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Species: Octostruma balzani   (Emery, 1894) 

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Taxonomic History (provided by Barry Bolton, 2022)

Rhopalothrix balzani Emery, 1894d PDF: 217, pl. 1, fig. 10 (w.q.) BOLIVIA. Neotropic. Primary type information: Primary type material: lectotype worker (by designation of Brown & Kempf, 1960 PDF: 194). Primary type locality: lectotype Bolivia: canton Coroico, 1891 (L. Balzan) (by restriction of Brown & Kempf, 1960 PDF: 196). Primary type depository: MSNG. Secondary type information: Secondary type material: paralectotype workers (number not stated), 1 paralectotype queen. Secondary type localities: Bolivia: canton Chilumani-Yungas, 1891 (L. Balzan), and Bolivia: Salinas sul Beni (no collector’s name, probably also L. Balzan). Secondary type depository: MSNG. Type notes: Longino, 2013b PDF: 20, nominates what appears to be a second lectotype, also from Coroico. AntCat AntWiki HOL

Taxonomic history

Lectotype designation: Brown & Kempf, 1960 PDF: 194.
[Misspelled as balsani by Perrault, 1988c PDF: 307 (in key).]
Combination in Octostruma: Brown, 1949f PDF: 92
Status as species: Emery, 1924f PDF: 328; Brown, 1949h PDF: 92; Brown & Kempf, 1960 PDF: 194 (redescription); Kempf, 1961b PDF: 516; Kempf, 1972b PDF: 169; Perrault, 1988c PDF: 307 (in key); Brandão, 1991 PDF: 362; Bolton, 1995b: 293; Palacio, 1997: 417 (in key); Wild, 2007b PDF: 34; Branstetter & Sáenz, 2012 PDF: 258; Longino, 2013b PDF: 20 (redescription); Bezděčková et al., 2015 PDF: 118; Fernández & Serna, 2019 PDF: 867.


A cryptic inhabitant of forest floor leaf litter and rotten wood; widespread in lowland forests; often common; ranging from northern Mexico to southern Brazil. // Distribution


  Geographic regions (According to curated Geolocale/Taxon lists):
    Americas: Argentina, Belize, Bolivia, Brazil, Colombia, Costa Rica, Dominica, Ecuador, El Salvador, French Guiana, Guadeloupe, Guatemala, Guyana, Honduras, Jamaica, Mexico, Nicaragua, Panama, Paraguay, Peru, Suriname, Trinidad and Tobago, Venezuela
  Biogeographic regions (According to curated Bioregion/Taxon lists):
    Nearctic, Neotropical

Distribution Notes:

Northern Mexico (Tamaulipas, Nuevo Leon) to southern Brazil (Paraná). Paraguay: Alto Paraná, Caaguazú, Canindeyú, Central, Concepción, Guairá, Itapúa.


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 m2 litter plot samples, within-sample 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.


Mandible with 8 teeth, 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; face matte, densely punctate rugulose throughout; face typically with ten erect spatulate setae; mesonotum typically with a pair of erect spatulate setae located at the juncture of pro- and mesonotum.


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 were reevaluated in Longino (2013) 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; Nahá 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 Nahá. 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 is the Lectotype (Longino 2013). 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."

15 Oct 2017: Molecular data now show that "balzani" as defined by Longino 2013 is polyphyletic. In Central America, two morphologically-indistiguishable clades are parapatric, roughly north and south of the Nicaragua-Honduras border. The northern clade is related to Central American amrishi and gymnogon. The southern clade is related to trithrix and South American amrishi.

Taxonomic Treatment (provided by Plazi)

Scientific Name Status Publication Pages ModsID GoogleMaps
Pyramica amrishi   Bolton, B., Sosa-Calvo, J., Fernà ¡ ndez, F. & Lattke, J. E., 2008, New synonyms in neotropical Myrmicine ants (Hymenoptera: Formicidae)., Zootaxa 1732, pp. 61-64: -1, (download) -1 21399
Octostruma balzani   Wild, A. L., 2007, A catalogue of the ants of Paraguay (Hymenoptera: Formicidae)., Zootaxa 1622, pp. 1-55: 34, (download) 34 21367

Specimen Habitat Summary

Found most commonly in these habitats: 100 times found in mature wet forest, 70 times found in tropical rainforest, 76 times found in lowland rainforest, 58 times found in lowland wet forest, 47 times found in tropical moist forest, 44 times found in montane wet forest, 33 times found in rainforest, 36 times found in 2º lowland tropical rainforest, 38 times found in 2ndary liquidambar forest, 9 times found in forest, ...

Found most commonly in these microhabitats: 683 times ex sifted leaf litter, 7 times litter, 9 times ex sifted litter, 7 times leaf litter, 5 times forest litter, 3 times Ex sifted leaf litter., 1 times rocky loam soil, ex sifted litter, 4 times sifted litter (leaf mold, rotten wood), 1 times ex log litter, 1 times cacao pod litter, 1 times Hojarasca, ...

Collected most commonly using these methods: 557 times MiniWinkler, 75 times Winkler, 49 times Berlese, 52 times MaxiWinkler, 4 times search, dung baited times PF, 1 times bait, 1 times Winkler sample, 1 times baiting, 1 times Winkler48h, 1 times direct collection, ...

Elevations: collected from 5 - 1700 meters, 556 meters average

Collect Date Range: collected between 1911-06-21 00:00:00.0 and 2022-01-17 00:00:00.0

Type specimens: Syntype Octostruma equilatera: mcz-ent00022446, mcz-ent00510924; syntype of Octostruma balzani: casent0904969; syntype of Rhopalothrix balzani: casent0915939; syntype of Rhopalothrix lutzi: casent0900926

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