Octostruma gymnosoma

Octostruma gymnosoma is known only from the three type workers, all from one Winkler sample of sifted litter and rotten wood from the forest floor. The sample was taken in mesophyll cloud forest in the Sierra Madre de Chiapas, at 1520 m elevation. (Longino 2013)

Identification
Face lacking transverse arcuate carina; basal five teeth of mandible acute; apex of labrum bilobed; face and mesosomal dorsum lacking erect setae. This is the largest Central American species. It is very similar to the allopatric Octostruma schusteri, from Volcán Atitlán in Guatemala. The two differ only in the presence of spatulate setae on the face of O. schusteri. (Longino 2013)

Distribution based on Regional Taxon Lists
Neotropical Region: Mexico.

Nomenclature

 *  gymnosoma. Octostruma gymnosoma Longino, 2013: 33, figs. 1B, 3B, 5M, 25, 43 (w.) MEXICO.

Worker
HW 0.88–0.98, HL 0.76–0.82, WL 0.91–1.07, CI 115–120 (n=3). Labrum triangular, about as long as wide, strap-like lateral portions converging from base to near apex, joined by thin translucent cuticle medially but leaving distinctly bilobed apex, with short median notch; mandible triangular, in profile view with mandible closed, in same plane as clypeus, apex of mandible not strongly down-turned; with mandible fully open, dorsal face remains in same plane as clypeus; mandible with 8 teeth, tooth 1 continuous with basal rim of dorsal surface, 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 very faintly sculptured; clypeus with broad, shallow emargination anteriorly; clypeus and face matte, with very faint sculpture, nearly smooth (difficult to determine sculptural detail; three known specimens have a thin layer of wax-like material on face); 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 matte, faintly foveolate anteriorly, grading to smooth posteriorly; occipital carina distinct, extending anteriorly on ventral surface of head nearly to hypostoma; postgenal suture visible as dark line on undersurface of head; undersurface sculpture similar to face.

Promesonotum moderately convex in profile, promesonotal suture moderately impressed, medial mesonotum with shallow longitudinal impression; metanotal groove moderately impressed, inconspicuous in profile view, stronger in dorsal view, with coarse longitudinal rugae in depression; propodeum with distinct dorsal and posterior faces; dorsal face flat, sloping; propodeal spines well-developed, in the form of acute flat translucent perpendicular plates, extending ventrally as thin carinae; a few thin carinulae extend between propodeal spines, separating dorsal and posterior faces of propodeum; propodeal spiracle small, diameter much less than width of base of propodeal spine, slightly projecting on low tubercle, located below propodeal spine and anterior to posterior margin of propodeum; all surfaces of mesosoma matte except posterior face of propodeum, which is shiny; dorsum of pronotum and lateral mesonotum irregularly rugose, medial impression of mesonotum smooth, dorsal face of propodeum faintly punctate, posterior face of propodeum smooth, lateral pronotum very faintly punctate; meso- metapleuron and side of propodeum confluent, smooth.

Petiole in profile with peduncle differentiated from node, node with differentiated anterior face; node triangular, with posterodorsal face long, sloping, medially impressed; anteroventral margin with acute tooth; postpetiole low, broad, crescent-shaped in dorsal view; dorsa of petiolar node and postpetiole faintly sculptured like frons; first gastral tergite covered with very faint, dense, confluent puncta, appearing shagreened; first gastral sternite anteriorly smooth, shiny to faintly shagreened, posteriorly punctate, interspaces subequal in width to puncta, smooth and shining.

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 10 spatulate setae; clypeus and face with fine, sparse fully appressed ground pilosity; face, mesosomal dorsum, petiole, postpetiole, and first gastral tergite lacking erect setae; mesotibia with moderately abundant ground pilosity, about 5 short spatulate setae at apex; second and subsequent gastral tergites with usual complement of clavate setae; first gastral sternite with abundant short clavate setae on posterior half to two thirds, anterior portion devoid of setae.

Color dark brown.

Type Material
Holotype worker: Mexico, Chiapas: 2km SE Custepec,15.72099, -92.95054, ±200 m, 1520 m, mesophil forest, ex sifted leaf litter, 17 May 2008 (LLAMA, Wm-A-02-1), unique specimen identifier JTLC000015356]. Paratype workers: same data ECOSCE, CASENT0609703;, CASENT0627366.

Etymology
The name refers to the general lack of spatulate setae on the dorsal surfaces. It is a noun in apposition and thus invariant.