Rhopalothrix isthmica

This species occurs in moderately seasonal to aseasonal wet forest, from sea level to 2000m elevation. All recent specimens are from Winkler or Berlese samples of sifted leaf litter. It is rare for ant species to occur across such a broad elevational spectrum, and given the considerable variability in setal pattern and wide geographic range, it may comprise multiple allopatric populations with unknown degrees of genetic divergence. In Honduras it was a rare cloud forest species at Comayagua and La Muralla and a rare lowland species along the north Caribbean coast. At La Union, in the mountains southeast of Zacapa, Guatemala, it was a rare species sympatric with the much more abundant Rhopalothrix megisthmica. In cloud forest on the slopes of Volcán Atitlán in western Guatemala it was abundant—occurring in 29 of 100 miniWinkler samples—and sympatric with the smaller Rhopalothrix atitlanica. (Longino & Boudinot, 2013)

Identification
Key to Rhopalothrix workers

Distribution based on Regional Taxon Lists
Neotropical Region: Costa Rica, Guatemala, Honduras, Nicaragua, Panama.

Castes
Known only from the worker caste.

Nomenclature

 *  isthmica. Acanthidris isthmicus Weber, 1941a: 188, figs. 4-7 (w.) PANAMA. Combination in Rhopalothrix: Brown & Kempf, 1960: 235.

Worker
Length 2.2 mm. (of thorax, in straight line from anterior pronotal margin to apex of episternal angles, 0.51 mm.). Head angular; in front view, excluding mandibles, one-sixth broader than long, occipital margin distinctly concave, occipital angles evenly rounded, sides of head produced as two convexities back of antennal insertions and an even convexity at insertions extending to clypeus, anterior clypeal margin feebly convex; frontal lobes small but covering antennal insertions, convex, and partially roofing a distinct fossa for the accommodation of the antennae; eye minute, apparently of a single facet, situated beneath a gibbosity at the upper margin of the antennal scrobe at a level about opposite the middle of the scape; antennal scrobes complete, divided into a proximal part for the antennal insertions and a distal part for the entire scape and proximal joints; mandibles porrect, stout, outer margin strongly convex, with a very long sub-apical spine-like tooth, three apical denticles, and on the inner surface about seven acute denticles alternating in two sizes; antennal scapes strongly elbowed with a slender rod-like pedicel and a massive, much larger distal portion, scape failing to reach the occipital angles by a distance equal to over half its length, terminal funicular joint longer than all preceding joints but shorter than the scape.

Thorax with a short neck, in profile with pronotum slightly angulate in front and separated from mesonotum by a feeble depression, the latter feebly convex; meso-epinotal impression shallow, epinotum with a high translucent lamina on either side starting from the basal surface and prolonged as distinct spines separating basal and declivous regions, the lamina produced beneath the spines as an irregular lobe. Thorax from above, excluding neck, broader through pronotum than its length to meso-epinotal impression, convex laterally, mostly flat on top, meso-epinotal impression laterally distinct. Petiole in profile with a distinct peduncle whose anterior dorsal surface is convex and whose mid-ventral surface bears a distinct hooked process directed anteriorly; node slightly convex above, feebly pedunculate behind; node from above transversely elliptical except for the nearly straight posterior margin, about two-fifths broader than long. Postpetiole in profile longer than petiolar node, convex above, highest at posterior half; from above kidney-shaped, the anterior margin strongly concave, the posterior convex, two and one-half times broader than long. First gastric segment covering more than three-fourths of gaster when viewed from above and with anterior margin strongly concave, flattened dorsally, remaining segments much smaller. Legs of moderate proportions, median legs including coxae much the smallest, tibiae massive.

Opaque, densely punctate, integument largely obscured by an apparent glandular deposit extending even to the mandibles.

Pilosity diverse, of sparse, short clavate hairs over the body generally, much longer and slenderer hairs confined largely to the mouthparts, terminal gastric segments and appendages; and coarse squamate-clavate hairs confined largely to the scapes, tarsi, tibiae and posterior half of gaster.

Reddish-ferruginous, appendages but slightly paler.

Longino and Boudinot (2013) - HW 0.58–0.68 (n=13); mandible with three teeth on masticatory margin, middle tooth largest; subapical tooth with distinct reclinate denticle at base; subapical tooth about twice as long as apical tooth; intercalary teeth prominent, one closest to apical tooth about half as long as apical tooth; labrum trapezoidal, anterior lobes triangular, inner margins of lobes shallowly sloping to semicircular median notch; propodeal tooth acute to right-angled, infradental lamella evenly and shallowly concave; squamiform setae abundant on first gastral tergite, either uniformly covering entire tergite or covering varying extent of posterior portion, at least posterior half with abundant setae.

Type Material
Holotype: One worker taken July 29, 1938, on Barro Colorado Island, Panama Canal Zone, by Mr. E. C. Williams, Jr. (No. 313 (131). It probably belonged to the floor fauna of the rain forest covering the island.