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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.
|Pyramica amrishi||Bolton, B., Sosa-Calvo, J., FernÃ ¡ 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|
Found most commonly in these habitats: 98 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, 36 times found in 2º lowland tropical rainforest, 31 times found in rainforest, 38 times found in 2ndary liquidambar forest, 9 times found in forest, ...
Found most commonly in these microhabitats: 674 times ex sifted leaf litter, 6 times litter, 7 times leaf litter, 5 times forest litter, 1 times rocky loam soil, ex sifted litter, 3 times ex sifted litter, 3 times Wet forest. Ex sifted leaf litter., 4 times sifted litter (leaf mold, rotten wood), 1 times Hojarasca, 1 times ex log litter, 1 times cacao pod litter, ...
Collected most commonly using these methods: 557 times MiniWinkler, 48 times Berlese, 58 times Winkler, 52 times MaxiWinkler, 5 times search, 1 times litter, 1 times Malaise, 1 times Maxi Winkler, dung baited times PF, 1 times bait, 1 times baiting, ...
Elevations: collected from 5 - 1700 meters, 559 meters average
Type specimens: Syntype Octostruma equilatera: mcz-ent00022446, mcz-ent00510924; syntype of Octostruma balzani: casent0904969; syntype of Rhopalothrix balzani: casent0915939; syntype of Rhopalothrix lutzi: casent0900926