad gap present between mid- and hind coxae.

Petiole seen from above much longer than broad, slightly narrowed posteriad, and laterally weakly convex, in profile much longer than high, weakly converging posteriad. Gaster with a long and up-curved sting.

Whole body only weakly sculptured and moderately shining; mandible with sparse large punctures which generally bear setae; dorsum of head superficially punctate; clypeus with posterior portion almost unsculptured and shining; mesosoma more weakly sculptured than dorsum of head, with posteroventral portion of its side irregularly sculptured; petiole and gaster almost smooth and shining.

Head with dense short hairs that are erect or suberect; mandible when closed with lower margin bearing relatively long and sparse standing hairs; antennal scape with sparse erect hairs in addition to denser short pubescence; hairs on funiculus generally short, especially on apical segments; mesosoma and petiole dorsally with sparser standing hairs; erect hairs on tibiae and tarsi shorter than those on femora; gastral terga dorsally with standing hairs that are denser than those on mesosoma; gastral sterna each with isolated erect hairs.

Whole body light brown, with antennae and legs slightly yellowish.

Etymology. The genus name Opamyrma is an anagram of Apomyrma for the first three letters. The specific name ( hungvuong ) derives from the legendary king Hung Vuong who founded the first Vietnamese state Van Lang.

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FIGURES 1-3. Opamyrma hungvuong gen. & sp.n., holotype (worker) — 1, body in profile; 2, head in full-face view (as-II: antennal segment 2); 3, anterior part of head in anterodorsal view (alc: anterolateral corner, lpd: peg-like denticles on labrum, mtl: mandibular tooth 1, pcc: posterior carina of clypeus, trl: trulleum).

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FIGURES 4-6. Opamyrma hungvuong gen. & sp.n., holotype (worker) — 4, head in profile (poc: preoccipital carina); 5, left antenna and left mandible in outer view; 6, mesosoma in profile (msf: mesosomal furrow).

Discussion

Opamyrma is similar to Apomyrma with several shared characteristics: the outer face of the labrum bears peg-like teeth; the frontal lobe is absent; the antennal socket is directed almost dorsad; the sternite of the petiole is reduced to a small posteroventral sclerite, bounded by the conspicuous tergo-sternal sutures; and the third abdominal segment above the helcium has a free anterior face. All this may support the inclusion of the new genus within the separate tribe Apomyrmini with Apomyrma (but see below for the current status of this tribe).

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FIGURES 7-9. Opamyrma hungvuong gen. & sp.n., holotype (worker) — 7, mesosoma and petiole in dorsal view; 8, tibial spurs of left hind leg in anterior view (ats: anterior spur, pts: posterior spur); 9, pretarsal claws of left hind leg.

The features which support the erection of the new genus Opamyrma independent of Apomyrma (see Brown et al 1971; Bolton 1990, 2003 for characterization of Apomyrma ) are: preoccipital carina complete, almost encircling the head slightly before its posterior margin; clypeus posteriorly margined with a distinct continuous carina; petiole without a distinct anterior peduncle; abdominal segment III longer than IV, V and VI; segment VII longest among the segments III-VII; anteriormost part of abdominal sternite III produced anteriad to the same level as the anteriormost part of tergite III; segment IV with differentiated presternite.

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FIGURES 10-12. Opamyrma hungvuong gen. & sp.n., holotype (worker) — 10, petiole in profile (tss: tergo-sternal suture); 11, gaster in profile (abs: abdominal sternite, absg: abdominal segment, abt: abdominal tergite, ps: presternite); 12, gaster in dorsal view.

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Opamyrma also shares some features with members of Leptanillinae , which, however, clearly differ from all the amblyoponines sensu Saux et al. (2004) in having a 2-segmented waist. For example, the morphology of clypeus resembles that of Protanilla and Anomalomyrma , and the structure of the metapleural gland orifice and its surroundings is also rather similar to that seen in Leptanillinae . The extremely hypertrophied pygidium of Opamyrma also reminds us of the condition in some leptanillines.

The complicated history summarized below indicates the difficulty in deciding the systematic position of Apomyrma . Apomyrma was established as a monotypic genus under the subfamily Ponerinae by Brown et al. (1971) from Ivory Coast (Afrotropical region). Wheeler and Wheeler (1985) placed it under the tribe Amblyoponini of Ponerinae, and later Dlussky & Fedoseeva (1988) established the tribe Apomyrmini under Ponerinae. However, Bolton (1990) transferred Apomyrmini to Leptanillinae listing 16 characteristics shared by the traditional Leptanillinae and Apomyrma (but only with the size and location of the spiracle on abdominal segment III as apomorphies), and then Baroni Urbani et al. (1992) established the subfamily Apomyrminae . Bolton(2003), in his morphology-based classification of the family Formicidae, followed their treatment. More recently Saux et al. (2004), based on their molecular phylogenetic analysis, placed the genus under the subfamily Amblyoponinae , and partly modified Bolton's (2003) definition of Amblyoponinae . Although the three recent molecular analyses of these taxa (Saux et al. 2004, Moreau et al. 2006, Brady, et al. 2006) all gave different results, in each of them Apomyrma is the sister group of one or more genera of Amblyoponinae sensu Bolton (2003).

As mentioned above, and also as pointed out by Ward (2007), Apomyrma has some similarities in morphology and behavior with both Leptanillinae and Amblyoponinae (see also Brown et al. 1971, Bolton, 1990, Masuko 1990, Ward 1994). The discovery of the new genus Opamyrma would further require a reexamination of the relationship of these two subfamilies, since Opamyrma possesses additional features shared with Leptanillinae . Here we do not intend to propose any new classification system in which the position of this new genus is appropriately settled. Here Opamyrma is tentatively placed in Amblyoponinae . Since the resolution within Amblyponinae is still insufficient to address relationships among the genera (Saux et al., 2004), we follow the single tribe-rank classification for this subfamily.