Formicinae, Formicidae, Hymenoptera, Insecta, Arthropoda, Animalia
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).
Face smooth, with abundant erect setae; scapes surpass vertex margin by length of first funicular segment; pronotum, mesonotum, and propodeum with dorsal setae; first gastral tergite with abundant erect setae, no appressed pubescence; metanotal groove weakly impressed; color dark brown; large size.
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 Josˇ. 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?
John T. Longino, The Evergreen State College, Olympia WA 98505 USA.firstname.lastname@example.org
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