Subfamily FORMICINAE
Tribe PLAGIOLEPIDINI

Brachymyrmex obscurior Forel 1893

by Joe MacGown, uploaded on 19 July 2009, last updated on 26 November 2013.

Brachymyrmex obscurior, frontal view of the head of a worker (Photo by Joe A. MacGown)
Brachymyrmex obscurior, side view of a worker (Photo by Joe A. MacGown)
Brachymyrmex obscurior, frontal view of the head of a worker (Photo by Joe A. MacGown)
Brachymyrmex obscurior, side view of a worker (Photo by Joe A. MacGown)
Brachymyrmex obscurior, full face view of a worker
Photo courtesy of http://www.antweb.org/.
Brachymyrmex obscurior, side view of a worker. Note the much smaller eye and the much denser pubescence than in B. patagonicus
Photo courtesy of http://www.antweb.org/.
Brachymyrmex obscurior, side view of a male
Photo courtesy of http://www.antweb.org/.
Brachymyrmex obscurior, side view of a female
Photo courtesy of http://www.antweb.org/.
Brachymyrmex obscurior, side view of a male . Photo courtesy of Lyle Buss.
Brachymyrmex obscurior, side view of a male Photo courtesy of Lyle Buss.

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 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).

Identification
HW, 0.41–0.46; HL, 0.43–0.52; CI,83.36-93.88; EL, 0.09–0.11; OMS, 0.12–0.13; SL, 0.35–0.40; WL, 0.40–0.47
Brachymyrmex obscurior is a small yellowish-brown to darker brown species with similarly colored appendages. The head is longer than wide. Eyes are relatively small; much shorter than the length of the malar space. Antenna 9-merous, lacking a club; scape surpasses posterior margin of head by more than 1/5th of its length. Mesonotal dorsum lacking deep, broad concavity (present in some species). The promesonotum usually has four or more erect hairs. Petiolar node is somewhat flattened and often hidden by the overhanging gaster. The gaster has dense, appressed pubescence.

Brachymyrmex obscurior is 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.

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). In Florida, this species has been reported to be widespread, but rare (Deyrup 2003). Despite its supposed rarity in that region, it is listed by Klotz et al. (1995) as being a minor urban pest. 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. Morrison (2006) observed an absence of B. obscurior from larger islands in the Caribbean that had more diverse ant faunas than the smaller islands, which suggested that this species might be a poor competitor against most other ants.

Distribution
Native range: Brachymyrmex obscurior is thought to be an exotic species native to Central America and South America.

Introduced range in US: Confirmed MEM records include specimens from FL, HA, 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. I (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. I have not confirmed records from GA or MO.

Introduced range in other countries: Samoa, Solomon Islands, and Netherlands (from AntWeb). Morrison (2006) reported this species to be ubiquitous on some of the smaller islands in the Caribbean region.

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

AntWeb Images
Discover Life Images
Insects of Hawaii: Brachymyrmex obscurior