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Argentina, Brazil (type locality), Costa Rica, Guyana. Costa Rica: widespread but low density, from sea level to 1200m. I have records from Guanacaste (Isla San Jose, Estacion Cacao); Puntarenas (Corcovado, Wilson Botanical Garden, 5km SW Las Alturas).
This is a mysterious species. It appears to be widespread, occurring in many areas and habitats, but it is rarely collected. The few records suggest it might be more common in mid-elevation sites than in the lowlands. Although many Brachymyrmex might be rare in collections due to their small size and general inconspicuousness, this is not the case for coactus. Brachymyrmex coactus is one of the largest and most conspicuous species in the genus, with a proclivity for foraging or at least moving in dense columns.
At Sirena, in Corcovado National Park, I saw what looked like a colony migration in progress. A slow-moving, double-file column was moving across a trail. The column was more than 20m long, and I could find no clear beginning or end. Workers were carrying mostly pupae, and a few males were traveling with the column. This was the only time I saw coactus at the site, in spite of working there over a two-year period. I found another column, also crossing a trail on the ground, during a trip to the small Cerro Rinc—n cloud forest, at 700m elevation in the center of the Osa Peninsula.
At Wilson Botanical Garden, a 1200m elevation site in the southern mountains, I observed a dense column on a recent treefall, and I found a nest space with some brood in the base of a bromeliad. Similarly, at a site near Las Alturas in the Cordillera de Talamanca, I found a column on the trunk of a recently felled tree in a small pasture area.
Jenny Jacobs, a student carrying out a project on the leaf litter fauna of the small islands off the northwest coast of Costa Rica, found specimens on Isla San JosŽ. John Noyes captured specimens in sweepnet samples from Estaci—n Cacao, at 1100m in the Cordillera de Guanacaste.
Brachymyrmex coactus obviously has some interesting biology. I have never seen isolated foraging workers, only these occasional dense columns. Is the species nomadic? What do they eat?
Found most commonly in these habitats: 1 times found in tropical wet forest, 1 times found in pasture at forest edge, 1 times found in pasture with recently felled trees, 1 times found in savanna, 1 times found in wet forest, 1 times found in 2° mesophil forest, 1 times found in cloud forest, 1 times found in mature scrub, 1 times found in montane wet forest, 1 times found in primary lowland rainforest.
Found most commonly in these microhabitats: 1 times nest under stone, 1 times general collecting, 1 times ex sifted leaf litter, 1 times Column on trunk of felled tree., 2 times column, 2 times at bait, 1 times under epiphytes, 1 times ex sifted litter.
Collected most commonly using these methods: 2 times search, 2 times baiting, 1 times miniWinkler, 1 times Winkler.
Elevations: collected from 10 - 1915 meters, 655 meters average
Collect Date Range: collected between 1981-01-16 and 2019-06-22
Type specimens: Lectotype of Brachymyrmex coactus: casent0915731, casent0915733; Lectotype of Brachymyrmex coactus robustus: usnment00757224; paratype of Brachymyrmex constrictus: casent0911598; syntype of Brachymyrmex constrictus: casent0905171; type of Brachymyrmex coactus: focol2901, focol2902