Meranoplus dlusskyi

Meranoplus dlusskyi is only known from Dong Nai Culture and Nature Reserve of Southern Vietnam. Specimens were collected from soil cores obtained from tropical rainforest. Reduced eyes and maxillary palps, depigmentation of the cuticle, superficial sculpture, shortened and flattened tibiae and an obsolete promesonotal shield collectively suggest this species has a hypogaeic lifestyle. M. dlusskyi workers also differ from epigaeic species by the followed features (for comparison were used M. bicolor workers): 1) enlarged metapleural gland bulla, 2) deep and curved mandibular grooves that seem to be connected via mandalus with ducts of mandibular glands and 3) concentration of trichoid curvata sensillae [sensu Dumpert, 1972] on the much enlarged club segments of antennae. These features are believed to amplify signal functions (namely of chemical signalization), important for underground lifestyle. The morphological distinctions and its apparent specialization as a underground forager are distinctive within the genus. (Zryanin 2015)

Identification
Zryanin (2015) - Monomorphic subterranean myrmicine ants but with clearly size-variable workers (TL 2.3–3) that have vestigial eyes (0–2 ommatidia per eye) and reduced/obsolete promesonotal shield, depigmented cuticle with superficial sculpture and shortened appendages. Other important characteristics are: broad frontal carinae and well-defined antennal scrobes that extend above eyes, 9-merous antennae with 3-merous club, mandible with 5 teeth, sessile petiole with cuneate node, aculeus with dorsal flange and spatulate lancets. From all known species of the genus as defined by Bolton [1981, 2003] and Boudinot and Fisher [2013] M. dlusskyi differs by palp formula 3.3 (5.3 in other species).

Distribution based on Regional Taxon Lists
Oriental Region: Vietnam.

Castes
Known only from the worker caste.

Nomenclature

 *  dlusskyi. Meranoplus dlusskyi Zryanin, 2015: 155, figs. 1-12 (w.) VIETNAM.

Worker
Habitus of worker as illustrated (Figs 1–3). Head in full-face view subquadrate, slightly broadened posteriorly (trapezoid above antennal scrobes so that genae and temples are distinctly prominent), posterior outline broadly convex. Median portion of clypeus narrowing anteriorly, not projecting beyond apices of frontal lobes, the margin with an anteclypeal carina but without an apron as well without conspicuous prominence or denticle where the anterior margin meets the longitudinal carinae which bound the side of the median portion of the clypeus (Figs 4, 6). Labrum cleft medially. Frontoclypeal suture continuing anteriad to malar space and forming clypeomalar suture separating narrow premalar space from malar space, which also differs in sculpture (Fig. 4). Frons anteriorly delimited by faint but moderately large frontal triangle. Frontal carinae rather long, diverging from clypeus to vertex, sinuate. Frontal lobes distinct. Antennal scrobes conspicuous in full-face view, dorsally delimited by frontal carinae and ventrally by anteocular costulae. Ventral surface of head with several subocular rugae on each side (Fig. 5). Eyes strongly reduced, situated ventral to antennal scrobes and slightly posteriad between torulus and occipital margin, consisting of 1 or 2 ommatidia or 1 ommatidium in one eye but none in the other; in some specimens eyes completely absent. Mandibles triangular, possessing deep margin with 5 teeth; apical tooth largest, preapical tooth half size of apical, 3rd tooth smaller than preapical and 4th teeth, basal tooth smallest, sometimes obsolete. Strong swelling (denticle) present on ventral side of mandible beneath third tooth of masticatory margin (Fig. 6, arrows) as in other Meranoplus [Boudinot, Fisher, 2013]. Palp formula: maxillary 3, labial 3 (Fig. 5). 1st and 2nd segments of maxillary palps with distinctive silhouettes, suggesting formation of 2 segments due to partial and complete fusion [sensu Ettershank, 1966], respectively. Stipes of maxilla lacking a distinct transverse crest, which is visible as trace. Antennae 9-merous with 3-merous club. Scapes thicker in apical than in basal halves; club strongly incrassate, more than half the length of flagellum.

Mesosoma (alitrunk) compact, dorsally forming a promesonotal shield that is characteristic of Meranoplus, but it is obsolete (Figs 2, 7). Promesonotum very slightly wider than long. Anterior pronotal corners weakly projecting, blunt; lateral margins of pronotum subparallel, promesonotal suture absent, lateral mesonotal margins narrowed posteriorly and produced into small foliaceous projections. Mesopleuron well impressed (mesosoma constricted) receiving femora; metapleural gland scrobe superior to metacoxa, extending from metapleural gland bulla to mesopleural-coxal excavation (Fig. 8). Inferior carinula of this scrobe flanks metapleural gland slit dorsally that is very narrow; bulla is rather large, extended almost to propodeal spiracle. Propodeum (dorsopropodeum) and notopropodeal suture visible in dorsal view. Propodeal spines are short, wide at base and pointed to apex, divergent slightly outwards, with dorsomedian costa reaching notopropodeal suture. Lateropropodeum with a large tubulose spiracle that is accommodated into excavation on it and directed posteriorly. Internal margin of spiracle fused to the excavation, lateral margin free; from above lateropropodeal excavation limited by ridge-like anteropropodeal process. Posterior declivity of propodeum concave, descending to glabrous propodeal lobes. Articulatory excavation of petiole shallow, barely reaching a line spanning posteriormost points of metacoxal cavities. Legs comparatively short. Procoxa twice size of meso- or metacoxa, foretibia 2.3 times as long as wide. Meso- and metatibia without spurs. Pretarsus bearing two curved claws, without arolium.

Metasoma. Petiole sessile, tapered from base to crest, markedly higher than long; anterior and posterior faces meeting in an acute angle, but posterior face more gently sloping. Subpetiolar process present, laminar-lobose. Postpetiole wider than long, slightly wider than petiole, in profile nodiform, with relatively flat and oblique dorsal surface. Boundary between tergite and sternite visible only in helcium. Posttergite and poststernite of the postpetiole clearly fused (Fig. 12). Sternite of postpetiole projecting anteromedially into a subpostpetiolar process that forms with second cinctus a transverse cavity ventrally, where posterior margin of petiolar sternite are coupled. 1st gastral segment (Abd IV) occupying nearly 3/4 of gaster. 2nd helcium situated in excavation at base of postsclerites. In ventral view posttergite on each side of Abd IV markedly projecting anteriad to poststernite, broadly overlapping it. As in other Meranoplus species sting shaft relatively long, with dorsal flanges and spatulate lancets, large lancet valves and V-shaped furcula without dorsal arm (Fig. 9–11). Spiracular plate rectangular. Anterior apodeme without any process. Body of the quadrate plate very weakly sclerotized in lower part, anterodorsal corner blunt, but prominent. Posterior arm of oblong plate relatively long and massive. Anterior apodeme short and blunt. No postincision separates the posterior and ventral arms. Gonostylus oblong-triangular with acute apex, single-segmented.

In general, body with superficial sculpture; sculpture mainly alveolate on ventral surface of head, sides of mesosoma, all coxae and metasoma; postsclerites of 1st gastral segment more superficially and sparsely sculptured. Frons including frontal lobes with sparse longitudinal and slightly sinuate rugae (costulae), without meshes and with glabrate main surface. On vertex and sides of head rugae connected and forming partly areolate-rugulose sculpture with finely alveolate interspaces. Antennal scrobes also finely alveolate. Clypeus predominantly smooth and shiny. Sculpture of promesonotum obscure, as if erased. A similar condition observed also on other dorsal surfaces (propodeal declivity, petiole and postpetiole). Legs without marked sculpture, excluding coxae (see above). Scapes slightly rimose in basal halves; mandibles striate.

Pilosity consisting of short decumbent and subdecumbent hairs (30–80 μm long) and sparse longer (ca. 100 μm) suberect hairs. Longest hairs (150–160 μm) situated on median clypeus and on dorsum of petiole immediately below crest. Hairs whitish and more or less arcuate.

Generally, exposed cuticle exhibits depigmentation. Color of head, mesosoma, appendages, petiole and postpetiole dull yellow, mandibles (especially masticatory margin) darker. Gaster brown to dark-brown with yellowish-brown edges of sclerites; sometimes dark pigmentation reduced to a broad median band or entirely absent.

Measurements and indices. Holotype worker: TL 2.73, HL 0.6, HW 0.6, SL 0.33, PML 0.42, PW 0.45, PTLL 0.2, PTLH 0.28, PPLL 0.17, WL 0.65; CI 100, PMI 108, PTI 71, SI 56.

Paratype workers (n=30, mean ± standard deviation and variability in parentheses): TL 2.64±0.14 (2.28–2.98), HL 0.59±0.02 (0.53–0.63), HW 0.57±0.02 (0.52–0.6), SL 0.32±0.01 (0.28–0.33), PML 0.39±0.02 (0.33–0.42), PW 0.43±0.02 (0.37–0.47), PTLL 0.18±0.01 (0.15–0.2), PTLH 0.27±0.01 (0.25–0.3), PPLL 0.15±0.01 (0.12–0.17), WL 0.61±0.03 (0.53–0.67); CI 97±2.5 (91–100), PMI 110±2.4 (108–117), PTI 64±3.5 (59–71), SI 56±1.7 (53–59).

Type Material
Holotype, worker: Vietnam, Dong Nai Province, Vinh Cuu District, Dong Nai Culture and Nature Reserve, 11°18′N / 107°04′E, lowland, on the plot “Dipterocarpus forest”, 22.10.2011, soil core 1-5 (leg. A. Anichkin). Paratypes: 40 workers from the same locality, 22.10 and 21.12.2011 (leg. A. Anichkin), (, IEBR,, , and the personal collection of the author).

Etymology
The species is named to the memory of the famous Russian myrmecologist, Prof. Gennady M. Dlussky, whom the author considers as his first mentor in the study of ants.