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Throughout the mainland Neotropics, from Panama to Argentina. Questionable records from Costa Rica, Belize.
At the time of Kempf's (1951) revision the genus Cephalotes was restricted to the species with the antennal scrobe extending above the eye and back to the vertex margin. In Andrade and Baroni Urbani the former Cephalotes is a monophyletic group with 4 relatively similar species. This group contains the largest species in the genus, and the large, spiny workers are a conspicuous element in lowland Neotropical rainforests. Kempf (1951:109) summarized the biology of C. atratus, and surmised that the biology of alfaroi and other related species would be similar. Kempf's summary is as follows:
C. atratus is usually found on large trees, running up and down within the crevices of the bark. I have once caught a few individuals on the bulbs of an orchid of the genus Cattleya, in Petropolis, near Rio de Janeiro. The species is xyloecete, establishing the nest within the cavities of dead or living trees. Forel (1899, 1912) relates how he himself discovered an immense nest of these ants within a large dead tree on the island of Trinidad. Through repeated knocking on the trunk, he was able to arouse them. As a consequence, the ants left the nest in great numbers and destroyed in a few moments the nest of a small polybiine wasp, not sparing its inhabitants. Prof. Bugnion and Santschi (Forel, 1912; Santschi, 1929) found a nest of the same species in a cavity of a living tree, of 40 cm. in diameter, not much above the ground, which was being attacked by a considerable number of Eciton (Nomamyrmex) crassicorne, a species of army ants. The majority of the Cephalotes atratus were inside the nest, some of them engaged in closing the entrance by placing their heads side on side, whereas a few others on the outside, were struggling with the invaders. Most probably the army ants were longing for the eggs and larvae of Cephalotes, since the heavy armor of the adults were invulnerable to their attacks.
According to Mann (1916) "the species nests generally in hollowed branches of high trees, though one nest was in the hollowed trunk of a small tree. It is omnivorous in habit, frequenting garbage and eating even carrion. Some dead macaws which I placed in the woods as bait for carrion-feeding insects were continually covered by C. atratus, to the exclusion of other insects. It is diurnal, and a striking form as it walks slowly about on tree trunks and logs. The hard spiny armor is sufficient to protect it from any ordinary enemy."
Eidmann (1936) gives a detailed description of the nest of a young colony of C. atratus, found near Mendes, State of Rio de Janeiro, Brazil. Wheeler (1942) reports that he discovered the same species nesting in a large branch of a Cecropia sciadophylla var. decurrens Sn.
L. Richter (1945), in a paper on Colombian Membracidae, presents an interesting account of the relationship between this ant and Tragopa peruviana Funkhouser, a neotropical membracid. According to this writer's observations, C. atratus is always found in company with Tragopa peruviana, which lives on the large leaves of Isertia haenkeana and, less frequently, on Vismia angusta. It is suggested that the ants take advantage of the sugary secretions of Tragopa. The interesting fact, however, consists both in the constant association of the two insects, and also in a certain "convergent adaptation" of the membracid which in its general habitus resembles the color and the shape of the gaster of C. atratus.
Regarding the occurrence of C. atratus north of Panama:
Cephalotes atratus is common throughout Amazonia north to Panama, and appears to be allopatric or parapatric with alfaroi. There are numerous collections of atratus from Barro Colorado Island, and no records of alfaroi. In contrast, C. alfaroi is the common species in Costa Rica, extending to Bugaba, Panama. However, Andrade and Baroni Urbani (1999) report one collection of C. atratus from Costa Rica, and Kempf (1951) reports a collection from Belize. The Belize specimen is an outlier, with no other known Cephalotes collections between Belize and Costa Rica. The Costa Rican collection is from Guapiles, collected by D. E. Harrower in 1915 (specimen or specimens at the MCZC). Various hypotheses may explain the Costa Rican collection. 1. It may be a case of misidentification or mislabeling. 2. alfaroi character distributions may overlap atratus (even if barely), and the Harrower collection is alfaroi but looks like atratus. 3. The two species are sympatric in Costa Rica, maintaining genotypic clusters sensu Mallet (1995), and C. atratus is very rare. Further collections may resolve this.
Andrade and Baroni Urbani (1999) consider the presence of a transverse pronotal carina diagnostic for major workers of Cephalotes. Cephalotes atratus exhibits large variation in worker size, but even the largest workers have an evenly curved promesonotal profile, with no development of a transverse crest. Thus Kempf (1951) and later Andrade and Baroni Urbani (1999) consider C. atratus to be "monomorphic," in the sense of lacking a distinct soldier caste. In contrast, major workers ofC. alfaroi have a transverse crest (in the form of an angularly produced pronotum with a pair of stout median teeth in addition to the lateral pair).
Andrade, M. L. de, and C. Baroni Urbani. 1999. Diversity and adaptation in the ant genus Cephalotes, past and present (Hymenoptera, Formicidae). Stuttgarter Beitrage zur Naturkunde Serie B (Geologie und Palaontologie) 271:1-889.
Eidmann, H. 1936. Okologisch-faunistische Studien an sudbrasilianischen Ameisen. Arb. Physiol. Angew. Entomol. Berl.-Dahl. 3:26-48, 81-114.
Forel, A. 1899. Biologia Centrali-Americana 3 (Formicidae). 169pp, London.
Forel, A. 1912. Formicides neotropiques. Part I. Ann. Soc. Entomol. Belg. 56:28-49.
Kempf, W. W. 1951. A taxonomic study on the ant tribe Cephalotini (Hymenoptera: Formicidae). Rev. Entomol. (Rio J.) 22:1-244.
Linnaeus, C. 1758. Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata. Holmiae [= Stockholm]: L. Salvii, 824 pp.
Mallet, J. 1995. A species definition for the modern synthesis. TREE 10:294-299.
Mann, W. M. 1916. The Stanford Expedition to Brazil, 1911, John C. Branner, Director. The ants of Brazil. Bull. Mus. Comp. Zool. 60:399-490.
Richter, L. 1945. Membracidae Columbianae. Rev. Acad. Colomb. Ci. Ex. Fis. Nat. Bogota 6:339-354.
Santschi, F. 1929. Nouvelles fourmis de la Republique Argentine et du Bresil. An. Soc. Cient. Argent. 107:273-316.
Wheeler, W. M. 1942. Studies of Neotropical ant-plants and their ants. Bull. Mus. Comp. Zool. 90:1-262.
Alto Paraguay, Alto Paraná, Boquerón, Caaguazú, Canindeyú, Central, Concepción, Cordillera, Itapúa, Ñeembucú, Paraguarí, Pte. Hayes, San Pedro, “Paraguay” (s. loc.) (ALWC, LACM, MCSN, MHNG, MZSP, NHMB, NHMW). Literature records: Caaguazú, Central, Cordillera, “Paraguay”(s. loc.) (de Andrade & Baroni-Urbani 1999, Emery 1896b, Forel 1906, Forel 1907d, Santschi 1920b).
Found most commonly in these habitats: 10 times found in Semideciduous seasonal forest, 6 times found in rainforest, 2 times found in secondgrowth rainforest, 1 times found in riparian scrub in savanna, 1 times found in 2nd growth rainforest, 1 times found in tropical moist forest, 2 times found in Research campus grounds, 1 times found in River edge second growth and highly degraded forest, 1 times found in second-growth riparian forest, 1 times found in shaded lawn, ...
Found most commonly in these microhabitats: 4 times foraging on low vegetation, 2 times general collecting, 2 times nest in tree branch, 2 times foraging on tree, 1 times Column on tree trunk, 1 times on tree trunk, 1 times ground forager, 1 times forest trail, 1 times foraging on tree in canopy (30 m), 1 times foraging on top of tree ~35m up, 1 times foraging on fruit, ...
Collected most commonly using these methods: 8 times Arboreal pitfall, 5 times search, 7 times direct collection, 3 times FIT, 2 times at light, 2 times Hand, 1 times malaise, 1 times malaise trap, 1 times Manual sampling, 1 times pan trap, 1 times sweeping, ...
Elevations: collected from 4 - 1760 meters, 360 meters average
Type specimens: syntype of Cephalotes atratus quadridens erecta: casent0912579, casent0912580; syntype of Cephalotes atratus nitidiventris: casent0912578; syntype of Cephalotes atratus quadridens dehnowi: casent0909259; syntype of Cephalotes atratus crassispina: casent0912576, casent0912577