Rostromyrmex pasohensis

These ants are small, cryptobiotic inhabitants of rainforest leaf litter. They are easily overlooked because of their small size, slow movements and dark colouration. They form small, apparently monogynous (single-queened) colonies in rotten wood on the ground. Their diet is not known, and ants kept in captivity refused to eat honey and dead insects.

Identification
The following details regarding the genus are useful for identification of this sole member of Rostromyrmex: Rosciszewski (1994) - The assignment of Rostromyrmex to any of the established tribes or genus groups is difficult. The characters of the clypeus, sting and wing venation rule out a placement in the Myrmica- or Tetramoriurn genus groups. The strongly carinate clypeus and the open radial cell plead against a placement in the Pheidologeton genus group (Ettershank 1966, Bolton 1987). According to Bolton (pers. comm.), Rostrornyrmex is very close to Lordomyrma, with which it shares a number of characters: frontal lobes, antennal scrobes, strong sculpturing of the body, conspicuous first gastral tergite, and absence of tibial spurs on middle and hind legs (Wheeler 1919). Also the shape of the clypeus can be derived from the Lordomyrma pattern. However. the fore wing venation in the male differs considerably from that illustrated by Wheeler (1919: 105) for Lordomyrma (still, its variability has never been assessed - Bolton, pers. comm.). Other important differences, which however may be interpreted as further transformation of the Lordomyrma characters (Ward, pers. comm.), are (in the females) the reduction of frontal carinae, 6- or 7 -dentate mandibles, 9-segmented antennae, and a long petiolar peduncle. The antennal scrobes in Lordomyrma are bordered with frontal carinae and are differently sculptured from the remainder of the head - which is not the case in Rostromyrmex. The sting in Rostromyrmex differs from that of Lordomyrma (Kugler 1978): not cuneiform, shaft long, sting base not vertical in profile. The strong basal ridge and well developed furcula with a long dorsal arm, as found in Rostromyrmex, are absent in the Lordomyrma (= Promeranoplus) genus group accodling to Kugler. The sting and furcula resemble those of some species of the Solenopsis genus group (Kugler 1978), but other characters of the sting apparatus are different: spiracular plate with dorsal notch, and oblong plate without noticeable posterior incision. The fore wing venation: cross-vein r-m absent, and radial cell open, except for the reduced stigma, is similar in the Solenopsis genus group and Megalomyrmex (Ettershank 1966, Bolton 1987). A strong median seta on anterior clypeal margin is characteristic of the Solenopsis genus group (also present in some Rogeria - Bolton, pers. comm.). A similar but weaker seta is also present in females of Rostromyrmex but, considering the peculiar development of clypeus, it is probably not homologous. Further differences from the Solenopsis genus group (see Bolton 1987) include mandibles with more than 5 teeth, antennal scrobes, well developed propodeal spines, strong sculpture on promesonotum, and 10-segmented antennae in the males.

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

Biology
Rosciszewski (1994) - The ants were collected in the litter layer of the forest floor. In one case three specimens were found (1 queen and 2 workers) below a piece of dead wood. Further, 5 workers and 1 queen were found with some brood in a piece of strongly rotten wood. In the third case a similar small colony (6 workers, 1 queen, and some brood) were found in the same microhabitat. The male was bred in captivity. Unfortunately, during a long absence of the author the colony died and the specimens were partly destroyed by mold (and therefore not useful as paratypes).

Nomenclature

 *  pasohensis. Rostromyrmex pasohensis Rosciszewski, 1994: 163, figs. 1-19 (w.q.m.) WEST MALAYSIA.

With characters of the generic diagnosis Rostromyrmex.

Worker
Holotype — TL 2.17, HL 0.54, HW 0.45, CI 83, SL 0.33, Si 73, PW 0.34, AL 0.59.

Strongly reduced eyes situated at about 1/3 head length on sides of head (measured from mandibular insertions to occipital margin) and measure 0.037 mm in diameter. Length of rostrum in dorsal view is 0.06 mm. Antennal segments 3-6 about 2 times as broad as long, segment 7 slightly broader than long. Apical club more than 2 times longer than remainder funiculus. Sides of head in face view slightly narrowing towards mandibles and rounded towards feebly and shallowly concave occipital margin. Propodeal spines length 0.10 mm. Propodeal declivity almost vertical, slightly sloped dorsad, meeting propodeal dorsum at an angle of about 45° at anterior level of spine bases. A thing, transverse ruga lies between propodeal faces. It may be followed by one or two rugae on propodeal declivity. Metanotal groove in dorsal view at narrowest point of alitrunk and continues laterally as a suture to ventrolateral incision above middle and hind coxae. The suture is interspersed with transverse cross-ribs. Petiolar peduncle relatively long and slender, subpetiolar process (anterior to and below petiolar spiracles) very small. Petiolar node distinct, high, ascending almost vertically, rounded dorsally, slightly broader than long, without a visible step posteriorly. Postpetiole in lateral view slightly lower than petiole, shallowly rounded dorsally, broader than long. Postpetiolar process small and rounded, with two pairs of hairs laterally.

As part of sculpture, small knobs occur on dorsum of head, alitrunk and nodes of petiole and postpetiole. They are readily seen by SEM, less easily under a light microscope. Sculpture on sides of head (including antennal scrobes and base of mandibles) foveate-reticulate with hairs in the pits, sculpture more diluted on ventral side of head capsule. Surface of rostrum with traces of sculpture laterally, moderately shining. Anterolateral portion of clypeus smooth and shining. Scape sculptured dorsally. Mandibles, except laterobasally, smooth and shining. Dorsum of head with irregular sculpture, partly diluted between pits and, particularly towards occiput, equipped with small knobs. Alitrunk sculpture similar, except pits around propodeal spiracle (except ventrad) and on pronotal dorsum small and without hairs. Propodeal declivity, beneath posterior level of spine bases, smooth and shining. Sculpture of nodes almost entirely consisting of knobs, petiolar peduncle foveate. Gaster smooth and shining. Hind parts of coxae and basal parts of femora with weak impressions. Distal parts of femora smooth and shining ventrally, this area flattened and without hairs. Remainder of legs, as well as antennal funiculus, without or with only traces of sculpture, moderately shining. Pilosity moderately dense on whole body, sparse only on meso- and metapleuron and on petiolar peduncle. Gaster with pilosity less dense than on head and alitrunk. Hairs mostly short, approx.. 0.025 mm long, hooked, decumbent to suberect, appressed on legs and antennae. Scattered long hairs present. Maximal hair length on head dorsum 0.036 mm, on alitrunk dorsum 0.086 mm and on gaster 0.049mm. Length of setae on rostrum 0.074 mm, setae between rostrum and projection of anterior clypeal margin 0.098 mm, single median seta 0.074 mm. Body colour uniformly dull red, legs, antennal funiculi, mandibles and rostrum somewhat lighter.

Paratype worker—TL 2.10, HL 0.52, HW 0.43, CI 82, SL 0.31, SI 73, PW 0.32, AL 0.57. One pair of hairs laterally on postpetiolar process. Petiolar node with a small step posteriorly. Other characters as in holotype worker. Two more paratype workers are present in, London.

Queen
Paratype — TL 2.34, HL 0.55, HW 0.47, CI 84, SL 0.34, SI 72, PW 0.36, AL 0.66. Differs from workers as follows. Slightly larger. Eyes well developed, maximal diameter 0.12 mm. Small black spots present around ocelli. Metapleural lobes somewhat weaker developed. Petiolar node provided with small step posteriorly.

Male
Paratype male—TL 1.74 [petiolar node and peduncle measured in combination], HL 0.38, HW [including eyes] 0.40, CI 104.84, SL 0.13, SI 33.85, PW 0.34, AL 0.63.

Median 3 setae on anterior clypeal margin about same length (ca. 0.06 mm), 4 lateral setae are somewhat shorter. Antennal scrobes relatively short (reaching about 2/3 of HL), shallow and poorly delimited, deepened beneath antennal sockets. Frontal carinae not bordering antennal scrobes dorsally, but following their declivity. A weak, carina on ventrolateral margin of head extends from occipital margin to about level of eyes. Eyes large (max. diameter 0.19 mm), occupying most of anterolateral head, extending posteriorly to about level of antennal scrobes. Antennae with all segments longer than wide (pedicellus only slightly so because distally somewhat thickened). Terminal segments slightly longer than the two preceding segments together. Scape relatively short, somewhat flattened dorsoventrally. Head slightly wider across eyes than long. Head in full face view broadest before midlength. Sides of head slightly narrowing towards occiput and rounded towards feebly and shallowly concave occipital margin. Mesonotum almost vertically raised anteriorly, concealing most of pronotum in dorsal view. Metanotum small, strap-like, slightly elevated, just visible in dorsal view beneath scutellum. Dorsal face of propodeum relatively short, declivitous face almost vertical. Two small elevations present on transition between dorsal and declivitous faces of propodeum, thus the faces meet in a sharp angle in lateral view. Sides of propodeum rounded. Petiolar node broadly rounded dorsally, posteriorly descending without a step. Subpetiolar process absent. Postpetiole somewhat flattened dorsally, distinctly wider than long and wider than petiolar node in dorsal view. Postpetiolar process small and rounded, with one pair of hairs.

Fore wing with on cubital cell, radial cell open. Mf1 arises distal of cu-a, lr and r-m absent. Stigma absent, replaced by thickening of the vein. 2r very long, possibly aligned with Rsf4. Hind wing with no closed cells, venation strongly reduced.

Sculpture on head as in female castes, i.e. foveate-reticulate with hairs in the pits. Knobs not recognizable under the light microscope (80x). Dorsum of mesocutum dull, punctate. Punctures shallow, scattered, much more sparse than foveae of head. Scutellum with several large pits. Propodeum foveolate posterolaterally. Pronotum punctate laterally, remainder of pronotum and lateral sides of alitrunk dull with scattered, very shallow pits, absent on smooth and shining anterior and ventrolateral parts of katepisternum. Petiolar peduncle foveolate, laterally with pits extending to ventral and posterior parts of node. Petiolar and postpetiolar nodes from dorsal view dull, sculpture diluted. Pilosity on head including clypeus moderately dense. Hairs decumbent to suberect, mostly short, about 00025 mm long, on vertex and genae longer, up to 0.06 mm. Mandibles with some decumbent to appressed hairs, antennae with dense, decumbent to suberect pilosity. Eyes with short, suberect pilosity. Alitrunk dorsum, sides of petiole and postpetiole, and gaster covered with relatively sparse, appressed to suberect pilosity, occasionally longer hairs present. Scutellum with several long hairs (up to 0.06 mm) which arise from large pits. Remaining alitrunk bare, except for some hairs on anterior and dorsal parts of pronotum and on propodeum. Some short appressed hairs ventrally on petiolar peduncle.

Genitalia not examined.

Type Material
Holotype worker, West Malaysia: Negeri Sembilan, Pasho Forest Reserve, in rotten wood on forest floor, 10.iii.1990, FN 27 (K. Rosciszewski), Paratypes: 2 workers with same data as holotype , 2 workers, 1 queen from same locality beneath rotten wood on forest floor, 6.iii.1990, FN 15 (K. Rosciszewski) SMNK, 1 male from same locality, colony found 31.iii.1992 in rotten wood on forest floor, bred in captivity, 23.iv.1992, FN 2430 (K. Rosciszewski) SMNK.