Proceratium silisili

At present, this species is only known from Savai island in Samoa, and is likely endemic to Samoa. The type locality is a montane rainforest on Mt. Silisili, situated at an elevation of 1200m. Only one single worker of this species was collected through leaf litter extraction. There is no additional information about its ecology due to the limited available material. (Liu et al., 2015)

Identification
Liu et al. (2015) - Proceratium silisili differs from the other Oceanian members of Proceratium by the following combination of characters: mandible with 4 distinct teeth; petiole node in dorsal face-view subrectangular, almost as long as wide (DPeI 128); abdominal segment IV in profile view strongly recurved (IGR 0.25), highly rounded and almost spherical in its appearance; whole body very densely punctate, except for small smooth and shiny spot posterior of frontal lobes; pilosity dense, uniformly short and decumbent, long and erect or suberect hairs completely absent. Using the above character combination, P. silisili can also be distinguished easily from its geographically closest congeners in Fiji. Proceratium oceanicum, Proceratium relictum and Proceratium vinaka all have elongate-triangular mandibles with relatively long masticatory margins and more than six teeth or denticles, petiole in profile either squamiform or narrow, transversally compressed subrectangular, abdominal segment IV not strongly recurved (IGR > 0.45), and long standing hairs present.

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

Biology
This species is morphologically distinct from all the other members in the Oceanian region. It is thus possible that the Samoa species has a different origin than the other species in the region and that it is a descendent of a New World ancestor from the micrommatum clade. Several of the observed morphological characters are in support of this hypothesis: the mandibles have four teeth only, clypeus medially narrow with triangular projection, and mesotibiae without pectinate spur present. Also the subrectangular shape of the petiole and the absence of a lamellate ventral process, as well as the strongly recurved and almost spherical shape of the abdominal segment IV point in the same direction, although a triangular to strongly reduced ventral process can also be observed in the Proceratium species present on Fiji. A more definitive placement of the new species within the genus phylogeny, however, has to be postponed until more conclusive (e.g genetic) data can be analysed.

Nomenclature

 *  silisili. Proceratium silisili Liu, Fischer & Economo, 2015: 40, fig. 5 (w.) SAMOA.

Worker
(N=1). TL 3.36; EL 0.04; SL 0.56; HL 0.83; HLM 0.99; HW 0.75; WL 0.97; HFeL 0.65; HTiL 0.52; HBaL 0.27; PeL 0.24; PeW 0.31; DPeL 128; LT3 0.4; LS4 0.2; LT4 0.8; OI 6; CI 90; SI 74; IGR 0.25; ASI 204.

In full-face view, head subrectangular, longer than wide (CI 90), sides and posterior head margin convex. Mandibles with four distinct, well developed teeth, curved triangular with short masticatory margin. Clypeus strongly reduced, anteromedially with a small, triangular projection, anterolaterally reduced to extremely narrow with a thin wall in front of antennal sockets. Frontal carinae absent or vestigial, frontal lobes narrow, not covering the antennal sockets, posteriorly strongly convergent, ending just after posterior limit of antennal sockets. Eyes very small (OI 6), consisting of single ommatidium.

Mesosoma in profile convex, almost as long as maximum head length including mandibles. Lower mesopleuron with well impressed sutures, propodeum without posterior teeth, propodeal lobes small, reduced and blunt, posterior declivity relatively steep, in posterolateral and posterodorsal view separated from lateral propodeum by a distinct margin, propodeal spiracle circular and facing posterior end of mesosoma, situated slightly above mid height. Front and hind tibia with pectinate spur present, both without calcar of strigil, mesotibial spur absent, pretarsal claws simple, arolia absent. Petiole node in profile about as high as long, anterior face almost vertical, the dorsum almost flat, anteriorly and posteriorly weakly rounded, in dorsal view subrec-tangular with convex sides and slightly wider than long (DPeL 128), ventral process a small, blunt tooth.

Abdominal segment III in dorsal view anteriorly wider than petiole, posteriorly diverging, in profile abdominal sternite III anterolaterally with small, angulate anterior projection on either side of shallow median depression. Constriction between abdominal segments III and IV distinctly impressed. Abdominal segment IV strongly recurved (IGR 0.25), highly rounded and almost spherical in its appearance, abdominal tergum IV about twice as long as abdominal tergum III (ASI 204). Remaining abdominal segments reduced and comparatively inconspicuous, curved forwards.

Whole body in profile and in dorsal view covered with uniform dense layer of short, decumbent hairs, longer erect hairs completely absent.

Sculpture on mandibles irregularly punctate, on remainder of body very densely punctate, except for small smooth and shiny spot posterior of frontal lobes. Punctation also less strongly developed on abdominal segment IV, tergum IV appearing more shiny.

Body color dark red, legs and flagella of lighter, reddish brown coloration.

Type Material
Holotype. Pinned worker, Samoa, Savai, 5.4km SSW A’opo vil, Mt. Silisili, 13°38'10"S, 172°30'23"W, 1200m, montane rainforest, leaf litter, 22.iv.2015 (E. Sarnat & C. Liu) (OSAKA: CASENT0741888).