Octostruma triangulabrum

Octostruma triangulabrum occurs in montane wet forest habitats, in both mesophyl cloud forest and wet pine-oak forests, throughout the central Chiapas highlands. It occurs from 900–2750 m elevation and can be moderately abundant (up to 13% of 1 m2 plot litter samples in quantitative sampling). It is only known from Winkler and Berlese samples of sifted leaf litter. (Longino 2013)

Identification
Longino (2013) - With the characters of Octostruma wheeleri and Octostruma triquetrilabrum. Differing from O. wheeleri in the presence of 8–12 spatulate setae on face (6 on O. wheeleri) and shallow reticulate rugulose sculpture on face and dorsal pronotum (nearly smooth on O. wheeleri). Differing from both species in a pair of spatulate setae on the mesonotum (lacking in O. wheeleri and O. triquetrilabrum).

Distribution based on Regional Taxon Lists
Neotropical Region: Mexico.

Biology
Longino (2013) - This species and Costa Rica's Octostruma triquetrilabrum are extremely similar allopatric forms, differing only in the presence or absence of mesonotal setae. Intensive sampling of the litter fauna in cloud forest sites in Nicaragua, Honduras, and Guatemala has failed to reveal additional populations. Both together are quite similar to the lowland Octostruma wheeleri. Octostruma triangulabrum is sharply parapatric with O. wheeleri (see Comments under O. wheeleri).

Castes
Known only from the worker caste.

Nomenclature

 *  triangulabrum. Octostruma triangulabrum Longino, 2013: 52, figs. 1A, 3A, 5J, 38, 43 (w.) MEXICO.

Worker
HW 0.73–0.78, HL 0.68–0.73, WL 0.82–0.89, CI 106–109 (n=5). Labrum sides straight, strap-like lateral portions converging from base to apex, fused apically (not bilobed, at most with minute notch apically), joined medially by thin translucent cuticle; mandible triangular, in profile view with mandible closed, in same plane as clypeus, apex of mandible not down-turned; mandible with 8 teeth, basal rim of dorsal surface slightly offset from base of tooth 1, forming small denticle, teeth 1–5 acute, similar in shape, teeth 5–8 forming an apical fork, with 5 and 8 large, 6 and 7 small partially confluent denticles; dorsal surface of mandible roughened; ventral surface flat and parallel to clypeus apically, twisting basally to nearly perpendicular orientation basally, smooth and shining; interior surface concave, smooth and shining; scape flattened, with pronounced anterobasal lobe, dorsal surface roughened, dull; clypeus with broad, shallow emargination anteriorly; clypeus and face shallowly, irregularly rugulose, generally matte, tops of face rugulae sublucid; frontal carinae faint, nearly obsolete; antennal socket deep, dorsal rim of socket continuous with pronounced dorsal margin of antennal scrobe; antennal scrobe deep, strongly delimited dorsally, posteriorly, and ventrally with sharply defined thin cuticular rim; compound eye small, circular, composed of about 7 ommatidia; distinct carina extends from ventral margin of antennal socket across floor of scrobe to compound eye; scrobe floor faintly foveolate, matte; vertex margin anterior to occipital carina smooth, sublucid (top of head, not visible in face view); occipital carina extends anteriorly on ventral surface of head to level of compound eye; undersurface coarsely rugose.

Promesonotum moderately convex in profile, promesonotal suture not impressed, promesonotum with broad, weak, longitudinal impression; metanotal groove not impressed; propodeum with distinct dorsal and posterior faces; dorsal face weakly convex; propodeal spines well-developed, in the form of acute flattened perpendicular plates, extending ventrally as thin carinae; single well-defined transverse carina extends between propodeal spines, separating dorsal and posterior faces of propodeum; propodeal spiracle medium-size, diameter less than width of base of propodeal spine, located below propodeal spine and abutting to somewhat separated from posterior margin; all surfaces of mesosoma matte except posterior face of propodeum, which is shiny; dorsum of promesonotum irregularly shallowly rugulose laterally, smooth medially, dorsal and posterior face of propodeum smooth, lateral pronotum smooth; meso- metapleuron and side of propodeum confluent, smooth.

Petiole in profile with peduncle differentiated from node, node with differentiated anterior face; node subquadrate to triangular, with long sloping dorsal face, posterior face short to absent; anteroventral margin with acute to peg-like tooth; postpetiole low, broad, crescent-shaped in dorsal view; dorsum of petiolar node and postpetiole shallowly rugulose, matte; first gastral tergite densely punctate, tergal puncta smaller posteriorly, interspaces sublucid; first gastral sternite smooth, matte anteromedially, punctate posteriorly.

Anterior labral lobe with radiating tuft of soft, thick, translucent, capitate setae of unequal length projecting from apex; each larger mandibular tooth with fully appressed seta running length of tooth; anterior margin of scape with about 8 stiff clavate setae; clypeus and face with fine, sparse fully appressed ground pilosity; face typically with 10 erect clavate setae arranged as in; pronotum lacking erect setae; mesonotum with a pair of erect clavate setae; mesotibia with about 5 clavate seta of variable length at apex; petiole, postpetiole, first gastral tergite lacking erect setae; ground pilosity of first gastral tergite short, sparse, fully appressed; first gastral sternite with abundant clavate setae clustered on posterior half, anterior half devoid of setae.

Color dark red brown.

Type Material
Holotype worker: Mexico, Chiapas: Nahá, 16.96291, -91.59335, ±2 km, 950 m, 10 Jun 2008, mesophil forest, ex sifted leaf litter (LLAMA, Wm-A-07-all), unique specimen identifier CASENT0608768. Paratype workers: same data, CASENT0608761; same data except 16.97417, -91.58592, ±50 m, 950 m, 14 Jul 2007 (J. Longino#6047-s) , JTLC000009758; , JTLC000009759]; 16.97417, -91.58592, ±50 m, 950 m, 14 Jul 2007 (R. S. Anderson#2007-013) , CASENT0602034; 16.96313, -91.59337, ±50 m, 985 m, 8 Jun 2008 (LLAMA, Wa-A-07-1-06) , CASENT0627348; , JTLC000014462]; 16.96318, -91.59337, ±50 m, 985 m, 8 Jun 2008 (LLAMA, Wa-A-07-1-07) , CASENT0627349; CASC, CASENT0627350; 16.94866, -91.59392, ±50 m, 930 m, 8 Jun 2008 (LLAMA, Wa-A-07-2-08) CASC, JTLC000014488.

Etymology
The name refers to the triangular labrum that is not bilobed at the apex. It is a noun in apposition and thus invariant.

References based on Global Ant Biodiversity Informatics

 * Dattilo W. et al. 2019. MEXICO ANTS: incidence and abundance along the Nearctic-Neotropical interface. Ecology https://doi.org/10.1002/ecy.2944
 * Longino J. T. 2013. A revision of the ant genus Octostruma Forel 1912 (Hymenoptera, Formicidae). Zootaxa 3699(1): 1-61.
 * Longino J. et al. ADMAC project. Accessed on March 24th 2017 at https://sites.google.com/site/admacsite/