Aenictus seletarius

Aenictus seletarius was collected from a tropical lowland primary and old growth secondary rainforest in Singapore proximally located (<100 m) to a freshwater catchment. As individuals were collected with subterranean pitfall traps set 15 cm beneath the soil surface, A. seletarius likely exhibits a hypogaeic lifestyle similar to many other Aenictus species. Additionally, we found over thirty specimens of a small (TL ca. 4 mm), eyeless unidentified Pseudolasius species in the same traps in which the A. seletarius individuals were collected. It is possible the A. seletarius were preying on these other ants as they recruited to the bait. (Wong and Guénard 2016)

Identification
A member of the minutulus species group.

Wong and Guénard 2016) - Worker caste with important size variation. Head almost as wide as long, with side margins broadly convex. Masticatory margin of mandibles, medium-sized subapical tooth followed posteriorly by a distinct medium-sized denticle, and both the subapical tooth and the posterior denticle are of similar size. Posterodorsal corner of the propodeum strongly angular and followed by a concave propodeal declivity. Subpetiolar process well-developed and plough-shaped.

The new species A. seletarius displays substantial variation in body size among workers (TL 2.31–3.18 mm). This was also observed by Jaitrong and Hashimoto (2012) in Aenictus minutulus (TL 1.7–2.4 mm) and Aenictus changmaianus (TL 1.95–2.6 mm). Among the A. minutulus species group, A. seletarius is morphologically most similar to A. minutulus. Excluding the latter, individual workers of A. seletarius can be distinguished from other species by the dentition on the masticatory margin of their mandibles, where the medium-sized subapical tooth is followed posteriorly by a distinct medium-sized denticle, and both the subapical tooth and the posterior denticle are of similar size. This is contrary to the pattern of mandibular dentition of A. changmaianus, Aenictus minimus, A. sp. 56 of WJT and Aenictus subterraneus, where a medium-sized subapical tooth is followed posteriorly by a distinctly smaller denticle, as well as Aenictus peguensis, where both the subapical tooth and subsequent denticles are small in size. However, important variation in mandibular patterns can be observed in ants as blunting of the denticles with usage what may result in slight variation in dentition patterns (as observed in some paratypes and non-type specimens). Therefore, relying on mandibular dentition alone for species determination is not ideal. In consideration of the above, A. seletarius may be further distinguished from the A. minutulus group species including the morphologically similar A. minutulus by several other notable characters outlined below.

In full face view, A. seletarius displays the most square-shaped head among all A. minutulus group species, as its head is almost as wide as it is long, CI 91–96; the side margins of its head are broadly convex and its posterior occipital margin is approximately 3/4 the length of its HW. Although A. peguensis also possesses a head that is almost as wide as it is long, CI 82–96, the head shape of this species as well as that of A. minimus and A. sp. 56 of WJT are all markedly rounded; the side margins of their heads are strongly convex and the respective lengths of their posterior occipital margins are no more than 2/3 the lengths of their HW. In relation to A. seletarius, the heads of the remaining A. minutulus group species are comparatively longer than wide (A. changmaianus, CI 75–89; A. minutulus, CI 76–90 A. subterraneus, CI 86–87) and their heads appear slightly more elongate than that of A. seletarius. The antennal scape of A. seletarius is also relatively short in comparison to its head width, SI 61–64, in contrast to most other species in the A. minutulus species group (A. changmaianus, SI 69–71; A. minutulus, SI 67–68; A. peguensis, SI 74–79; A. subterraneus, SI 75–79; A. sp. 56 of WJT, SI 67–74), but similar to that of A. minimus (SI 63–64).

In profile view, a strongly angular posterodorsal corner of the propodeum, a concave propodeal declivity, and a flat anterior face of the petiole distinguish A. seletarius from A. changmaianus and A. minutulus, whose individuals have a more rounded posterodorsal corner of the propodeum, a weakly concave propodeal declivity, and a more rounded or broadly convex anterodorsal face of the petiole. In addition to the flat anterior face of the petiole in A. seletarius, a less pronounced postpetiolar process also distinguishes this new species from A. subterraneus, which has a rounded anterodorsal face and a longer and slightly more acute postpetiolar process. Another subtle difference between the two species is the helcium, which appears to be more elongate in A. subterraneus than in A. seletarius.

Distribution
Described from and only known from Singapore.

Distribution based on Regional Taxon Lists
Indo-Australian Region: Singapore.

Nomenclature

 *  seletarius. Aenictus seletarius Wong & Guénard, 2016: 37, figs. 2–6 (w.) SINGAPORE.

Worker
Holotype: HL 0.57 mm; HW 0.52 mm; MaL 0.33 mm; SL 0.33 mm; WL 0.82 mm; PNW 0.32 mm; PNH 0.29 mm; MW 0.17 mm; PTL 0.24 mm; PTW 0.17 mm; PTH 0.27 mm; TL 2.83 mm (stinger not included); PPL 0.19 mm; PPW 0.16 mm; PPH 0.23 mm; CI 92, SI 64, MaI 65, PI 70, PPI 81. Paratypes (n=3 measured): HL 0.46–0.63 mm; HW 0.42–0.60 mm; MaL 0.27–0.36 mm; SL 0.26–0.37 mm; WL 0.67–0.91 mm; PNW 0.26–0.36 mm; PNH 0.22–0.34 mm; PTL 0.19–0.27 mm; PTW 0.14–0.19 mm; PTH 0.23–0.33 mm; TL 2.31–3.18 mm (sting not included); PPL 0.17–0.23 mm; PPW 0.14–0.18 mm; PPH 0.20–0.28 mm; CI 91–96, SI 61–64, MaI 59–64, PI 69–74, PPI 78–83.

Head. Head in full-face view almost as wide as long (CI 91–96), side margins broadly convex, posterior margin slightly convex to almost straight and approximately 3/4 of HW, posterior corners broadly rounded. Antennal scape curved and enlarged in their posterior half, relative size to head moderate (SI 61–64), slightly extending to over the midpoint of head length; antennal segments longer than broad; length of segments II–IX continuously increasing; apical segment X longer than VIII and IX combined; last two segments forming indistinct club. Frontal carina distinct, surpassing posterior margin of antennal torulus. Clypeus short, its anterior margin convex and without denticles. Basal margin of mandible with denticles that gradually reduce in size toward base of mandible. Masticatory margin of mandible with large acute apical tooth, followed posteriorly by a medium-sized subapical tooth, one medium-sized denticle and one small denticle, a medium-sized basal tooth; basal margin with 3–4 small denticles.

Mesosoma. In profile, promesonotum convex, sloping gradually to the metanotal groove; mesopleuron relatively short, demarcated from metapleuron by distinct groove. In profile, dorsal outline of propodeum flat to weakly convex nearing the posterior corner. Posterior part of propodeum forming the propodeal declivity nearly at right angle with propodeal dorsum, and separated from the latter by an angular edge; overhanging declivity of propodeum is strongly concave and encircled with thin but distinct rim. Metapleural gland bulla well-developed, its maximum diameter about 1.3 times as long as distance between propodeal spiracle and most proximate part of metapleural gland bulla.

Metasoma. In profile, petiole excluding subpetiolar process slightly higher than long and with triangular shape; petiole node with steep anterior face and broadly convex dorsal outline; subpetiolar process well-developed and of an irregular quadrilateral shape (plough-shaped) with roughly angular apex posteriorly oriented and a slightly concave posterior lateral margin. Size of subpetiolar process approximately 1/5 of petiole height and 2/5 of petiole length, its ventral outline broadly convex and its ventralmost part with thin almost transparent lamella. In profile postpetiole has a square shape with rounded corners; dorsal outline of postpetiole node flat to weakly convex; postpetiolar process developed and pointing anteriorly with rounded to weakly angular apex. First gastral tergite and sternite long, extending over half the total length of the gaster.

Sculpture. Head entirely smooth and shiny. Mandibles superficially striate at the base. Basal portion of antennal scape (approximately 1/3 of SL) reticulate transitioning to smooth and shiny on its last 2/3 portion. Mesosoma finely reticulate with exception of pronotum and parts of metapleuron; pronotum smooth and shiny on dorsum and sides but finely reticulate towards the posterior edge; metapleuron smooth and shiny on anterior median portion but otherwise finely reticulate. Petiole including subpetiolar process finely reticulate with the exception of a smooth and shiny spot anterodorsally. Postpetiole finely reticulate, with flat surface on dorsum smooth and shiny. Gaster entirely smooth and shiny. Legs entirely smooth and shiny.

Pubescence. Head and body, except sides of mesosoma, with abundant suberect standing hairs with lengths of 0.7–0.8 mm on head dorsum and 0.1–0.13 mm on dorsum of meso- and metasoma. Shorter decumbent pubescence also present in between longer hairs. Antennal scapes and legs with abundant, decumbent pilosity.

Colouration. Dark amber colouration on head, most of antennae, mesosoma, petiole and most of postpetiole, with darkest brown colouration on the reticulated propodeum. Tip of antennal segment X, entire legs, entire gaster and dorsum of postpetiole node with lighter yellow colouration.

Castes. Worker caste displays variation in body size. Apart from size variation, values of the different measurement indices are generally consistent among the workers measured, thus indicating an absence of allometric growth. Other morphological features such as sculpture, pubescence and colouration remain constant among the specimens examined. Male and female are unknown.

Type Material
Holotype.Worker from Singapore, Seletar Trail, 1°23’N; 103°48’E, ca. 40m, subterranean pitfall trap, 25.vii.2015 (Mark K. L. Wong), label “MW250715- 1.1” (ANTWEB1009000); deposited in. Paratypes. Three workers in total, all with the same collection data as holotype; deposited at.

Etymology
The species epithet is derived from the collection locality, Seletar forest, which contains some of Singapore’s last primary and old secondary habitats. The species epithet is a noun, and thus invariant.