Brachymyrmex

Ortiz and Fernández (2014) - Brachymyrmex is a genus of minute ants that at first glance exhibit little morphological variation. Currently only the 9-segmented antennae and lack of antennal club have been proposed to diagnose workers of the genus (Bolton 2003). The combination of small size, soft metasoma, and the simple morphology makes observations and interpretation of morphological characters difficult. These difficulties impede taxonomic revisions and even led Creighton (1950) to call Brachymyrmex a “miserable little genus”. Nevertheless, described species and subspecies are currently assigned to Brachymyrmex (Bolton, 2013). The genus has a mainly Neotropical distribution, ranging from the United States to Argentina and Chile, including the Caribbean islands (Kempf 1972, Brandão 1991, Bolton 1995, 2003), but some species have been introduced to Japan (M. Yoshimura, pers. comm.), and Madagascar (Dejean et al. 2010). More recently, Ortiz-Sepulveda et al. (2019) published a revision that: (1) redefined the limits of all of the described species, subspecies, and varieties in light of intra and interspecific morphological variation in workers; (2) documented this variation both qualitatively and quantiatively; (3) summarized these findings on diagnostic traits with a new, dichotomous, illustrated identification key to increase reproducibility and to make the diversity of Brachymyrmex more accessible for future research; and (4) examined the significance of our morphological identification system and the monophyly of the genus in light of molecular evidence. Finally, we also report on the biogeographical distribution of the recognized species and how our taxonomic framework compares with previous studies. In summary, we recognize a total of 40 species, 4 of which are newly described here. We also synonymize 25 previously described species/subspecies and raise two former subspecies to species status. This important revision was also stated to be work toward a comprehensive revision of Brachymymex. Remaining taxonomic work in the genus includes describing a number of putative new species. These remain undescribed due to there not being sufficient material and more collections are needed. There is also a need to determine if there are true intercastes present in a number of named species. This requires ovarian dissections and the collection of demographic data (Peeters 1991 ti insure these intercastes are not ergatoid queens. This problem also would be aided by additional collections of the Brachymyrmex species with intercastes.

Identification
Minute ants (maximum length ~ 3 mm) with an acidopore and 9-segmented antennae that lacks an antennal club.

The combination of small body size, soft metasoma, and at least superficially monotonous external morphology complicate the observation and interpretation of morphological variation.

LaPolla and Longino (2006) - The genus Brachymyrmex is most likely to be confused with Myrmelachista. The most obvious distinction between these two genera is the presence of a 3 to 4-segmented antennal club in Myrmelachista. Although Brachymyrrnex usually possess incrassate antennae, they never form a distinct antennal club. Both genera include species with 9-segmented antennae, although Myrmelachista also includes some species with 10-segmented antennae (species in the synonymized subgenus Hincksidris [Snelling and Hunt 1975]). No Brachymyrrnex have been recorded with 10-segmented antennae. Two other morphological characteristics separate the two genera. The clypeus of Myrmelachista is typically subguadrate and compact, whereas Brachymynnex possess a broadly rounded (along the anterior margin) and wide clypeus. The shape of the mandible also differs between the two genera. In Myrmelachista, the masticatory and basal margins form a right angle. Whereas in Brachymynnex the masticatory and basal margins form an obtuse angle.

Biology
LaPolla and Longino (2006) - In Neotropical forests, the common species of Brachymyrmex nest in a variety of small plant cavities, under epiphytes, or in the leaf litter. They seem quite generalized in choice of nest site, and the nests can be in relatively fragile or ephemeral substrates, suggesting frequent nest movement. Brachymyrmex species seem to feed mainly at carbohydrate sources, being common at extrafloral nectaries and at sugar water baits. Some species are known to tend Coccoidea (Hemiptera) in underground chambers (Wheeler 1910; Santschi 1923). Very little is known about the natural history for the vast majority of Brachymyrmex species.

Castes
Most species are monomorphic. The small number of dimorphic species (Brachymyrmex bicolor, Brachymyrmex giardi, Brachymyrmex heeri, Brachymyrmex micromegas, Brachymyrmex pilipes and Brachymyrmex santschii) have a caste intermediate between a typical Brachymyrmex worker and queen. New collections are needed for further study of their morphology and reproductive status.

Intercaste
Ortiz-Sepulveda et al. (2019) - In the original description of Brachymyrmex giardi, Emery (1895: 215) described a worker, a replete, and a queen, and the replete is what we consider here as a putative worker-queen intercaste, because a regular queen was also reported by Emery (1895). Note that this queen was indicated to be wingless; however, after studying the material, we confirm that it represents a real queen rather than an ergatoid, and the replete has, as mentioned above, a hybrid morphology between queen and worker. Upon dissection of the abdomen of the replete, Emery (1895: 215) reported that the crop is full of honey-like liquid, but also that the ovaries are more developed than in normal workers, and that these repletes likely have a reproductive function. Nevertheless, he considered nourishment their primary function, as is confirmed by De Zolessi et al. (1978). In summary, the exact affinity of these repletes is uncertain: if it were ergatoid queens we would not expect a regular queen to be present (Peeters 1991), which points to an intercaste, because intercastes co-exist with a regular queen. However, intercastes do not usually participate in reproduction (Peeters 1991). Given all the available data, we consider these specimens for now to be a putative workerqueen intercaste, as mentioned before, but the intriguing issue of the repletes in B. giardi requires further study.

Nomenclature

 *  BRACHYMYRMEX [Formicinae: Plagiolepidini]
 * Brachymyrmex Mayr, 1868b: 163. Type-species: Brachymyrmex patagonicus, by monotypy.
 * Brachymyrmex senior synonym of Bryscha: Smith, D.R. 1979: 1424.
 * BRACHYPONERA [junior synonym of Pachycondyla]
 * Brachyponera Emery, 1900c: 315 [as subgenus of Euponera]. Type-species: Euponera (Brachyponera) croceicornis, by monotypy.
 * [Brachyponera also described as new by Emery, 1901a: 43. Type-species not Ponera sennaarensis, unjustified subsequent designation by Emery, 1901a: 43, repeated in Wheeler, W.M. 1911f: 160, Emery, 1911d: 84, Wheeler, W.M. 1922a: 777, Donisthorpe, 1943g: 628, Wilson, 1958d: 346; Bolton, 1973a: 335 and Taylor & Brown, D.R. 1985: 23.]
 * Brachyponera as subgenus of Euponera: Emery, 1911d: 83; Wheeler, W.M. 1922a: 649.
 * Brachyponera raised to genus: Bingham, 1903: 101; Wilson, 1958d: 346.
 * Brachyponera junior synonym of Pachycondyla: Brown, in Bolton, 1994: 164.
 * BRYSCHA [junior synonym of Brachymyrmex]
 * Bryscha Santschi, 1923b: 652 [as subgenus of Brachymyrmex]. Type-species: Brachymyrmex pilipes, by original designation.
 * Bryscha junior synonym of Brachymyrmex: Smith, D.R. 1979: 1424.
 * Bryscha junior synonym of Brachymyrmex: Smith, D.R. 1979: 1424.