Tropidomyrmex

A monotypic genus, Tropidomyrmex elianae has been collected from a termite colony and from winkler litter samples.

Identification
Silva et al. (2009) - Tropidomyrmex can be easily distinguished from all other ant genera by the bilobed subpostpetiolar process, hence the generic name we chose for this ant. Bolton (2003) diagnosed the solenopsidine tribe group, which includes Solenopsidini, Stenammini and Adelomyrmecini (Bolton et al., 2006), as having, primarily, the clypeus constricted posteriorly, with median portion narrowed and elevated, without an isolated longitudinal carina, and the ventral surface of the metathorax simple. From the combination of characters that define Solenopsidini according to Bolton (2003), Tropidomyrmex does not have the short subtriangular mandibles, the dental count is different from 2-6, the clypeus is not bicarinate, and the maximum exposure of toruli is posterior to the maximum width of frontal lobes, characters that may vary in some genera. Still, according to Bolton (op. cit), most solenopsidine genera have a single stout median clypeal seta. Interestingly, although adult females of Tropidomyrmex do not have a stout median clypeal seta, the larvae of T. elianae clearly present a median seta accompanied by two similar ones at the anterior clypeal margin. The male has a row of setae at the anterior clypeal margin, although the median seta is not particularly different or stouter than the others in the row. Solenopsidines in general have a strongly differentiated antennal club of 2-4 segments. The funicular segments of Tropidomyrmex increase quite regularly in size until the obviously enlarged apex, different from all other solenopsidine or even stennamine ants; even the socially parasitic species, which tend to have very reduced dentition, retain a strong antennal club.

The mandibles of Tropidomyrmex are completely different from all other solenopsidine genera, with only a single curved apical tooth. Most of these differences may be related to the evident reductions that characterize Tropidomyrmex. The most striking character, however, is the presence in T. elianae workers of a clearly visible promesonotal suture, which is completely lacking in ergatoids.

Bolton (2003) recognized three genus groups within the Solenopsidini; Tropidomyrmex fits better within the Carebara genus-group. Judging from Ettershank's scheme (1966), Tropidomyrmex would be classified in his Megalomyrmex genus group within Solenopsidini, based on the antennal count, clypeus shape, and relative position of the anterior tentorial pits. Tropidomyrmex shares some characteristics with the solenopsidine Tranopelta Mayr and the recently described Dolopomyrmex Cover & Deyrup (2007): delicate and unpigmented female integument; strongly convex, non-carinate clypeus; antennal count; comparatively reduced wing venation; a broad attachment of postpetiole to gaster; specialized larval morphology (not known for Dolopomyrmex ), and the apparent cryptobiotic habits. However, Tropidomyrmex workers differ from these genera in having a single apical tooth on the mandibles instead of four to five, a palpal formula of 2,1 instead of 3,2 or 4,3, and an antennal club with a single segment instead of three. The venom apparatus of Tropidomyrmex elianae is extremely delicate, and it was very hard to dissect without losing the connections among plates. This is why in Figure 5, some parts do not have their ends depicted. Comparing the venom apparatus of T. elianae with other Myrmicinae studied by Kugler (1978), it seems closer to Solenopsis and related genera, by the square spiracular plate, the one-segmented gonostylus and the overall shape of the oblong, triangular and quadrate plates. However, Kugler's classification of genera is very different from Bolton's scheme (2003), which hinders these comparisons.

According to Wheeler & Wheeler (1976), Tranopelta larval mandibular shape is classified as pristomyrmecoid, shared with the Ponerinae Pachycondyla (Hagensia), Pseudomyrmecinae (Pseudomyrmex and Tetraponera), and several unrelated Myrmicinae. The closest mandibular shape to Tropidomyrmex, using Wheeler and Wheeler (op. cit.) criteria, is anergatidoid [recorded only in Pheidole (= Anergatides)], although in Tropidomyrmex the mandible does not bear an apical denticle. Following these authors, only Bothriomyrmex, Technomyrmex and Apterostigma present larval mandibles so short that they do not even meet, as in Tropidomyrmex.

Overall, most of the distinctive characters of this new genus appear to represent reductions in characters normally present in females of solenopsidine genera: fewer ommatidia, reduced mandibular dentition, reduced body size and shape, the fragility of the integument, fused and reduced structures of the venom apparatus, and reduced male wing venation. This may be related to the special habits adopted by these ants. It is noteworthy that T. elianae has been collected inside a chamber within a ground termite nest, but it is known also from two free-living workers extracted from the leaf-litter of two very close sites in central Brazil. From the very scanty information we have, it is not possible to ascertain whether the type series represents a whole colony or a fragment, and also it is not possible to be sure whether these ants always live inside termite nests.

Biology
See Tropidomyrmex elianae for details.

Castes
Ergatoid queen differing from the conspecific worker by the slightly larger size, presence of three relatively small ocelli and wing buds, and absence of a promesonotal suture (Silva et al. 2009). Gaster is noticeably bulkier in the ergatoid queen.



Nomenclature

 *  TROPIDOMYRMEX [Myrmicinae: Solenopsidini]
 * Tropidomyrmex Silva, Feitosa, et al. 2009: 34. Type-species: Tropidomyrmex elianae, by original designation.

Worker
Monomorphic myrmicine ants. Size relatively small (TL 2.15–2.57). Body color pale-yellowish, with appendages somewhat lighter. Integument extremely thin, predominantly smooth and rather opaque, with the inferior portions of meso- and metapleuron finely striate. Pilosity composed mainly of short to long,flexuous, suberect hairs.

Head subquadrate, in full face view; lateral borders and dorsal surface slightly convex; vertexal margin strongly concave medially; posterolateral corners rounded. Mandibles narrow, weakly convex dorsally, and with external margins considerably curved inwards; masticatory margins very short and mostly edentate, except for the apical tooth which is extremely developed. Palpal formula 1,2. Maxillae stipes without transverse crest. Clypeus very broad and convex dorsally, posteriorly emarginate; lateral portions set lower than central disc; anterior margin very pronounced anteriorly at the median portion. Frontal lobes very reduced, exposing the antennal insertions. Toruli visible in full-face view, their maximum exposure anterior to point of maximum width of frontal lobes. Frontal triangle indistinct. Compound eyes relatively small, occupying some 5% of the head capsule, and set at the same level as the surrounding surface of head. Antennae 11-segmented, segments gradually increasing in size without a noticeable club; apical segment as long as the four preceding ones; antennal condyle exposed; scapes with dorsal surface convex apically and failing to reach the vertexal margin.

Mesosoma compact; pronotum relatively large; promesonotal suture distinct under SEM; propleuron well developed; dorsal surface of mesonotum separated from anepisternum by a distinct suture; metanotal suture vestigial; propodeum unarmed and with the dorsal surface strongly sloped posteriorly, propodeal spiracle relatively large, posterior declivity strongly concave, so that it can receive the entire petiole when it is closed against the mesosoma, propodeal lobes reduced, not visible in side view. Legs relatively short and robust, middle and hind tibiae without apical spurs; tarsal claws simple.

Petiolar peduncle short, without a ventral carina or processes, node distinct, high, and dorsally convex in profile; postpetiole broader than long in dorsal view and relatively narrow in lateral view, ventral process well developed, as broad as postpetiole, and formed by two divergent lobes, when seen in ventral view. Gaster moderately elongate, with the first segment (IV abdominal) larger than the posterior ones; tergite of abdominal segment IV (first gastral) broadly overlapping sternite on ventral surface of gaster; gastral shoulder inconspicuous.

Sting apparatus. Spiracular plate subrectangular; dorsal notch absent; spiracle relatively wide; anterior apodeme narrow; ventral edge weakly pronounced. Quadrate plate with the dorsal region as broad as ventral region; dorsal margin convex; posterior margin complete. Oblong plate with posterior apodeme long. Triangular plate longer than broad. Gonostylus with body virtually fused to the oblong plate, with a single sclerotized anterior segment. Anal plate small and subtriangular. Lancet short, with functional valves; dorsal and ventral margins converging towards the apex. Sting sclerotized, with acute apex; body of bulb slender in lateral view. Furcula with dorsal arm fused to the base of the sting bulb.

Queen
Ergatoid. Virtually identical to the conspecific worker except for the presence of the ocelli and wing buds, and the absence of a promesonotal suture.

Male
Body dark brown. Dorsum of head densely punctate; body predominantly smooth and weakly shiny. Short, subdecumbent, whitish hairs sparsely covering the body; appendages with fine apressed pubescence.

Head rounded. Mandibles falcate and relatively well developed, with a single tooth at apex. Palpal formula 1,2. Frontal lobes obsolete exposing the basilar condyle of antennae. Antennae 12-segmented; scape very short, pedicel enlarged. Ocelli present.

Pronotum reduced; scutum large and rounded, parapsidial lines distinct; prescutellum narrow; propodeum unarmed, with short dorsal face and distinctly concave declivous face in dorsal view, convex in side view; propodeal spiracle small, propodeal lobes reduced. Legs more elongate than in the conspecific females. Wing venation reduced; longitudinal veins Sc+R, Rs, M, Cu, and A present; cells C, R and SR closed. Hind wing with R cell only; four sub-median hamuli.

Petiole pedunculate, ventral processes absent. Postpetiole broader than long in dorsal view, attached to gaster by almost its full width. Gaster elongate, with rounded gastral shoulder.

Larva
Profile pheidoloid, although the head does not protrude ventrally as much as in Pheidole larvae. Head subcircular; irregular, very reduced mandibles, apices do not meet, edentate; prothorax and abdomen relatively short, no protuberances. Body pilosity sparse and formed by smooth unbranched and short anchortipped hairs.

Etymology
The generic epithet refers to the keels under the female postpetiole; from Greek tropidos (τροπιδοσ) = keel, which, by virtue of being double, clearly differentiate these ants from all others, as far as we know.