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Extant: 16 valid species
The rotten wood and leaf litter of tropical wet forest floors are abundantly inhabited by a diversity of tiny predaceous ants in the tribes Dacetini and Basicerotini. Rhopalothrix is one of the rarer and more distinctive basicerotine genera.
Neotropical mainland from southern Mexico to Argentina, Cuba, New Guinea, Australia.
From Longino and Boudinot 2013 (with some expansion):
Knowledge of the biology of R. isthmica clade of Rhopalothrix is conjectural; a nest has never been recovered and a live one never seen. What we know is based on locations and frequencies of capture using various mass-sampling methods. Specimens are known from wet to moderately seasonal forest, from sea level to 2140 m elevation. At higher elevation, they are found in diverse mesophyl forest and in forests with various combinations of Liquidambar and montane oak. In Costa Rica, they are restricted to the wet forests of the Atlantic slope, to 1500 m on the Barva Transect in the Cordillera Volcánica Central and to 800 m in the Cordillera de Tilarán. The genus is unknown from the Monteverde cloud forest at 1500 m, the lowland wet forests of the Osa Peninsula, and the lowland tropical dry forests of Guanacaste, in spite of intensive collecting efforts in these areas. Further north in Central America they can occur at higher elevations.
In quantitative sampling at La Selva Biological Station, in the Atlantic lowlands of Costa Rica, occurrences were relatively more frequent in soil/litter cores than in samples of sifted litter from the soil surface. This suggests that nests are subterranean, with workers only occasionally venturing up into the litter layer. Dealate queens are known for a few species, occurring occasionally in Winkler or Berlese samples. Alate queens of one La Selva species were found in canopy fogging samples, one each in two separate fogging events. Oddly, alate queens have not been found in the many Malaise samples from La Selva. Males remain unknown.
Rhopalothrix workers probably exhibit contraction and thanatosis (immobility) in response to disturbance, like other small dacetines and basicerotines. We propose a funiculus-concealment hypothesis wherein several parts of the headcapsule act as an anatomical complex to functionally elongate the scrobe—enhancing a thanatosis display—to explain several characteristics of the head. When the antenna is completely tucked into the ventrolateral scrobe of the headcapsule, antennomeres 2–6 are dorsally concealed by the broadened scape, leaving the apical antennomere projecting beyond the scape base. The scape often has a shoulder-like basolateral elongation which functions to increase coverage of the funiculus, according to our hypothesis. The anterolateral lobe of the headcapsule, which forms the ventral wall of the antennal fossa, is broad enough to cover the basalmost portion of the terminal antennomere. The apical portion of the terminal antennomere is hidden either completely by the mandibular shaft, or by the shaft and the labrum. In some species, especially R. apertor, the base of the mandible has a strong ventral arch in which the apical antennomere fits perfectly. Moreover, we hypothesize that the labrum of Rhopalothrix has dual derived functions. The first function, as indicated by the funiculus-concealment hypothesis, is concealing the apicalmost portion of the terminal antennomere. The second function we propose for the labrum is raising the trigger hairs (Bolton 1999) towards the mandibular apices, such that the hairs are in contact, or nearly in contact, with the apical and subapical teeth. With the trigger hairs in such a position, a snap- or kill-zone is formed when the mandibles are fully open. Thus two natural selection pressures may influence labral shape and function: protection of the antenna during thanatosis and improved efficiency during hunting.
From Longino and Boudinot 2013:
While most basicerotines have triangular mandibles, Rhopalothrix mandibles are arched shafts with an apical fork similar to many dacetines.
Among the Rhopalothrix species, we propose a monophyletic R. isthmica clade with the following synapomorphies: (1) absence of squamiform setae on the face, and (2) development of shallow arcuate grooves and ridges on the face. All the species share a similar habitus, being small, nearly eyeless, pale brown, with uniformly matte surface, and a characteristically broad, lumpy face. The R. isthmica group is strictly Neotropical, with the greatest abundance and diversity in Central America. The only Rhopalothrix species outside of the R. isthmica clade are R. ciliata from Colombia, R. diadema from New Guinea, and R. orbis from Australia. These latter three species all have squamiform setae on the face and their heads are more elongate and less flattened, more like other basicerotines.
Species of the R. isthmica clade vary in size, armature of the mandible, shape of the labrum, shape of the propodeal tooth and infradental lamella, and the distribution and abundance of squamiform setae on the gaster. Size is measured as head width (HW), the maximum width of the head capsule in full-face view. There is little variation in allometry among traditional metric characters (head length, scape length, mesosoma length, etc.), obviating the need for extensive measurements. The inner surface of the mandible curves through the masticatory margin and has a variable number of teeth or small denticles. The apical fork is perpendicular to the dorsal surface of the mandible and comprises two long spiniform teeth: the subapical and apical. The subapical tooth is visible as the apparent apex of the mandible in dorsal view; the shorter apical tooth is below it. There are usually two small intercalary denticles between the subapical and apical teeth. A small, usually reclinate denticle occurs on the inner base of the subapical tooth, with a diastemma between it and the more basal teeth of the masticatory margin. This denticle varies in size and distinctness and may be extremely minute or absent. In one species it is pronounced and recurved, directed back toward the base of the mandible. The shape of the labrum is highly variable and of great diagnostic value (Fig. 2). In some species the apical lobes of the labrum are secondarily divided, thence bilobed; we term these the ectal and mesal lobules (Fig. 2A, B). When preparing specimens, it helps to open at least one mandible by inserting an insect pin between the mandibles and labrum and wedging open a mandible. This allows clear viewing of the mandibular dentition and the labrum.
The posterior face of the propodeum always has a pair of translucent longitudinal lamellae, of varying width, and with the dorsal portion forming an obtuse, right-angle, or acute tooth. The infradental lamella is the portion of the lamella ventral to the tooth. Varying numbers of squamiform (paddle-shaped) setae occur on the gaster (Fig. 3). Setae are always present on the second through fourth gastral tergites (abdominal segments 5–7, small sclerites at the apex of the gaster). The first gastral tergite, which covers most of the gaster, varies from being devoid of setae to having a dense, even coverage. The setae vary in the length of the stem and the degree of inclination. The meso- and metabasitarsus and the apices of the tibiae also have squamiform setae varying in number and size.
From Longino and Boudinot 2013:
Baroni Urbani & de Andrade (2007) proposed a synonymization of the tribe Basicerotini with the Dacetini and that all basicerotine genera, including the genus Rhopalothrix, be placed in the single genus Basiceros Schulz, 1906. This was an incorrect synonymization, because Rhopalothrix Mayr, 1870 has priority over Basiceros Schulz, 1906 (Basiceros was a replacement name for the homonym Ceratobasis Smith 1860). Baroni Urbani & de Andrade's concept of Rhopalothrix (assuming the nomenclatural correction) is much broader than the one we adopt here. There is currently variable acceptance of Baroni Urbani & de Andrade's reclassification, and it is clear that a larger study that incorporates morphological and molecular data will be necessary before genus boundaries within the tribe are understood. ... we follow the classification of Brown & Kempf (1960) and Bolton (2003).
Central, Itapúa (BMNH).