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Taxonomic History (provided by Barry Bolton, 2023)

Overview:

Brachymyrmex obscurior is a small, dull brown, non-descript and soft-bodied ant that is associated with marginal and synanthropic habitats. Wilson and Taylor (1967) raised B. obscurior to species status, qualifying their decision as a "purely provisional measure, contingent upon a fuller revision of the large and difficult genus it which it belongs." It is native to the Neotropics and introduced to regions including the Pacific Islands, and the Netherlands.

// Distribution

Distribution:

  Geographic regions (According to curated Geolocale/Taxon lists):
    Americas: Argentina, Bahamas, Barbados, Brazil, British Virgin Islands, Chile, Colombia, Costa Rica, Cuba, Dominican Republic, French Guiana, Grenada, Guatemala, Guyana, Haiti, Jamaica, Martinique, Mexico, Panama, Puerto Rico, Saint Vincent and the Grenadines, United States
    Oceania: Guam, Hawaii, Marshall Islands, New Caledonia, Northern Mariana Islands, Samoa, Solomon Islands
  Biogeographic regions (According to curated Bioregion/Taxon lists):
    Australasia, Nearctic, Neotropical, Oceania
  Native to (according to species list records):
    Neotropical bioregion

Distribution Notes:

Native Range. Neotropics.
Introduced Range. USA: Hawaii (Maui, Oahu, Hawaii), Florida, Georgia, Louisiana, Missouri, Washington (King Co.). Samoa. Solomon Is. (Makira). Netherlands.

Biology:

The species is known to nest in the ground (Tschinkel & Hess, 1999), is especially well adapted to extreme, marginal environments (Morrison, 2006), and can be a common visitor to extrafloral nectaries (Koptur et al., 2010; Piovia-Scott, 2011). Brachymyrmex obscurior is considered a minor urban pest in Florida (Klotz et al., 1995), where it is rare but widespread (Deyrup, 2003a). Brachymyrmex obscurior is native to the Neotropics, and is ubiquitous across the Caribbean. The species is so common among those islands, that it is often the only single ant species to occur on the smallest cays (Morrison, 2006). However, the absence of B. obscurior from larger islands with more diverse ant faunas suggests that the species may be a poor competitor against most other ants (Morrison, 2006). The introduced range of B. obscurior includes Hawaii (Krushelnycky et al., 2005), Samoa (Wilson & Taylor, 1967), and the Solomon Islands (E.M. Sarnat, pers. obs.) in the Pacific, the southern United States (Dash et al., 2005; Deyrup, 2003a; Ipser et al., 2005), and the Netherlands (Boer & Vierbergen, 2008). Records from Antweb also indicate the species occurs in greenhouses and butterfly houses in the state of Washington.

In Costa Rica (Jack Longino)
This is a widespread species, common in other parts of the world in synanthropic habitats. In Costa Rica it is less common than the related B. heeri, a similarly synanthropic species. I know B. obscurior from two collections in Costa Rica, both coastal. One collection is from a beach site 25km North of Cahuita on the Atlantic coast, the other is from the town square of Sierpe, a small port town in a mangrove estuary on the Pacific coast.

Identification:

Diagnosis among workers of introduced and commonly intercepted species.  Antenna 9-segmented. Antennal club indistinct. Antennal scapes surpassing the posterior margin of head by more than 1/5th their length. Eyes of moderate size (greater than 5 facets). Eye length less than malar distance. Head with or without distinct ocelli. Antennal sockets and posterior clypeal margin separated by a distance less than the minimum width of antennal scape. Dorsum of mesosoma lacking a deep and broad concavity. Metanotal groove present. Pronotum and mesonotum with pairs of erect hairs. Propodeum and petiolar node both lacking a pair of short teeth. Propodeum lacking posteriorly projecting protrusion. Metapleuron with a distinct gland orifice. Waist 1-segmented (may be hidden by gaster). Petiolar node appearing flattened. Gaster armed with acidopore. Gaster (especially first segment) with dense pilosity, giving it a dull appearance. Color dull brown.

Brachymyrmex obscurior can be distinguished from most other introduced members of the genus by the following combination of characters: (1) dense pubescence on the first gastral tergite, (2) antennal scapes exceeding posterior margin of head by at least 1/5 their length, (3) erect hairs on the pronotum and mesonotum, (4) eye length distinctly less than malar length, and (5) dull brown in color. MacGown (2012) offers that the species, is most similar to B. patagonicus, but can be differentiated by the much denser pubescence, especially on the gaster. Males of B. obscurior are concolorous dark brown; whereas males of B. patagonicus are bicolored with the head and gaster dark brown and the rest of the body, including the appendages, being pale yellowish-brown. Additional characters cited by MacGown (2007) to separate the two include the smaller eye of B. obscurior which is shorter than the malar space, compared to the larger eye of B. patagonicus which is approximately equal to the malar length.  

Notes:

(Dutch vernacular name: Mexicaanse negenspriet)

References:

Dash, S.T., Hooper-Bui, L.M. & Seymour, M.A. (2005) The pest ants of Louisiana. A guide to their identification, biology, and control. Louisiana State University, Louisiana Agricultural Experiment Station, Louisiana Cooperative Extension Service, Pub. 2915

Deyrup, M. (2003) An updated list of Florida ants (Hymenoptera: Formicidae). Florida Entomol., 86, 43-48.
Ipser, R.M., Brinkman, M.A. & Gardner, W.A. (2005) First report of Brachymyrmex obscurior Forel (Hymenoptera: Formicidae) from Georgia. J. Entomol. Sci., 40, 250-251.

Klotz, J.H., Mangold, J.R., Vail, K.M., Davis, L.R., Jr. & Patterson, R.S. (1995) A survey of the urban pest ants (Hymenoptera: Formicidae) of peninsular Florida. Florida Entomol., 78, 109-118.

Koptur, S., Pascale, W. & Zuriany, O. (2010) Ants and plants with extrafloral nectaries in fire successional habitats on Andros (Bahamas). Florida Entomol., 93, 89-99.

Krushelnycky, P.D., Loope, L.L. & Reimer, N.J. (2005) The ecology, policy, and management of ants in Hawaii. Proc. Hawaii. Entomol. Soc., 37, 1-22.

MacGown, J.A. (2012) Ants (Formicdae) of the Southereastern United States
Morrison, L. (2006) The ants of small Bahamian cays. Bahamas Nat. J. Sci., 1(2), 27-32.

Piovia-Scott, J. (2011) The effect of disturbance on an ant-plant mutualism. Oecologia, ??, ??

Santschi, F. (1923) Revue des fourmis du genre Brachymyrmex Mayr. An. Mus. Nac. Hist. Nat. B. Aires, 31, 650-678; pl. 1-3.

Smith, M.R. (1951) Family Formicidae. In: Muesebeck, C.F., Krombein, K.V. & Townes, H.K. (Eds.) Hymenoptera of America north of Mexico. Synoptic catalogue. United States Department of Agriculture. Agriculture Monograph 2:1-1420778-875.

Tschinkel, W.R. & Hess, C.A. (1999) Arboreal ant community of a pine forest in northern Florida. Ann. Entomol. Soc. Am., 92, 63-70.

Specimen Habitat Summary

Found most commonly in these habitats: 3 times found in Sand Pine Scrub, 1 times found in beach edge, 1 times found in dist. habitat, 1 times found in pine/oak/thorn scrub, 1 times found in thorn scrub, 0 times found in in tropical hammock, 1 times found in tropical dry scrub, 1 times found in dry secondary, 1 times found in rainforest, 1 times found in Flatwoods, ...

Found most commonly in these microhabitats: 7 times ground forager, 3 times sifting leaf litter, 2 times foraging on tree, 1 times nest under rock, 1 times leaf litter, 1 times foragers under rock, 2 times foragers, 1 times ex sifted leaf litter, 1 times tree buttress litter, 1 times tending Hemiargus ceraunus on Mimosa-like plant, 1 times tending Hemiargus ceraunus on "Mimosa" sp., ...

Collected most commonly using these methods: 11 times search, 0 times honey trap, 3 times Winkler, 1 times on spam bait, 2 times hand collecting, 2 times DaJuns sifting, 1 times maxiWinkler, 1 times litter sample, 1 times Pitfall72h, 0 times bait trap, 1 times beating vegetation (1 hour), ...

Elevations: collected from 1 - 1780 meters, 169 meters average

Collect Date Range: collected between 1951-07-01 00:00:00.0 and 2022-05-21 00:00:00.0

Type specimens: Lectotype of Brachymyrmex obscurior: usnment00757132; Not Provided: casent0248882; syntype of Brachymyrmex obscurior: casent0903120

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