Pseudomyrmex

Arboreal ants in the genus Pseudomyrmex are widely distributed in warm temperate and tropical regions of the New World, occurring from southern United States to Chile and Argentina. The genus is the largest genus in its subfamily, with more than 200 valid specific and infraspecific names. Nesting habits vary widely, with the majority of the species nesting nonspecifically in dead hollow twigs of various plants, but with a substantial minority being obligate inhabitants of specialized ant-plants such as Triplaris, Tachigali and swollen-thorn Acacia. The species living in domatia usually have a mutualistic relationship with their host, defending the plant from herbivores and plant competitors, and receiving shelter and sustenance in return (Janzen 1966; Benson 1985; Davidson et al. 1988; Ward 1991). (Ward 1990, 2017).

Identification
Workers of Pseudomyrmex may be recognized by their large compound eyes, closely set frontal carinae and antennal insertions, and short scapes. There is a distinct postpetiole and a well developed sting (Ward 1985).

It is relatively simple to determine you have Pseudomyrmex specimens when you collect these ants but it can be difficult to sort out what species you have found. Phil Ward has been steadily improving our understanding of this genus for decades. His lab webpage includes an indispensable set of identification resources (Ward Lab Pseudomyrmex species identification resources). If you are trying to identify Pseudomyrmex information on the Ward Lab webpages to understand the measurements and characters used in the keys. There are also keys (as noted below) from various Ward Pseudomyrmex revisions here on Antwiki, including a contemporary provisional key to the species groups and those species that stand apart from these groups:

Key to Pseudomyrmex species groups

Distribution
Southern United States to south-central Chile and Argentina.

Biology
Probably the best known “plant-ants” are the members of the Pseudomyrmex ferrugineus group that occupy and protect swollen-thorn acacias (Vachellia spp.) of Mexico and Central America (Wheeler 1942; Janzen 1966, 1967, 1973; Kautz et al. 2009; Heil et al. 2009, 2014). These ants were revised taxonomically more than twenty years ago, with the consequent recognition of ten species (Ward 1993). Since that time no additional species of obligate acacia-ants have been discovered, but recent studies have revealed that two undescribed species of generalist twig-nesting Pseudomyrmex are nested phylogenetically within the P. ferrugineus group (Kautz et al. 2009; Chomicki et al. 2015; Ward & Branstetter, 2017).

Nomenclature

 *  PSEUDOMYRMEX [Pseudomyrmecinae]
 * Pseudomyrmex Lund, 1831b: 106. Type-species: Formica gracilis, by subsequent designation of Smith, M.R. 1952a: 98.
 * Pseudomyrmex senior synonym of Pseudomyrma (and its junior synonyms Leptalea and Myrmex): Smith, M.R. 1952a: 98.
 * Pseudomyrmex senior synonym of Apedunculata, Clavanoda, Latinoda, Ornatinoda, Triangulinoda: Ward, 1990: 469.
 * APEDUNCULATA [junior synonym of Pseudomyrmex]
 * Apedunculata Enzmann, E.V. 1944: 62 [as "branch" of Pseudomyrma]. Type-species: Pseudomyrma sericea, by original designation.
 * Apedunculata junior synonym of Pseudomyrmex: Ward, 1990: 469.
 * CLAVANODA [junior synonym of Pseudomyrmex]
 * Clavanoda Enzmann, E.V. 1944: 61 [as "branch" of Pseudomyrma]. Type-species: Formica gracilis, by original designation.
 * Clavanoda junior synonym of Pseudomyrmex: Ward, 1990: 469. [Pseudomyrmex and Clavanoda share the same type-species; synonymy is therefore absolute.]
 * LATINODA [junior synonym of Pseudomyrmex]
 * Latinoda Enzmann, E.V. 1944: 62 [as "branch" of Pseudomyrma]. Type-species: Pseudomyrma latinoda, by original designation.
 * Latinoda junior synonym of Pseudomyrmex: Ward, 1990: 469.
 * LEPTALEA [junior synonym of Pseudomyrmex]
 * Leptalea Erichson, 1839: 309. Type-species: Formica gracilis, by subsequent designation of Wheeler, W.M. 1911f: 166.
 * [ Leptalaea Spinola, 1851b: 52 and Spinola, 1853: 68; unavailable names, incorrect subsequent spellings.]
 * Leptalea junior synonym of Pseudomyrma: Smith, F. 1858b: 153; Roger, 1863b: 24.
 * Leptalea junior synonym of Pseudomyrmex: Smith, M.R. 1952a: 98. [Pseudomyrmex and Leptalea share the same type-species; synonymy is therefore absolute.]
 * ORNATINODA [junior synonym of Pseudomyrmex]
 * Ornatinoda Enzmann, E.V. 1944: 61 [as "branch" of Pseudomyrma]. Type-species: Formica tenuis, by original designation.
 * Ornatinoda junior synonym of Pseudomyrmex: Ward, 1990: 469.
 * PSEUDOMYRMA [junior synonym of Pseudomyrmex]
 * Pseudomyrma Guérin-Méneville, 1844a: 427 [as subgenus of Formica]. Type-species: Pseudomyrma bicolor, by monotypy.
 * Pseudomyrma senior synonym of Leptalea, Myrmex: Smith, F. 1858b: 153; Roger, 1863b: 24.
 * Pseudomyrma junior synonym of Pseudomyrmex: Smith, M.R. 1952a: 98.
 * TRIANGULINODA [junior synonym of Pseudomyrmex]
 * Triangulinoda Enzmann, E.V. 1944: 61 [as "branch" of Pseudomyrma]. Type-species: Pseudomyrma spinicola, by original designation.
 * Triangulinoda junior synonym of Pseudomyrmex: Ward, 1990: 469.

Diagnosis, worker Monomorphic, varying greatly in size (worker HW 0.45-2.86). Basal and masticatory margins of mandibles distinct, the former bearing a proximal tooth and 1 (rarely 2) mesial tooth (teeth); masticatory margin with 5 - 10 teeth or denticles. Venter of mandible with a sharp ridge or escarpment, preceding the masticatory margin, which terminates at the apico-basal tooth; corresponding ridge behind basal margin weak or absent. Palp formula: 6,4; with reductions to 6,3; 5 ,4; 5,3; and 4,3. Distal margin of labrum with a broad, v-shaped cleft; labrum without prominent teeth or protuberances. Upper (anterodorsal) surface of median clypeal lobe discontinuous and (at least laterally) truncate. Antennae 12-segmented. Median lobes of antennal sclerites slightly to moderately expanded laterally, the basal condyles of the antennae correspondingly exposed (ASI 0.40-0.74). Frontal carinae often rather closely contiguous, but not always so (FCI 0.01-0.13). Compound eyes relatively large, and elongate (012 <0.65). Ocelli (3) always present. Metanotal groove varying from distinctly impressed to entirely absent. Petiolar node varying in shape, distinct anterior peduncle present or absent. Mid and hind basitarsal sulcus always absent. Median connection of spiracular plates (of sting apparatus) sclerotized.

Diagnosis, queen Deciduously winged, agreeing with most worker characters, except for caste-specific differences in mesosomal morphology. Mandibles and median clypeal lobe occasionally modified from the condition seen in worker. Forewing with two cubital cells.

Diagnosis, male Basal and masticatory margins of mandibles distinct, former one-half or less the length of the latter; basal margin lacking teeth entirely; masticatory margin with 6-18 teeth or denticles, those preceding the pre apical tooth often quite small in size. Palp formula and labrum as in workers. Anterior margin of median clypeal lobe varying from broadly convex to medially angulate; upper (anterodorsal) surface non-truncate. Antennae 12-segmented. Median lobes of antennal sclerites little developed; frontal carinae absent or obsolete. Antennal sclerites often (but not always) remote from the posteromedial margin of the clypeus (i.e. separated by notably more than the maximum diameter of the antennal fossa). Paramere usually with posterodorsal lobe and mesial dorsoventral lobe, and frequently other adornments. Internal face of inner valve (aedeagus) typically with a flat, plate-like structure separated from the dorsal margin by a membranous groove or strip. Wing venation generally as in queen; some males of P. viduus group exhibit intra- and inter-individual variation in the number of cubital cells (1-2). Male pupa with unique antennal configuration: the antennae pass ventrally over the mandibles, and are frequently (but not in the P.gracilis group, P. sericeus group, and a few other species) curled in a semi-circular position.