Holotype (THNHM-I-02612, THNHM), 55 paratype workers (THNHM-I-02614, MHNG, SKYC, THNHM, USNM) and queen (THNHM-I-02613, THNHM), N Thailand, Chiang Mai Province, Omkoi District, Omkoi Forest, DDF (dry dipterocarp forest), 17.89583333°'N, 98.40750000°E, ca 1000 m a.s.l., 16.VII.2016, W. Jaitrong leg., Colony no. TH16-WJT-859.

28 workers, Thailand, Chiang Mai Province, San Sai District, San Sai Luang Sub-district, near Maejo University Campus, mixed deciduous forest, collected from leaf litter, 18.92777778°N, 99.05083333°E, ca 350 m a.s.l., 15.X.2015, N. Likhitrakarn leg., Colony no. NL151015-1 (THNHM).

Holotype: HL 0.53; HW 0.40; ML 0.69; PH 0.17; PL 0.18; SL 0.41; TL 2.28; CI 75; PI 91; SI 104. Paratype workers (n = 10): HL 0.50–0.53; HW 0.38–0.43; ML 0.66–0.73; PH 0.13-0.17; PL 0.17–0.18; SL 0.40–0.43; TL 2.24–2.41; CI 77–83; PI 90-91; SI 94–104.

(paratype). HL 0.92; HW 01.06; ML 1.55; PH 0.53; PL 0.53; SL 0.64; TL 5.31; CI 114; PI 100; SI 61.

(Holotype and paratypes; Fig. 1). Head in full-face view elliptical, clearly longer than broad with slightly convex sides and almost straight posterior margin. Antennal scape short, extending beyond the mid-length of the head but not reaching the posterolateral corner of the head; antennal segment II slightly longer than III–VI; the last (X) almost as long as VIII and IX combined and as long as II and III combined. Frontal carina thin and short, not extending beyond level of posterior margin of torulus. Clypeus short, with its anterior margin roundly convex, bearing 7 denticles. Mandible with an apical tooth large and curved, followed by a medium-sized subapical tooth and a series of 10–12 minute teeth on masticatory margin. Mesosoma in profile with pronotum strongly convex dorsally, demarcated from mesonotum by a shallow transverse groove; mesonotum convex, sloping gradually to metanotal groove; mesopleuron demarcated from metapleuron by a shallow groove. Propodeum in profile lower than promesonotum, with a weakly convex dorsal outline; propodeal junction angulate; declivity of propodeum widely and shallowly concave, encircled by a thin rim. Petiole in profile slightly longer than high, with a dorsal outline convex; seen from above relatively narrow with sides almost parallel; subpetiolar process present, its ventral outline convex, without angle or tooth; postpetiole slightly shorter than petiole but seen from above slightly broader than petiole; its node short, clearly shorter than high.

Head, antennal scapes, pronotum, petiole, postpetiole, gaster, femora and tibiae of legs entirely or extensively smooth and shiny. Antennal flagellum densely punctate; mesothorax and propodeum with dense punctures; metapleuron partly or extensively smooth.

Body with relatively sparse standing hairs mixed with sparse short hairs over surface; longest pronotal hair 0.10–0.13 mm long. Head, mesonotum, propodeum and gaster dark brown; pronotum, waist, antennae and legs reddish brown.

(Paratype, Fig. 2). Head in full-face view subrectangular, posteriorly narrow and gradually widening anteriorly, slightly shorter than broad, with sides weakly convex and posterior margin concave; upper frons weakly concave. Antennal scape flat, relatively short, about half as long as head, basally narrow, widening considerably apicad; flagellum of antenna missing (for this specimen). Frontal carina indistinct. Parafrontal ridge absent. Anterior clypeal margin concave, without denticles. Mandible half as long as head length, with a slender, inner margin that is convex while lateral margin weakly concave; masticatory margin without denticles. Mesosoma elongate; in profile, pronotum convex dorsally; mesonotum weakly concave; propodeal dorsum almost straight; seen from above pronotum and propodeum broader than mesonotum; propodeal junction low, roundly convex; propodeal declivity weakly convex, not encircled by a rim. Petiole longer than high, with its dorsal outline slightly elevated posteriorly, with petiole in profile posterodorsal corner bluntly angulate; seen from above petiole with a distinct longitudinal furrow running from anterior face to posterior face; subpetiolar process large, subtriangular, with its apex pointed downwards. Gaster large and elongate; first tergite narrower and shorter than second, its anterior slope weakly concave; second tergite largest; third as long as first; tip of gaster missing in this specimen. Legs relatively long and slender; femora and tibiae clavate.

Figure 2. 

Aenictus nuchiti sp. n. (paratype, queen, THNHM-I-02613). A Body in profile B Body in dorsal view C Head in full-face view.

Entire body smooth and shiny, with relatively dense standing hairs; hairs slightly shorter on pronotum than on head, mandible and antennal scape; longest pronotal hair 0.08 mm long. Head dark brown; lateral and ventral faces of head and mandible reddish brown; scapes and legs yellowish brown. Mesosoma with ground colour reddish brown; lateral faces of pronotum and mesonotum, entire mesopleura and propodeal declivity dark brown. Petiole with ground colour reddish brown; lower portion of petiole, posterior slope of petiole and subpetiolar process dark brown; gaster with ground colour dark brown; lateral faces of second tergite reddish brown.

The species is named after Mr Supachai Nuchit (Royal Forest Department, Thailand) who kindly helped us with ant collecting at Pa Omkoi National Forest, Chiang Mai Province.

Northern Thailand (Chiang Mai Province).

Aenictus nuchiti sp. n. is most similar to A. biroi, A. camposi, A. gutianshanensis and A. vieti in having dense punctures on the mesosoma and an angulate propodeal junction. However, A. nuchiti is much smaller than the latter four (TL 2.24–2.41 mm, HW 0.38–0.43 mm in A. nuchiti; TL > 2.6 mm, HW > 0.43 mm in the latter four). It has a short antennal scape that reaches only two-thirds the head length (in contrast, reaching or extending beyond the posterolateral corners of the head in the latter four). This species can be distinguished from A. gutianshanensis and A. vieti by the configuration of the subpetiolar process (ventral outline roundly convex and without anterior angle in A. nuchiti; ventral outline with anterior angle in A. biroi, A. gutianshanensis and A. vieti). Aenictus nuchiti is similar to A. biroi and A. camposi in the unarmed subpetiolar process. In A. nuchiti, however, the body size is much smaller than that of A. biroi and the head is clearly longer than broad in A. nuchiti (almost as long as broad in A. biroi). The body colour is dark brown in A. nuchiti, whereas it is entirely yellow in A. camposi. The propodeal declivity is broader and widely rounded above in A. nuchiti but is narrow and tapers distinctly above in A. camposi.

Holotype (THNHM-I-02615, THNHM) and 15 paratype workers (THNHM-I-02616, MHNG, SKYC, THNHM, USNM), Thailand, Tak Province, Um Phang District, Thung Yai Wildlife Sanctuary, Yuyi Junction, DEF (dry evergreen forest), 15.44861111°N, 99.04694444°E, ca 1100 m a.s.l., 25.IX.2016, W. Jaitrong leg., TH16-WJT-1069.

One worker, Thailand, Tak Province, Near Myanmar border, Tung Yai [Thung Yai] W.S., 23.V.1999, W. Jaitrong leg. (THNHM).

Holotype: HL 0.41; HW 0.31; ML 0.53; PH 0.17; PL 0.12; SL 0.20; TL 1.75; CI 76; PI 143; SI 63. Paratypes (n = 11): HL 0.40–0.43; HW 0.31–0.33; ML 0.51–0.53; PH 0.17–0.18; PL 0.12–0.13; SL 0.20–0.23; TL1.72–1.78; CI 76–80; PI 138–143; SI 63–70.

(holotype and paratypes; Fig. 3). Head in full-face view clearly longer than broad, with its sides weakly convex and posterior margin almost straight or feebly concave. Antennal scape short, only slightly extending mid-length of head; antennal segment II (pedicel) clearly longer than each of III–VI; X longer than VII, VIII and IX combined. Frontal carinae fused at level of anterior margin of torulus, extending beyond level of posterior margin of torulus. Clypeus short, with its anterior margin bearing 7 denticles. Mandible subtriangular, with apical tooth large and curved, followed by a medium-sized subapical tooth, 4–5 minute teeth and a medium-sized basal tooth on masticatory margin. Mesosoma in profile almost flat dorsally; metanotal groove distinct. Propodeum in profile with a nearly straight dorsal outline; propodeal junction rounded; propodeal declivity weakly convex, not encircled by a rim. Petiole including subpetiolar process shorter than high, with its dorsal outline convex; subpetiolar process rather developed, with its ventral outline roundly convex, without angle or tooth; postpetiole shorter than petiole and shorter than high, in profile slightly elevated posteriorly.

Figure 3. 

Aenictus samungi sp. n. (holotype, worker, THNHM-I-02615). A Body in profile B Body in dorsal view C Head in full-face view.

Head, antennal scape, promesonotum, propodeal dorsum, petiole, postpetiole, gaster, femora and tibiae of legs entirely or extensively smooth and shiny; mesopleuron, metapleuron and lateral face of propodeum superficially reticulate. Antennal flagellum densely finely punctate.

Body with relatively sparse standing hairs mixed with sparse short hairs over surface; longest pronotal hair 0.07–0.08 mm long. Head, mesosoma, waist, gaster, antennae and legs yellowish brown; mandible dark brown.

The species is named after Mr Yuthana Samung (Faculty of Tropical Medicine, Mahidol University, Thailand) who kindly helped us in taking pictures of Thai ants, including the two new species discovered in the present study.

Western Thailand (Tak Province).

Aenictus samungi sp. n. can be easily distinguished from the other members of the A. wroughtonii species group by the following characteristics: smallest species of the group (HW 0.31–0.33 mm in A. samungi; HW > 3.7 mm in other members); petiole shorter than high (as long as or longer than high in other members); promesonotum with almost flat or straight dorsal outline (convex and sloping gradually to metanotal groove in other members); antennal scape short only just reaching mid-length of the head (at least two-thirds of the head length or beyond the posterolateral corner of the head in other members). Both Aenictus nuchiti sp. n. and A. samungi sp. n. have small bodies and short antennae, but can be easily separated from each other by the different conditions of the propodeum (Figs 1A vs. 3A).