Introduction
Brachymyrmex is a New World genus of minute, soft-bodied ants. Workers have a small petiolar node, a nine-segmented (9-merous) antenna without a club, well-developed eyes, and an acidopore. Color ranges from pale light-yellow to dark brown. Species is this genus usually nest in soil or rotting wood. This group is in need of revision.

Brachymyrmex species can be identified by their one-segmented petiole, the presence of an acidopore, and their nine-segmented antennae.

Brachymyrmex obscurior is a minute yellowish-brown to brown species native to the Neotropics. In the US, it is considered to be an introduced species. In southern Florida, this species is an occasional nuisance pest (Pers. Comm. Mark Deyrup).

Taxonomic History (provided by Barry Bolton, 2013)
Described as Brachymyrmex heeri var. obscurior by Forel in 1893; raised to subspecies of Brachymyrmex heeri by Forel (1897); raised to species by Wilson and Taylor (1967).

Diagnosis
Workers of B. obscurior are most similar to B. patagonicus Mayr, another exotic species. It differs in the size of the eye, which is much smaller in B. obscurior than in B. patagonicus, and in the much denser pubescence, especially on the gaster (much sparser in B. patagonicus). The males of these two species differ in color. with males of B. obscurior being concolorous dark brown and males of B. patagonicus being bicolored with the head and gaster dark brown and the rest of the body, including the appendages, being pale yellowish-brown. Brachymyrmex sp.-01, recorded from Florida, is also dark brown in color, but lacks erect hairs on the body. An undescribed brown species known only from queens collected from Arkansas (Lloyd Davis, pers. comm.) is unusual in that the queens are tiny, about the size of typical workers. All other known species present in this region are yellowish in color. 

Identification
Worker: Minute (TL 1.0–1.50 mm, HW 0.41–0.46 mm, HL 0.43–0.52 mm, EL 0.09–0.11 mm, SL 0.35–0.40 mm, MeSL 0.40–0.47 mm) (n=5, MEM specimens). Yellowish-brown to brown. Head slightly longer than wide, with abundant, fine pubescence; eye length 1/4 to 1/5 of the head length; ocelli present; antennae 9-segmented, lacking a club; mandibles with five teeth. Mesosoma compact; metanotal suture impressed; propodeum lacking spines; dorsum of promesonotum with four or more stiff, semi-erect setae, dorsum of promesonotum and propodeum with scattered pubescence; sides of mesosoma shiny and mostly lacking setae. Waist with a single petiolar node, node somewhat flattened and often hidden by gaster from above. Gaster with dense appressed pubescence and scattered semi-erect setae; acidopore present.

Queen: (Description based on MEM photos). Approximately three times longer than worker (TL 4.0–4.3 mm). Concolorous reddish brown. Head about as wide as long, with dense pubescence; eyes large, length more than 1/4 head length; ocelli present; antenna 9-segmented, lacking club. Mesosoma enlarged, elongated, roundly flattened dorsally; mesosoma with dense, fine pubescence, with scattered, longer, erect setae dorsally. Wings large, clear, with strong venation; fore wing with one submarginal cell, pterostigma present; hind wing lacking closed cells. Waist with a single petiolar node, often hidden by gaster from above. Gaster enlarged, with fine, dense pubescence, longer, stiff erect setae present especially along tergite margins; acidophore present.

Male: (Description based on MEM photos). Approximately the same size as workers to slightly larger (TL about 1.5 mm). Color, dark reddish brown to blackish brown. Head slightly wider than long; eyes large, at least 1/2 the length of the head; three large, raised ocelli present; antenna 10-segmented, first funicular segment enlarged, globular; mandibles simple. Mesosoma enlarged, squared (in profile), mesoscutum enlarged, overhanging pronotum; mesosomal dorsum with dense, fine pubescence, larger erect setae absent; pronotum, mesopleura, and propodeum shiny, with reduced setation. Wings large, clear; forewing with one submarginal cell; hindwings with reduced venation. Waist with a single petiolar node, often hidden by gaster from above. Gaster shiny, with scattered fine pubescence and a few larger erect setae along tergite margins.

Biology and Economic Importance
Tschinkel and Hess (1999) reported that Brachymyrmex obscurior nests in the ground. At least in its introduced range, it appears to be well adapted to extreme, marginal habitats (Morrison 2006). Similar to many members of the Formicinae, this species appears to be attracted to sweet liquids and has been reported to visit extrafloral nectaries (Koptur et al. 2010). Some specimens in the MEM collection have been collected at honey traps, which supports this observation. In Florida, this species has been reported to be widespread, but rare (Deyrup 2003). Only isolated records are known from Mississippi (MEM records), where this species may or may not be established. In recent years, B. patagonicus has become widespread and abundant in the Southeast and could potentially out compete this species in regions where they co-occur, such as southern Florida and Mississippi. In the Caribbean, Morrison (2006) reported that this species is ubiquitous on some of the smaller islands, but observed an absence of B. obscurior from larger Caribbean islands that had more diverse ant faunas. Based on this observation Morrison (2006) suggested that this species might be a poor competitor against most other ants.

Despite its supposed rarity in Florida, B. obscurior is listed by Klotz et al. (1995) as being a minor urban pest.

Pest Status

Distribution
Native range: Neotropics (antweb.org)

Australian: Guam, Hawaii, Marshall Islands, Northern Mariana Islands, Samoa, Solomon Islands, Vanuatu (antweb.org and antwiki.org).
Nearctic: Canada, United States (antweb.org and antwiki.org).
Neotropical: Bahamas, Barbados, Bermuda, British Virgin Islands, Colombia, Costa Rica, Cuba, Dominican Republic, Greater Antilles, Grenada, Guatemala, Haiti, Jamaica, Lesser Antilles, Mexico, Panama, Puerto Rico, Saint Vincent and the Grenadines, Trinidad and Tobago, Venezuela (antweb.org and antwiki.org).
Palearctic: Netherlands (antweb.org).

U.S. Distribution: AZ, FL, HI, (LA), MS, MO, TX, WA (antweb.org and MEM).
Southeastern U.S. Distribution: FL, (LA), MS (antweb.org and MEM).

Introduced range in US: Confirmed MEM records include specimens from FL, HI, and MS. Antweb also lists GA, LA, MO, and WA. The Louisiana records on AntWeb appear to be based on Dash et al. (2005), who reported this species from that state. However, these records may be in error as identifications were based on taxonomic information for this group that was available at that time, which has been updated since. Joe MacGown examined voucher specimens identified as B. obscurior in the Louisiana State Arthropod Museum and determined them all to be B. patagonicus. Therefore, at this time, records of B. obscurior from Louisiana should be considered dubious. According to Antweb, specimens from Washington were collected from inside greenhouses and butterfly houses, and therefore, this species is not likely established in that region. MacGown has not confirmed records from GA or MO.

Acknowledgments
Thanks to Ryan J. Whitehouse for help with measuring specimens, comments on descriptions, photography of some specimens, and proofreading. Funding for the ant work being done by the MEM in Alabama and Mississippi is from several sources including the National Institute of Food and Agriculture, United States Department of Agriculture, under Project No. MIS-012040, the Mississippi Agricultural and Forestry Experiment Station at Mississippi State University, with support from State Project MIS-311080, NSF Grants BSR-9024810 and DFB-9200856, the Tombigbee National Forest (U.S. Forest Service), the Noxubee Wildlife Refuge, Mississippi Natural Heritage Program Research Grant, USDA Forest Service Agreement No. 08-99-07-CCS-010, the William H. Cross Expedition Fund, and primarily by the USDA-ARS Areawide Management of Imported Fire Ant Project (2001-2014) and USDA-ARS Areawide Management Invasive Ants Project. Additionally, special cooperation has been provided by State Parks, National Forests, National Wildlife Refuges, the Natchez Trace Parkway, and from various private landowners in both Alabama and Mississippi..

Literature Cited

Bolton, B. 2013.  Bolton World Catalog Ants. Available online: http://www.antweb.org/world.jsp. Accessed 25 November 2013.

Dash, S.T., L. M. Hooper-Bui, and M. A. Seymour. 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 Entomologist 86: 43-48.

Deyrup, M., S. Cover, and L. Davis. 2000. Exotic ants in Florida. Transactions of the American Entomological Society 126 293-325.

Forel, A. 1893. Formicides de l'Antille St. Vincent, récoltées par Mons. H. H. Smith. Transactions of the Entomological Society of London 1893: 333-418.

Forel, A. 1897. Quelques Formicides de l'Antille de Grenada récoltés par M. H. H. Smith. Transactions of the Entomological Society of London 1897: 297-300.

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

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

Morrison, L. W. 2006.  The ants of small Bahamian cays.  Bahamas Naturalist and Journal of Science. 1:2 7-32.

Tschinkel, W. R. and C. A. Hess. 1999. Arboreal ant community of a pine forest in northern Florida. Annal of the. Entomological Society of America 92: 63-70.

Wilson, E. O. and R. W. Taylor. 1967. The ants of Polynesia (Hymenoptera: Formicidae). Pacific Insects Monograph 14: 1-109.

Links

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Insects of Hawaii: Brachymyrmex obscurior