Brachymyrmex coactus

Specimens from UNA-ESMAI in Bahia (Brazil) were found in dwarf coconuts (: USNMENT00757557, 00757558), from Canindeyú (Paraguay) on shrubs (: USNMENT00757561), and from St. Catharina underneath bark in association with a beetle of the genus Claviger (Mayr 1887). Longino has observered long columns of workers on the ground in various locations in Costa Rica.

Identification
Ortiz-Sepulveda et al. (2019) - Brachymyrmex coactus is morphologically very similar to Brachymyrmex degener as both species have scapes that surpass the posterior margin of the head, faint sculpture on the mesosoma, a mesonotum that is inflated and bulges dorsally above the pronotum in lateral view, a wide metanotal groove, metathoracic spiracles that are slightly protruding dorsally, and a gaster with scarce pubescence. However, B. coactus has a brown yellowish head and mesosoma, but a darker gaster, whereas B. degener has a uniformly brownish body.

Body size is very variable even within localities (including among specimens mounted on the same pin).

Jack Longino - 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.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina, Bolivia, Brazil , Costa Rica , Guyana, Panama, Paraguay, Peru.

Biology
Jack Longino: 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 travelling 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 Rincón 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 José. John Noyes captured specimens in sweepnet samples from Estación 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?

Nomenclature

 * . Brachymyrmex coactus Mayr, 1887: 523 (w.q.m.) BRAZIL (Santa Catarina).
 * Status as species: Emery, 1888c: 364; Emery, 1890b: 56; Dalla Torre, 1893: 174; von Jhering, 1894: 377; Emery, 1894k: 61; Forel, 1895b: 106; Forel, 1899c: 123; Forel, 1912i: 62; Mann, 1916: 473; Luederwaldt, 1918: 48; Santschi, 1923b: 669; Santschi, 1923c: 272; Emery, 1925b: 41; Borgmeier, 1927c: 141; Kempf, 1972a: 38; Bolton, 1995b: 82; Branstetter & Sáenz, 2012: 255; Fernández & Ortiz-Sepúlveda, 2019: 728; Ortiz-Sepúlveda, et al. 2019: 480 (redescription).
 * Senior synonym of constrictus: Ortiz-Sepúlveda, et al. 2019: 481.
 * Senior synonym of nictitans: Ortiz-Sepúlveda, et al. 2019: 480.
 * Senior synonym of robusta: Ortiz-Sepúlveda, et al. 2019: 481.
 * constrictus. Brachymyrmex constrictus Santschi, 1923b: 671, figs. 5, 38, 61 (w.) BOLIVIA.
 * Status as species: Emery, 1925b: 41; Kempf, 1972a: 38; Bolton, 1995b: 82.
 * Junior synonym of coactus: Ortiz-Sepúlveda, et al. 2019: 480.
 * nictitans. Brachymyrmex coactus var. nictitans Emery, 1906c: 178 (w.) COSTA RICA.
 * Subspecies of coactus: Santschi, 1923b: 670; Emery, 1925b: 41; Kempf, 1972a: 38; Bolton, 1995b: 82.
 * Junior synonym of coactus: Ortiz-Sepúlveda, et al. 2019: 480.
 * robusta. Brachymyrmex coactus var. robusta Santschi, 1923c: 272 (w.) BRAZIL (Santa Catarina).
 * Subspecies of coactus: Emery, 1925b: 41; Borgmeier, 1927c: 141; Kempf, 1972a: 38; Bolton, 1995b: 82.
 * Junior synonym of coactus: Ortiz-Sepúlveda, et al. 2019: 481.

Worker
Ortiz-Sepulveda et al. (2019) - Lectotype and paralectotypes (n = 3). HL1 0.72–0.84; HL2 0.44–0.55; HL3 0.21–0.25; HW 0.64– 0.82; SL 0.68–0.80; EL 0.16–0.21; WL 0.53–0.88; PnL 0.21; PnW 0.43–0.55; ML 0.14–0.20; MW 0.23–0.35; Indices CI 94.29–97.67; SI1 97.62–106.06; SI2 144.00–152.17; OI1 24.24–28.57; OI2 29.73–31.43. Additional material (n = 10). HL1 0.52–0.88; HL2 0.34–0.60; HL3 0.16–0.25; HW 0.50–0.82; SL 0.51–0.82; EL 0.13–0.21; WL 0.53–0.98; PnL 0.18–0.25; PnW 0.36–0.57; ML 0.13–0.23; MW 0.18– 0.35; Indices CI 93.33–101.45; SI1 92.86–105.36; SI2 126.67–165.38; OI1 23.53–35.71; OI1 26.67–32.5.

Head. Slightly longer than wide in full face view; posterior cephalic margin flat or slightly concave. Dorsum of the head has scattered apressed hairs. Clypeus with a rounded anterior margin and five long, erect hairs of which a single, usually conspicuous hair is near the anterior margin, two hairs are in mediolateral position, and two more near the toruli; other hairs on the clypeus are markedly shorter and appressed or decumbent. Toruli surpassing the posterior clypeal margin in oblique anterodorsal view. The scapes extend beyond the posterior margin of the head by a length that is equal to the maximal diameter of the eye or larger, and they bear appressed and decumbent hairs. Three ocelli are present. Eyes are positioned on the cephalic midline and have 10–14 ommatidia along their maximal diameter.

Mesosoma. Typically with two erect hairs on the pronotum and two on the mesonotum; sometimes with additional suberect hairs, mainly on pronotum. Dorsum of the mesosoma with imbricate sculpture. The mesonotum is inflated and bulges dorsally above the pronotum in lateral view. Metanotal groove wider than the diameter of the metathoracic spiracles. Metathoracic spiracles in dorsolateral position, slightly protruding, and not touching any sutures. Dorsum of the propodeum strongly convex and shorter than the propodeal slope. Propodeal spiracles conspicuous and circular, positioned on the propodeal margin, anterior of the middle of the propodeal slope. Legs with appressed hairs. Petiole short and inclined forward.

Gaster. With scattered pubescence and several scattered long erect hairs.

Color and sculpture. Head and gaster are smooth and shiny, but the dorsum of the mesosoma usually has imbricate sculpture. Head and mesosoma are brown yellowish, whereas the gaster is darker.

Type Material
Ortiz-Sepulveda et al. (2019) - The worker on pin USNMENT00757191 is designated here as lectotype (. Paralectotype workers, males, queens (NHMW: USNMENT00757191–00757195; here designated): three workers, three males, two queens [examined]. BRAZIL: Santa Catharina (Hetscko).

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