Aenictus concavus

Known only in from the lowlands. The type series was collected in a wet evergreen forest near a stream. A colony from Khao Yai National park was collected in a bamboo forest.

Identification
A member of the ceylonicus group. Jaitrong and Yamane (2013) - Aenictus concavus is separated from the other species of the group by the following characteristics: head in profile relatively flattened with occipital corner convex bearing a distinct protuberance, in full-face view with posterior margin strongly concave (occipital corner round in the other species except A. gonioccipus); promesonotum almost flat or feebly convex dorsally. The single specimen collected from Cat Tien National Park (Eg04-VN-526) is extremely similar to the type series in all characters except for slightly more developed sculpture on mesopleuron, metapleuron, and propodeum and a slightly more angulated propodeal junction.

Distribution based on Regional Taxon Lists
Oriental Region: Thailand, Vietnam.

Castes
Known only from the worker caste.

Nomenclature

 *  concavus. Aenictus concavus Jaitrong & Yamane, 2013: 178, figs. 4A-C (w.) THAILAND.

Worker
(holotype and paratypes, n = 7). TL 2.20–2.30 mm; HL 0.54–0.56 mm; HW 0.48–0.50 mm; SL 0.33–0.35 mm; ML 0.75–0.78 mm; PL 0.20–0.25 mm; CI 84–89; SI 68–70.

Head in full-face view clearly longer than broad, with anterior portion narrower than posterior portion, sides convex, posterior margin strongly concave; with head in profile occipital corner convex, with a distinct protuberance on occipital corner; occipital margin bearing a distinct carina. Antennal scape relatively short, not reaching 2/3 of head length. Frontal carinae relatively short, fused at the level of antennal base to form a single carina, reaching the level of posterior margin of torulus. Parafrontal ridge feeble and incomplete. Anterior clypeal margin strongly concave, concealed by curved anterior extension of frontal carinae. Masticatory margin of mandible with 3 teeth including a large apical tooth; basal margin convex. Maximum width of gap between anterior clypeal margin and mandibles about 3.3 times as broad as maximum width of mandible. Promesonotum almost flat or feebly convex dorsally; metanotal groove indistinct; mesopleuron relatively short, demarcated from metapleuron by an indistinct groove; metapleural gland bulla relatively large, its maximum diameter 3 times as long as distance between propodeal spiracle and metapleural gland bulla. Propodeum in profile with feebly convex dorsal outline; propodeal junction bluntly angulate; declivity of propodeum widely and shallowly concave, encircled with a distinct rim. Petiole slightly longer than high, with its dorsal outline convex; subpetiolar process weakly developed, its anteroventral corner acutely angulate and ventral outline almost straight or feebly concave. Postpetiole distinctly shorter than petiole, subrectangular.

Head including mandible and antennal scape entirely smooth and shiny. Promesonotum smooth and shiny except for anteriormost portion reticulate; mesopleuron, metapleuron, and propodeum reticulate; in addition mesopleuron and metapleuron with relatively irregular longitudinal rugae; petiole and postpetiole reticulate except dorsa smooth and shiny. Legs entirely smooth and shiny.

Head with relatively sparse standing hairs mixed with dense short hairs over surface; mesosoma dorsally with relatively sparse standing hairs mixed with sparse decumbent hairs; longest pronotal hair 0.15–0.18 mm long. Head, mesosoma, petiole, and postpetiole reddish brown; gaster and legs yellowish brown.

Type Material
Holotype. THAILAND: Worker from E. Thailand, Chanthaburi Prov., Pong Nam Ron Dist., Hin Dad Waterfall, 15.V.2008, leg. W. Jaitrong, WJT08-E094 (THNHM). Paratypes. Nine workers, same data as holotype (SKYC, THNHM)

Etymology
The specific name refers to the strongly concave vertex of the head.