Octostruma balzani

Longino, John T, 2013, A revision of the ant genus Octostruma Forel 1912 (Hymenoptera, Formicidae), Zootaxa 3699, pp. 1-61 : 19-22

Octostruma balzani (Emery, 1894)

(Figs 1E, 3D, 5B, 7B, 8B, 9A, 14D, 18, 42)

Rhopalothrix balzani Emery 1894: 217, pl. 1, fig. 10. Lectotype worker (here designated): Bolivia; cantoni di Coroico e Chilumani-Yungas, 1600m, 1891 (Balzan) [presumed; see Comments] [MCSN, CASENT0904969] (examined); queen: same data as lectotype [MCSN, examined]. Combination in Octostruma : Brown 1949: 92.

Rhopalothrix (Octostruma) barberi Mann , 1922: 42. Syntype workers: Guatemala, Alta Vera Paz, Cacao, Trece Aguas (Barber & Schwarz) [USNM] (examined). Combination in Octostruma : Brown, 1949: 92. Junior synonym of O. balzani : Brown & Kempf, 1960: 194.

Rhopalothrix (Octostruma) equilatera Weber , 1934: 52, fig. 10. Syntype workers: Nicaragua, "Tuli Creek near San Mejuel" [assumed Rio Tule near San Miguelito; see Noble, 1918], Nov 1916, from stomach of Dendrobates tinctorius (C. R. Halter) [MCZC] (examined). Combination in Octostruma : Brown 1949: 92. Junior synonym of Octostruma balzani : Brown & Kempf, 1960: 194.

Geographic range. Northern Mexico (Tamaulipas, Nuevo Leon) to southern Brazil (Parana).

Description. Worker. HW 0.50-0.63 (lectotype HW 0.58), HL 0.46-0.58, WL 0.51-0.63, CI 104-113 (n=26). Labrum as in Fig. 1E, sides concave, strap-like lateral portions converging from base to mid-length, then becoming porrect, joined by a thin translucent lamella but leaving distinctly bilobed apex; mandible triangular, in profile view with mandible closed, in same plane as clypeus, apex of mandible not down-turned; with mandible fully open, dorsal face remains in same plane as clypeus; mandible with 8 teeth (Fig. 3D), tooth 1 continuous with basal rim of dorsal surface, tooth 1 a broad blunt lamella, strongly differentiated from tooth 2, teeth 2-5 acute, similar in shape, with denticles between them; 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 concave, smooth and shining; interior surface narrow, concave, smooth and shining; scape flattened, with pronounced anterobasal lobe, dorsal surface shallowly rugulose punctate; clypeus with broad, shallow emargination anteriorly; face matte, densely punctate rugulose throughout; 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; short feeble carina extends from ventral margin of antennal socket across floor of scrobe toward compound eye; scrobe matte but smoother than sculpture on face; vertex lobes and undersurface of head densely punctate; occipital carina obsolete.

Promesonotum forming continuous convexity in profile; metanotal groove forming a thin impressed notch visible in profile view; posterodorsal propodeum a single, concave, sloping surface, not differentiated into dorsal and posterior faces; propodeal spines in the form of acute flattened perpendicular plates, extending ventrally as very thin carinae, not forming ventral lamellae; propodeal spiracle medium-size, diameter less than width of base of propodeal spine, located below propodeal spine and abutting posterior margin; entire mesosoma uniformly matte, densely punctate.

Petiole in profile with peduncle differentiated from node, anterior surface concave from petiolar foramen to node, node subquadrate, with long sloping dorsal face and short vertical posterior face; anteroventral margin with acute tooth; postpetiole low, broad, crescent-shaped in dorsal view; entire petiole and postpetiole densely punctate; entire gaster densely punctate, tergal puncta denser than sternal puncta .

Anterior labral lobe with about three thin stiff setae on side, tuft of soft, thick, translucent, capitate setae of unequal length projecting from apex (like Fig. 2); each larger mandibular tooth with fully appressed seta running length of tooth; anterior margin of scape with about seven stiff spatulate setae; clypeus and face with very sparse fully appressed ground pilosity; face typically with ten erect spatulate setae arranged as in Fig. 5B; mesonotum typically with a pair of erect spatulate setae located at the juncture of pro- and mesonotum; mesotibia with 5 clavate seta of variable size at apex; petiole usually lacking erect setae; postpetiole with 0-2 erect setae; first gastral tergite with 4-16 erect setae, these clustered posteriorly, relatively broadened apically; ground pilosity fully appressed, sparse (length of setae less than distance between them); first gastral sternite with moderately abundant, somewhat clavate erect setae over much of surface except narrow area near postpetiolar insertion.

Color red brown.

Queen. HW 0.53-0.67, HL 0.50-0.61, WL 0.64-0.80, CI 104-112 (n=11). Labrum, mandible, scape, antennal scrobe, and head sculpture similar to worker; face with 8-10 erect setae distributed symmetrically around lateral and posterior margins of head, a seta on low ridge in front of each compound eye, 2-4 setae across vertex between compound eyes; ocelli distinct; compound eye large, multifaceted, about 12 ommatidia in longest row.

Mesosoma with queen-typical alar sclerites; sculpture like in worker; anepisternum and katepisternum separated by strong sulcus; posterodorsal propodeum concave; propodeal spines pronounced, in the form of flattened perpendicular plates, acute in profile; pronotum with about 4 erect setae, mesoscutum with 8-16, axilla with 1, scutellum with 2, metanotum with 2, petiolar node with 2, postpetiolar disc with 2-4, first gastral tergite with 20-40. Other characters similar to worker.

Biology. Octostruma balzani is a widespread Central and South American species that is common in many localities. It occurs in a variety of forested habitats: wet to seasonal dry, second growth to mature, lowland to montane. It usually occurs from sea level to the lower edges of cloud forests, typically around 1400 m. The highest elevation record is 1650 m in Nuevo León, Mexico. Almost all collections are from Berlese and Winkler samples of sifted litter and rotten wood from the forest floor. In quantitative 1 m 2 litter plot samples, withinsample abundance is tens of workers or fewer, but the species can occur in nearly every sample, suggesting a high density of small colonies. Dealate queens and intercaste workers often occur together with workers in litter samples.

Comments. Species in the Octostruma balzani complex share the following characters: the basal tooth of the masticatory margin of the mandible is broad and lamelliform, and the following four teeth are contrastingly narrow and acutely pointed; the surfaces of the face and dorsal mesosoma are uniformly punctate, with no rugae or smooth areas; and HW is 0.50-0.68. They have sparse spatulate setae and very inconspicuous appressed pubescence, and unlike many other Octostruma they rarely have a layer of soil adhering to the surface. In many wet forest areas of Central America they are among the most common ants in litter samples.

Brown and Kempf (1960) treated the group as a single polytypic species, O. balzani , with the three synonyms O. barberi, O. equilatera , and O. lutzi . They had no evidence of sympatry of forms but acknowledged the high variability among the specimens they examined. Perrault (1988) discovered the occurrence of sympatric forms in French Guiana, identifying one of them as O. balzani s.s. and the other as a new species O. betschi . The inadequately characterized O. amrishi was described from Suriname and quickly synonymized with O. balzani . All of these taxa have been reevaluated in this study and several new species described from the O. balzani complex.

Quantitative inventory projects in Central America have produced hundreds of individual collections from Costa Rica to Mexico, revealing multiple localities with evidence of two or three sympatric species. All the specimens involved are extremely similar in size, shape, mandibular dentition, labrum structure, and surface sculpture. They mainly differ in the disposition of spatulate setae on the face and mesosoma and the degree of impression of the metanotal groove. They also vary to an extent in color and habitat preference. Local bimodal distributions of morphological characters are used as evidence of sympatric species, but there is always a small percentage of specimens that are intermediate and cannot be reliably identified.

Octostruma batesi, O. betschi , and O. stenognatha share a somewhat triangular head shape and a reduced anterior lobe on the base of the scape. They appear to be allopatric or parapatric in South America, with O. stenognatha occurring in southern Brazil, O. batesi in the Andes, and O. betschi occurring throughout Amazonia and into the eastern foothills of the Andes. Octostruma lutzi is restricted to the islands of Dominica and Guadeloupe, and has two pairs of erect setae on the promesonotum (a trait shared with O. batesi and O. betschi ).

The remaining five species-O. amrishi, O. balzani, O. gymnogon, O. megabalzani , and O. trithrix-occur in Central and South America and are all extremely similar. Of the five, O. balzani is the most widespread and the most variable. It is sympatric with O. trithrix from Honduras northward, with O. amrishi and O. gymnogon in lowland wet forest areas from eastern Honduras to Panama, and with O. megabalzani in the highlands of southern Costa Rica and western Panama.

The overlap of O. balzani with O. trithrix occurs throughout eastern Mexico, south through the Petén region and southeastern Guatemala and into the northern Honduran lowlands, as far east as La Ceiba. In Chiapas, four Project LLAMA community samples suggest that O. trithrix prefers warmer or more disturbed habitats. These four LLAMA sites in eastern Chiapas fell on a disturbance and temperature gradient: Salto de Agua at 100 m elevation was scrubby second growth forest surrounded by a largely agricultural landscape; Playón de la Gloria at 160 m elevation was an ecotone between second growth vegetation and a large primary forest reserve; Metzabok at 575 m elevation was also ecotonal, like Playón de la Gloria; Naha at 985 m elevation was a large area of mature wet forest. Only O. trithrix was found at Salto de Agua; both species were similarly abundant at Playón de la Gloria and Metzabok; and only O. balzani was found at Naha. The evidence for two sympatric species was less clear in two community samples from the Atlantic coast of Honduras, Lancetilla Botanical Garden and a site near La Ceiba. The strength of the metanotal groove was variable, and sometimes the face had the O. balzani setal pattern on one side and the O. trithrix pattern on the other.

Octostruma balzani overlaps with O. amrishi in lowland rainforest habitats from the La Moskitia region of eastern Honduras south to Amazonia. In Central America, O. balzani is more likely to be in the warmer and/or more disturbed habitats, while O. amrishi favors cooler or mature forest habitats. For example, on Cerro Saslaya, an isolated mountain range in eastern Nicaragua, a community sample from lowland rainforest around 300 m elevation was a mix of O. balzani and O. amrishi , while a sample from montane forest around 1000 m elevation was pure O. amrishi . Octostruma amrishi is the dominant species in lowland wet forests of Panama and northern South America.

Octostruma gymnogon is a darker, montane version of O. amrishi . Allopatric populations occur in the mountains along the Guatemala-Honduras border and in Costa Rica. In Costa Rica, O. gymnogon has a sharp elevationally parapatric distribution with O. amrishi . On the Barva transect in Costa Rica, O. amrishi occurs from sea level to 300 m elevation, and O. gymnogon occurs from 500 m to 1100 m elevation.

The five similar species in Central America appear to segregate by climate and habitat, with O. trithrix favoring the warmest, driest, and most disturbed habitats; O. amrishi, O. gymnogon , and O. megabalzani favoring the coolest, wettest, least disturbed habitats (and segregating by elevation); and O. balzani falling in between.

The O. balzani complex is undersampled in many parts of its range. Given the complexity revealed in Central America and the paucity of characters separating sympatric forms, the true diversity of this group is undoubtedly greater than the simple arrangement proposed here, but molecular data will be needed to further reveal patterns.

The type locality of O. balzani is Bolivia. In the original description, specimens from two localities are described: "Bolivia; cantoni di Coroico e Chilumani-Yungas (Balzan); un esemplare di Salinas sul Beni è un poco più piccolo, ma non altrimenti differente." The Emery collection has a queen and several workers from Coroico and the single worker from Salinas. A worker from the Coroico series was imaged by the California Academy of Sciences and the images posted on Antweb; this worker was chosen as the Lectotype. The pin with the lectotype bears the label " Rhopalothrix balzani Em. " in Emery's handwriting but there is no locality label. However, it is clearly part of the Coroico series.

Octostruma balzani was named for Luigi Balzan, for whom Emery wrote this moving and poignant tribute:

After a long journey across Bolivia, made very uncomfortable for lack of funds, Luigi Balzan returned to Italy a few months ago, bringing important zoological and anthropological collections.

His sturdy physique, that had resisted the hardships and tropical climates, surrendered to a pernicious fever this past 20 September, in Padova, his homeland. For many years I was in correspondence with Balzan, who came to see me in Bologna before leaving; his unexpected death at a young age deeply saddened me.