Parvimyrma, Eguchi, Katsuyuki & Bui, Tuan Viet, 2007

Eguchi, Katsuyuki & Bui, Tuan Viet, 2007, Parvimyrma gen. nov. belonging to the Solenopsis genus group from Vietnam (Hymenoptera: Formicidae: Myrmicinae: Solenopsidini), Zootaxa 1461, pp. 39-47 : 40-44

Parvimyrma gen. nov.

Type-species. Parvimyrma sangi sp. nov. (Figs. 1–10)

Worker diagnosis. Monomorphic myrmicine ants with the following combination of characteristics. Frontal lobe in full-face view only partially concealing the toruli, not extending posteriorly as a frontal carina; antennal scrobe absent; posteromedian portion of clypeus narrowly inserted between frontal lobes; median clypeal seta well developed; 1st paracarinal seta well developed; lateral portions of clypeus not forming a raised rim or shield wall in front of the antennal insertions; the position of anterior tentorial pit as in Fig. 4; mandible triangular, overlapping but not crossing over at full closure, with 5 distinct teeth on the masticatory margin but without any tooth/denticles on the basal margin; trulleum open; hypostoma with a conspicuous lateral tooth just mesal to each mandibular base; anterior margin of labrum broadly concave medially; palpal formula: maxillary 2 and labial 2; antenna 11-segmented, with a 2-segmented club ( Fig. 6 ); the apical antennal segment elongated much more than the preapical segment; eye completely absent. Promesonotum low, in profile almost flat or very weakly convex dorsally, without conspicuous humerus; promesonotal suture completely absent dorsally; mesosoma in dorsal view constricted between promesonotum and propodeum; metanotal groove relatively shallowly impressed dorsally; propodeum unarmed but with a narrow cuticular rim on each posterolateral corner of the dorsum; the rim running downward and connecting with metapleural lobe; each meso and metasternum without a conspicuous process; metapleural lobes low and round; propodeal spiracle small, situated a little behind the midlength of the sides of propodeum; metapleural gland large. Fore basitarsus with a long and thick seta on the posteroinner margin (sbt in Fig. 8 ); meso and metatibial spur absent. Petiole pedunculate anteriorly and with a distinct node; the peduncle with a small anteroventral process; postpetiole much shorter than petiole, in dorsal view a little broader than petiolar node, narrowly attached to the anteriormost end of gaster. Abdominal tergite IV (= gastral tergite I) broadly overlapping the sternite IV on the ventral surface of abdomen; gastral shoulder present (gs in Fig. 9 ); sting poorly developed (st in Fig. 9 ). Body smooth to very weakly sculptured.

Notes. The characteristics highlighted above in italics are good grounds for placing Parvimyrma gen. nov. in the Solenopsis genus group (sensu Bolton, 2003). The morphology of Parvimyrma , however, disagrees with the other known genera belonging to the genus group as follows.

1) Allomerus — posteromedian portion of clypeus broadly inserted between frontal lobes; eye quite well developed; palpal formula (PF) 3, 2; antenna with a 3-segmented club in which the proximal end of each segment forms a clear neck; promesonotum forming a dome (see Ettershank, 1966; Bolton, 1987).

2) Anillomyrma — masticatory margin of mandible with 3–4 teeth; mandibular blades crossing over at full closure; PF 2, 1; antenna 10-segmented, with a 3-segmented club; petiolar peduncle lacking an anteroventral process; postpetiole in dorsal view broadly attached to anterodorsal part of gaster; sting large (see Bolton, 1987).

3) Bondroitia — mandibular blades crossing over at full closure; antenna with a 3-segmented club; metapleural gland small and inconspicuous; propodeal spiracle enormous, very close to margin of posterior face of propodeum; petiolar peduncle lacking an anteroventral process (see Bolton, 1987).

4) Carebarella — median portion of clypeus sharply defined by a pair of lateral carinae and forming a raised, oblong, flat region; masticatory margin of mandible with 4 teeth; PF 1, 2; eye present (but poorly developed); metanotal groove deeply impressed on the dorsum of mesosoma (see Ettershank, 1966; Bolton, 1987).

5) Diplomorium — median portion of clypeus not suddenly raised; posteromedian portion of clypeus broadly inserted between frontal lobes; antenna with a weakly differentiated club of 3 segments; eye present and conspicuous; promesonotum convex but not flat; postpetiole very narrowly attached to gaster; sting well developed (see Bolton, 1987).

6) Epelysidris — anterior clypeal margin with a pair of stout triangular teeth; basal margin of mandible with two broad-based bluntly triangular lobes; PF 3, 2; antenna 12-segmented, with a strongly differentiated club consisting of 3 segments; eye present (but small); promesonotum strongly convex; sting long and strong (see Bolton, 1987).

7) Megalomyrmex — PF 4, 3 or 3, 2; antenna 12-segmented, with a 3-segmented club; eye well developed; metapleural lobes connected above by a distinct carina (see Ettershank, 1966; Bolton, 1987).

8) Monomorium — antennal club never of 2 segments; eye present (but reduced to a single ommatidium in the fossulatum -group) (see Bolton, 1987).

9) Nothidris — PF 4, 3; antenna 12-segmented, with a 3-segmented club; eye well developed (see Ettershank, 1966; Bolton, 1987).

10) Oxyepoecus — masticatory margin of mandible with 4 teeth; antenna with a 3-segmented club; eye well developed; propodeum with a pair of sharp angles or dents (propodeal spine) (see Bolton, 1987).

11) Phacota — masticatory margin of mandible with 4 teeth; eye present. The diagnosis of the genus is poor because the single specimen of the genus (the holotype of the single member, Phacota sichelii ) appears to have been lost or destroyed (see Bolton, 1987).

12) Solenopsis — masticatory margin of mandible at most with 4 teeth; antenna 9- or 10-segmented (see Ettershank, 1966; Bolton, 1987, 2003).

FIGURES 1–3. Parvimyrma sangi , worker — 1, holotype (point-mounted), body in profile; 2, paratype (pointmounted), head in full-face view; 3, paratype (slide-mounted), anterior part of head in ventral view. labr, labrum (anterior margin); ms, median seta of clypeus; spp, subpetiolar process; ten, tentorium; tr, trulleum; 1st pcs, 1st paracarinal seta of clypeus.

The general habitus of Parvimyrma is very similar to that of some members of Carebara . Because the concept of the genus Carebara was broadened by Fernández (2004), the presence of a median clypeal seta is the only characteristic separating Parvimyrma from Carebara . Fernández (2004) subdivided Carebara into three species complexes: C. concinna species complex (= Oligomyrmex sensu Ettershank, 1966 ), C. escherichi species complex (= Paedalgus sensu Bolton & Belshaw, 1993) and C. lignata species complex (= Carebara sensu Ettershank, 1966 ); and furthermore, he considered Afroxyidris , a monotypical genus established by Belshaw & Bolton (1994), to be highly apomorphic within Carebara . Below we provide characteristics of the three species complex and C. crigensis (= Afroxyidris crigensis ) which disagree with the worker diagnosis of Parvimyrma .

FIGURES 4, 5. Parvimyrma sangi , worker — 4, paratype (point-mounted), anterior part of head in anterodorsal view; 5, paratype (slide-mounted), mouthparts in ventral view. atp, anterior tentorial pit; lht, lateral hypostomal tooth; lp, labial palp; mp, maxillary palp; ms, median seta of clypeus; 1st pcs, 1st paracarinal seta of clypeus.

1) C. concinna species complex — worker dimorphic, major worker with massive head and mesosomal segmentation relatively well developed (see Ettershank, 1966; Fernández, 2004).

2) C. escherichi species complex — eye present (but reduced to 1–4 ommatidia); antenna 8- or 9-segmented; dorsum of propodeum very short, followed by a long steep posterior slope; sting strongly developed (see Ettershank, 1966; Bolton and Belshaw, 1993).

3) C. lignata species complex — antenna 9-segmented (see Ettershank, 1966; Fernández, 2004).

4) C. crigensis — Mandible with two apical teeth followed by a long oblique edentate margin and a smaller basal tooth; median portion of clypeus with a transverse step; antenna 10-segmented, with a 2-segmented club; sting well developed (see Belshaw & Bolton, 1994).

We found one Indo-Chinese species (worker, queen and male) and one Indo-Malayan species (worker only) which agree well with the concept of the C. lignata species complex despite the two species showing a critical diagnostic charasteristic of the Solenopsis genus group, i.e., a developed median clypeal seta present. The two species should be determined as Solenopsis by following Bolton (1994), but they show a series of characteristics which is seen in the typical worker of C. lignata species complex: antenna 9-segmented, with a 2-segmented club; eye completely absent; the median portion of clypeus in profile roundly and strongly swollen; clypeal carina evanescent or absent; clypeal teeth completely absent; the masticatory margin of their mandible with the apical and two distinct subapical teeth which are followed by one to three much reduced teeth (5–6 teeth in total); promesonotum in profile rather flat dorsally; in alates radial cell completely closed (see Ettershank, 1966; Bolton, 2003).

Fernández (2004) examined a Carebara anophthalma worker from Ecuador having a developed median clypeal seta, but he concluded that it is a local variation ( C. anophthalma is a member of the Carebara lignata species complex). Furthermore, recent molecular phylogenetic studies of ants ( Moreau et al., 2006; Brady et al., 2006) do not support the monophyly of the tribe Solenopsidini and the Solenopsis genus group. These facts mean that the presence or absence of a median clypeal seta has been over-weighted in the classification of myrmicine ants. Because “lumping genera” seems to be a trend, it may be inevitable that Carebara , Solenopsis and their neighboring taxa (including Parvimyrma ) will be combined into a single genus which may be too large and too heterogeneous. However, for the present, our opinions are as follows: 1) the proposition of Parvimyrma under the Solenopsis genus group is valid; 2) the two “ Carebara -like” species are tentatively treated as undetermined species of the genus Solenopsis (a comprehensive revision of Old World species of Carebara lignata species complex is needed); 3) Parvimyrma is distinguished from Solenopsis , the morphologically closest genus, by its 11-segmented antenna and its triangular mandible with 5 distinct teeth on the masticatory margin; 4) the concept of the tribe Solenopsidini and the delimitation between its two genus groups are preserved because of its large practical value for descriptive taxonomy.

Parvimyrma is easily distinguished from the other myrmicine genera known from the Indo-Chinese subregion by a combination of the following features: median clypeal seta well developed; posteromedian portion of clypeus narrowly inserted between frontal lobes; masticatory margin of mandible with 5 distinct teeth; antenna 11-segmented, with a 2-segmented club; eye completely absent.

Figures

FIGURES 6 – 8. Parvimyrma sangi, worker — 6, paratype (slide-mounted), right antenna in ventral view; 7, holotype (point-mounted), mesosoma and waist in dorsal view; 8, paratype (slide-mounted), left foreleg in posterior view. as, antennal scape; sbt, seta on the posteroinner margin of fore basitarsus.

FIGURES 9, 10. Parvimyrma sangi, worker — 9, paratype (slide-mounted), the base of gaster in ventral view; 10, paratype (slide-mounted), the apex of gaster in profile. gs, gastral shoulder; st, sting.