Polyrhachis ypsilon

Identification
Polyrhachis ypsilon is easily recognised by its large size, rather massive pronotal spines, petiolar spines that widely diverge from their bases and the abundant hairs and rich golden pubescence covering the body. Morphologically it is a relatively stable species with specimens from throughout its range being closely similar.

Distribution based on Regional Taxon Lists
Indo-Australian Region: Borneo, Philippines, Singapore. Oriental Region: Thailand.

Polyrhachis ypsilon is known from peninsular Malaysia, Singapore, Borneo and Sumatra. (Kohout, 2014)

Nomenclature

 *  ypsilon. Polyrhachis ypsilon Emery, 1887a: 239 (w.) SINGAPORE. Wheeler, G.C. & Wheeler, J. 1990b: 764 (l.). Senior synonym of synacantha, victoris: Hung, 1970: 19.
 * victoris. Polyrhachis ypsilon var. victoris Santschi, 1925f: 93 (w.) INDONESIA (Sumatra). (Misspelled as vecticortis by Chapman & Capco, 1951: 304.) Junior synonym of ypsilon: Hung, 1970: 19.
 * synacantha. Polyrhachis (Polyrhachis) ypsilon ab. synacantha Santschi, 1933d: 2 (w.) BORNEO. Junior synonym of ypsilon: Hung, 1970: 19.

Two subspecific forms have been described, P. ypsilon var. victoris Santschi from Sumatra and P. ypsilon ab. synacantha from Borneo, by Santschi in 1925 and 1933 respectively. Both were synonymised with the nominal form by Hung (1970: 20), who considered the former to be a geographical variant while he was unable to locate the type of the latter. I have previously examined the unique holotype of P. ypsilon ab. synacantha in the collection of Prince Léopold (IRSN). Based on its extremely aberrant petiolar node, a huge, pillar-like structure terminating dorsolaterally in rather asymmetrical, dentiform angles, bearing two minute, posteriorly directed, uneven, acute spines, I considered it an undoubtedly anomalous variant (Kohout 1998: 509) and concurred with Hung’s (1970) opinion that it is a synonym of P. ypsilon. (Kohout, 2014)

Worker
Description by Kohout (2014).

Dimensions (lectotype cited first): TL c. 12.85, 12.10 – 13.10; HL 2.97, 2.67 – 2.97; HW 2.57, 2.32 – 2.68; CI 86, 86 – 93; SL 3.83, 3.43 – 3.93; SI 149, 139 – 150; PW 1.71, 1.40 – 1.71; PeH 3.07, 3.02 – 3.61; PeI 103, 103 – 124; MTL 5.44, 4.74 – 5.44 (1+15 measured).

Dimensions (holotype of synacantha): TL c. 13.71; HL 3.02; HW 2.67; CI 88; SL 3.93; SI 147; PW 1.71; PeH 3.83; PeI 129; MTL 5.39 (1 measured).

Dimensions (syntypes of victoris): TL c. 12.65 – 12.75; HL 2.84 – 2.90; HW 2.53-2.60; CI 89 – 90; SL 3.63 – 3.73; SI 143 – 144; PW 1.31 – 1.41; PeH 3.43 – 3.58; PeI 119 – 123; MTL 4.84 – 5.04 (3 measured).

Queen
Description by Kohout (2014).

Dimensions: TL c. 13.25 – 14.82; HL 2.82 – 3.22; HW 2.42 – 2.82; CI 86 – 87; SL 3.78 – 4.23; SI 150 – 156; PW 2.87 – 3.12; PeH 1.31 – 1.56; PeI 46 – 48; MTL 4.79 – 5.44 (2 measured).

Queen larger than worker and with usual characters identifying full sexuality, including three ocelli, complete thoracic structure and wings. Mandibles with four teeth; apical tooth much longer than other teeth which are greatly reduced or often vestigal. Eyes only moderately larger than in worker; sides of head in front of eyes virtually parallel towards mandibular bases. Frontal triangle distinct; frontal carinae sinuate with distinctly raised margins; central area concave with frontal furrow almost reaching median ocellus. Pronotal spines reduced to blunt angles. Mesoscutum wider than long; lateral margins converging anteriorly into moderately rounded anterior margin; median line distinct; parapsides flat; mesoscutum in profile relatively high, with anterior face widely rounding onto moderately convex, relatively short dorsum. Mesoscutellum moderately convex, only marginally elevated above dorsal plane of mesosoma. Propodeal dorsum not marginate, convex in outline, longer than declivity; posterior angles only poorly raised as medially directed, blunt ridges with propodeal dorsum between them descending into declivity in medially uninterrupted line. Petiole with pair of basally stout, relatively short, widely diverging spines with extreme tips curved backwards.

Mandibles finely longitudinally striate with piliferous pits. Head, mesosoma and petiole very finely reticulate-punctate; sides of mesosoma and coxae distinctly more coarsely sculptured; extreme tips of petiolar spines smooth and polished. Gaster shagreened.

Mandibles at masticatory borders with several curved golden hairs. Anterior clypeal margin with numerous, rather long setae medially and fringe of marginally shorter setae laterally. Generally abundant, relatively long, golden hairs on all dorsal body surfaces, including appendages, some hairs almost as long as greatest diameter of eyes. Closely appressed, rather long, rich golden pubescence in various densities over most body surfaces; pubescence somewhat longer on sides of head and mesosoma and virtually absent from tips of petiolar spines.

Colour. Black, with only base of petiole, subpetiolar process and most of first gastral tergite medium reddish-brown.

Male and immature stages unknown.