Rhopalothrix

The genus Rhopalothrix is a rare inhabitant of wet forest leaf litter and soil.

Identification
Rhopalothrix belong to the tribe Basicerotini. While most basicerotines have triangular mandibles, Rhopalothrix mandibles are arched shafts with an apical fork similar to many dacetines.

The Basicerotini tribe is taxonomically problematic as there is no single accepted classification of its species into genera. Baroni Urbani and de Andrade (2007) synonymized the tribe Basicerotini with Dacetini and proposed that all basicerotine genera be placed in the genus Basiceros. Longino and Boudinot (2013) revised a distinctive Rhopalothrix isthmica clade and recognized 13 species. The Rhopalothrix isthmica clade was defined by two 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 clade is strictly Neotropical, with the greatest abundance and diversity in Central America. They also noted there were an additional 3 species, outside of this clade, in the genus: Rhopalothrix ciliata from Colombia, Rhopalothrix diadema from New Guinea, and Rhopalothrix 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.

Key to Rhopalothrix isthmica clade Rhopalothrix workers

Biology
Longino (2013) - Knowledge of the biology of the Rhopalothrix isthmica clade of Rhopalothrix is conjectural; a nest has never been recovered and a live specimen 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 mesophyll 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.

Nomenclature

 *  RHOPALOTHRIX [Myrmicinae: Basicerotini]
 * Rhopalothrix Mayr, 1870a: 415. Type-species: Rhopalothrix ciliata, by subsequent designation of Wheeler, W.M. 1911f: 172.
 * Rhopalothrix senior synonym of Acanthidris, Heptastruma: Brown & Kempf, 1960: 230.
 * Rhopalothrix junior synonym of Basiceros: Baroni Urbani & De Andrade, 2007: 88.
 * ACANTHIDRIS [junior synonym of Rhopalothrix]
 * Acanthidris Weber, 1941a: 188. Type-species: Acanthidris isthmicus, by original designation.
 * Acanthidris junior synonym of Rhopalothrix: Brown & Kempf, 1960: 230.
 * HEPTASTRUMA [junior synonym of Rhopalothrix]
 * Heptastruma Weber, 1934a: 54. Type-species: Heptastruma wheeleri (junior secondary homonym in Rhopalothrix, replaced by Rhopalothrix weberi), by original designation.
 * Heptastruma junior synonym of Rhopalothrix: Brown & Kempf, 1960: 230.

Longino and Boudinot (2013) provided the following regarding key characters for the genus:

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 was 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 diastema 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. In some species the apical lobes of the labrum are secondarily divided, thence bilobed; we term these the lateral and medial lobules. 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.