Sphinctomyrmex siamensis Jaitrong

W. Jaitrong, D. Wiwatwitaya & W. Sakchoowong, 2016, Review of the Thai Species of the Genus Sphinctomyrmex Mayr, 1866 (Hymenoptera: Formicidae, Dorylinae), with Description of a New Species, Far Eastern Entomologist 305, pp. 1-9 : 6-8

Sphinctomyrmex siamensis Jaitrong , sp. n.

Figs 5–8

MATERIAL. Holotype – worker ( THNHM-I-00007 ), Thailand: Chiang Mai Province, Mae Tang District, secondary forest, 26.IV 2000, W. Jaitrong leg., colony no. WJT00-TH01 . Paratypes: 5 workers ( THNHM-I-00008 , THNHM-I-00009 , THNHM-I-00010 , THNHM-I-00011 , THNHM-I-00012 , THNHM), same data as holotype .

DESCRIPTION (Holotype and paratypes). Head in full-face view clearly longer than broad, subrectangular, slightly broader anteriorly with side weakly convex and posterior margin concave; posterior corners of head bluntly angular; occipital margin bearing a distinct carina. Eyes absent. Antenna 12 -segmented; antennal scape relatively short, not reaching midlength of head; antennal segment II almost as long as broad; III-X each shorter than broad; XII almost as long as VIII+IX+X+XI or slightly longer. Frontal carina short, extending beyond level of posterior margin of torulus, well developed anteriorly and poorly developed posteriorly, curved anterior extension of frontal carinae bearing 2 denticles in front of antennal socket; parafrontal ridge extending posteriorly less than 1 / 3 of head length. Clypeus narrowly inserted between frontal carinae; median portion of clypeus with a distinct tubercle. Mandible subtriangular, with basal margin rounding into masticatory margin; the latter apparently edentate, but there may be a few small inconspicuous denticles near basal angle.

Mesosoma twice as long as broad, in profile with almost flat dorsal outline or feebly convex; promesonotal suture and metanotal groove obliterated; katepisternum clearly demarcated from anepisternum by a distinct carina; metapleuron demarcated from mesopleuron and lateral face of propodeum by carinae; propodeal junction nearly right-angled; declivity of propodeum shallowly concave and encircled with a thin rim.

Petiole sessile, slightly longer than broad, its dorsal outline weakly convex, anterior face flat encircled with an indistinct ridge but posterior face slightly convex and tergite clearly demarcated from sternite by a distinct ridge; subpetiolar process well developed, subtriangular with a small window at middle, its apex truncate and directed downward, anterior margin convex, while posterior margin strongly concave.

Gaster elongate, gastral segments I and II clearly separated by distinct constrictions, segments III, IV, and V separated from each other by deeply impressed, short constrictions; in dorsal view, gastral segment I (abdominal segment III) narrower than segments II, III, and IV but slightly broader than petiole; segments II, III, and IV nearly equal to one another in width and length; dorsum of pygidium flat with 1–2 rows of spines laterally, pygidial apex truncate as seen from above.

Dorsum and lateral face of head with dense foveolae, areas between foveolae smooth and shiny; venter of head smooth and shiny; dorsum of mesosoma smooth and shiny with sparse, very shallow foveolae; lateral face of pronotum partly superficially shagreened but shiny; mesopleuron, metapleuron and lateral face of propodeum smooth and shiny; propodeum declivity superficially reticulate but shiny; petiolar node smooth and shiny; each gastral tergite with dense hair pits but areas between hair pits smooth and shiny.

Pilosity comparatively dense; dorsum and lateral faces of head with dense apprised to decumbent hairs; venter of head with sparse apprised hairs; dorsum of mesosoma with dense, very short decumbent hairs; petiole and gaster with dense, longer decumbent hairs; pygidium and hypopygium with dense erect hairs.

Body reddish brown to dark brown; legs, terminal segment of antenna ( XI) and tip of gaster yellowish brown; 2 / 3 of head, dorsa of mesosoma and petiole darker than elsewhere.

MEASUREMENTS. Holotype: TL 3.95 mm, HL 0.66 mm, HW 0.54 mm, SL 0.36 mm, WL 0.86 mm, PL 0.36 mm, PW 0.30 mm, CI 83, SI 67. Paratypes (n = 5): TL 3.85-3.95 mm, HL 0.66–0.73 mm, HW 0.54–0.58 mm, SL 0.35–0.36 mm, WL 0.89–0.92 mm, PL 0.33–0.36 mm, PW 0.30–0.36 mm, CI 77–83, SI 62–65.

DISTRIBUTION. Thailand (Chiang Mai Province).

DIAGNOSIS. Sphinctomyrmex siamensis sp. n. has been compared with high resolution images of syntypes and paratypes of all the related forms which were described from Australasian and Oriental regions (Antweb, 2015). New species is most similar to Sphinctomyrmex trux Brown, 1975 (Australian species) in general appearance as they share the pygidium truncate when seen in dorsal view, 12 -segmented antenna, subtriangular subpetiolar process ( Fig. 7), and somewhat smooth and shiny dorsa of mesosoma and petiolar node. However, S. siamensis sp. n. is easily separated from S. trux by the following conditions: much smaller body ( HW 0.54-0.58 mm, WL 0.89-0.92 mm in S. siamensis while HW 0.74-0.84 mm, WL 1.12-1.26 mm in S. trux ); relatively longer head ( CI 77-83 vs 87-89); complete lack of eyes (in S. trux the eyes are present as pigmented dots in the middle of the side of the head); possession of a distinct tubercle in median portion of clypeus (tubercle absent in S. trux ); and relatively longer petiole.

HABITAT. The type series of the new species was collected from soil surface in a shifting agricultural area at an elevation of about 800 m above sea level near a hill-evergreen forest during the dry season.

ETYMOLOGY. The scientific name is an adjective meaning ‘of Siam (old name of Thailand)’.