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.

Ortiz-Sepulveda et al. (2019) - Other formicine genera by having workers with nine antennal segments. Some species of Myrmelachista also have nine antennal segments, but these have a well-defined antennal club, whereas such a club is absent in Brachymyrmex. Some Agraulomyrmex species from Africa also have nine antennal segments without club (unpublished results), but Brachymyrmex differs from these by the presence of a mesometanotal suture. Workers are monomorphic to dimorphic; some species have a putative worker-queen intercaste.

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 Brachymyrmex 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
Ortiz-Sepulveda et al. (2019) - The biology and natural history of the genus are poorly known although habitat information exists for some species, such as the arboreal Brachymyrmex nebulosus. Some Brachymyrmex species are notorious invaders which are considered pests (MacGown et al. 2007). Brachymyrmex sometimes occur in association with other insects. Santschi (1923a) mentioned associations of Brachymyrmex depilis, Brachymyrmex giardi, and Brachymyrmex heeri with mealybugs (Hemiptera: Coccidae) and observed that some species live in or very close to termite nests (Brachymyrmex fiebrigi, Brachymyrmex modestus, Brachymyrmex myops, Brachymyrmex termitophilus).

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.

Worker Morphology
These characters are collated in the Worker Morphology table.

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.

Worker
Ortiz-Sepulveda et al. (2019) - Head. Usually longer than wide, cordate in some species, with sparse to dense pubescence and hairs in variable orientation (appressed, decumbent, erect). Mandibles with five teeth, of which intercalar (central) and basal teeth are smaller than the others and the apical tooth is largest. Maxillary palps and labial palps with six and four segments, respectively. Maxillary palps usually reach the occiput and bear several long ventral hairs. The clypeus has a rounded anterior margin or in some taxa, notably ‘’Brachymyrmex nebulosus’’, its medio-anterior portion forms a “lip.” In monomorphic species, the clypeus bears five long, erect hairs of which one usually conspicuous hair is near the anterior margin, two are in mediolateral position and the other two close to the toruli; other hairs are markedly shorter and appressed or decumbent. In dimorphic species, the clypeus is larger and with a row of long thick hairs near the anterior margin. Toruli either touch the posterior clypeal margin in oblique anterodorsal view or surpass it. Compound eye conspicuous, positioned usually on the cephalic midline or anterior to it; with 3–14 ommatidia along its maximal diameter. Number of ocelli either 0, 1, or 3, but when present often inconspicuous. Antennae with nine segments, without antennal club; flagelomeres sometimes gradually increasing apically in diameter; scapes variable in length, with appressed, decumbent or erect hairs.

Mesosoma. With sparse or dense pubescence and hairs in variable orientation. The pronotum and mesonotum typically bear two erect hairs each, but sometimes additional suberect hairs on one or both are present, or erect hairs may be absent from the mesonotum. The pronotum is slightly to strongly convex, and the promesonotal suture, i.e., the line of junction between the pronotum and mesonotum, is always present. The mesonotum may bulge dorsally above the propodeum, and the mesometanotal suture, i.e., the line of junction between the mesonotum and the metanotum, is usually conspicuous, although the mesonotum and metanotum appear fused in some species. The metanotum is reduced to a transverse groove, the metanotal groove, which separates the mesonotum from the propodeum on the mesosomal dorsum. The metanotal groove is variable, from absent to wide and deep. The metathoracic spiracles are dorsal near the midline or dorsolateral, and not, slightly or very strongly protruding, i.e., tumiliform. The propodeal suture, i.e., the line of junction between either the mesonotum (if the metanotal groove is absent) or the metanotal groove anteriorly and the propodeum posteriorly, is present as a dorsal fold with variable lateral extension. Dorsum of the propodeum flat or convex and usually shorter than the propodeal slope. Propodeal spiracles circular and positioned near the posterior propodeal margin. Petiole usually with a low scale, reduced to a narrow subcylindrical segment that is overhung from behind by the gaster, but in dimorphic species the scale of the petiole may be high and visible in dorsal view. Hairs on the legs may be appressed, decumbent, or erect.

Gaster. Of variable size, with five segments that bear sparse or dense pubescence and usually erect hairs, mainly but not exclusively along the posterior edges of the segments. Color and sculpture. Body color ranges from light yellow to dark brown and black; most often, it is uniform, but some species display markedly contrasting patterns, e.g., with the head and/or the gaster darker than the rest of the body. Body usually smooth and shiny, but in some species the head and/or mesonotum bear microsculpture.

Queen
Ortiz-Sepulveda et al. (2019) - Head wider than long, with abundant, fine pubescence, and with long erect hairs; eyes large, located laterally along the cephalic midline; three ocelli present; frontal lobes well-developed; scapes usually extending beyond the posterior margin of the head; palpal formula: 6,4. Mesosoma with moderately dense, fine pubescence, and several erect hairs; anepisternum and katepisternum separated by a distinct suture. Anterior wing with a single dark brown cell, i.e., pterostigma, the first submarginal cell is closed, others open. Posterior wing with five to seven hammuli. Gaster with moderately dense, fine pubescence, and erect hairs along the posterior edges of the segments. Body color ranges from yellow to dark brown, and it is uniform or sometimes with the head and/or gaster darker than the rest of the body.

Male
Ortiz-Sepulveda et al. (2019) - Head wider than long, with fine, sparse pubescence, lacking erect hairs except on mouthparts, and with smooth, shiny integument; maxillary palps with four segments, labial palps with two; mandibles unidentate; frontal lobes reduced; ocelli and eyes well-developed; antennae with ten segments. Mesosoma with sparse pubescence and shiny integument, without erect hairs. Gaster shiny, lacking pubescence, with scattered erect hairs on the last few segments. Head dark brown to almost black, rest of body, including appendages, very light brown or concolorous. Wilson et al. (2016) described the morphology of the male genitalia in detail.