Pseudomyrmex gracilis

A wide ranging species that is quite variable in its appearance. This ant can also be found in many different habitats. Pseudomyrmex gracilis typically nest in dead twigs or branches but will occasional be found in myrmecophytic plants. When nesting in a myrmecophyte, they play the role of a parasite of the ant-plant mutualism.

Identification
Similar to Pseudomyrmex hospitalis. See P. hospitalis and the nomenclature section below.

Distribution
Southern United States (introduced into Florida) to Argentina and Brazil (Ward 1993).

Distribution based on Regional Taxon Lists
Indo-Australian Region: Hawaii. Nearctic Region: United States. Neotropical Region: Argentina, Barbados, Belize, Bolivia, Brazil, Colombia, Costa Rica, Ecuador, El Salvador, French Guiana, Greater Antilles, Guadeloupe, Guatemala, Guyana, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru, Suriname, Trinidad and Tobago, Uruguay, Venezuela.

Biology
Ward (1993) - Befitting its wide distribution and variable phenotype, P. gracilis can be found in a variety of habitats from mangroves and thorn scrub to rainforest. It is often particularly common in disturbed situations such as old fields, roadsides, and secondary forest. Nests are usually located in dead twigs or small branches, but there are a substantial number of records of colonies occupying swollen-thorn acacias in Central America (Mexico to Panama). In a few localities P. gracilis is a common acacia inhabitant and under these circumstances it may exhibit local adaptation and phenotypic differentiation (see also Wheeler 1942:107). For example, Janzen collected a series of specimens from Acacia gentlei in Belize (15 mi. S Santa Elena) which have somewhat distinctive morphology: the workers are large, dark, abundantly hairy, and possess rather short petioles (PU 0.55), although none of these features is outside the total range of variation for the species. Janzen (1974:98) notes that the workers of this large black morph have atypically aggressive behavior. Given the kind of ecotypic variation to which P. gracilis is prone, it is not surprising to find a tendency of some populations to specialize on acacias. The ecology of this species is reminiscent of other animal species which show broad ecophenotypic variation, e.g. fish with trophic polymorphisms (Kornfield et al. 1982; Grudzien and Turner 1984; Sandlund et al. 1992).

Fonseca-Romero et al. (2019) - This ant opportunistically inhabits Turnera velutina (previously Acacia hindsii) plants, effectively exploiting the plant by using the hollow thorns, extrafloral nectaries, and beltian bodies while not providing robust defenses to the plant at the level of the aggressive ant Pseudomyrmex ferrugineus. The lack of a strong defensive functions provided by P. gracilis alters the plant through their producing thicker leaves and reducing their production of ant food rewards.

Brazil
Schmid et al. (2014) found this ant nesting in infructescences (the stem and remains of buds and fruits above the level of the water reservoir in the rosette) of the bromeliad Vriesea friburgensis on Santa Catarina Island, Brazil.

De Oliveira et al. (2015), studying ant occupancy of Cecropia trees in southwest Bahia, Brazil, found a colony of  nesting in a Cecropia pachystachya tree.

DaRocha et al. (2015) studied the diversity of ants found in bromeliads of a single large tree of Erythrina, a common cocoa shade tree, at an agricultural research center in Ilhéus, Brazil. Forty-seven species of ants were found in 36 of 52 the bromeliads examined. Bromeliads with suspended soil and those that were larger had higher ant diversity. Pseudomyrmex gracilis was found in 7 different bromeliads but was associated with twigs and bark cavities, rather than suspended soil or litter, of the plant.

Sanz-Veiga et al. (2017) observed this species visiting extrafloral nectaries of Tocoyena formosa plants in a southeastern Brazilian cerrado study-site.

Koch et al. (2018) sampled this species in Caryocar barsiliense trees, in southeastern Brazil cerrado, as part of a study examining species interactions in ant-plants.

Florida
Deyrup, Davis & Cover (2000): This introduced species is much larger than any of the seven native species of Pseudomyrmex. It might compete with these native species for food, but seems to nest in larger diameter holes in twigs and stems. It is more likely to exclude various native carpenter ants: Camponotus decipiens, Camponotus snellingi, Colobopsis impressa, Camponotus nearcticus and Camponotus discolor. It is a very abundant species in south and central Florida, and does not seem to distinguish between disturbed and undisturbed habitats. Aside from competition with native arboreal ants, this species could affect native phytophagous species, especially butterflies and moths. The buildup of populations of P. gracilis in Florida make it highly probable that it will be accidentally introduced into the West Indies, where it could have greater impact than in Florida.

Nomenclature

 *  gracilis. Formica gracilis Fabricius, 1804: 405 (w.) CENTRAL AMERICA. Wheeler, W.M. & Bailey, 1920: 256 (l.); Wheeler, G.C. & Wheeler, J. 1956: 385 (l.). Combination in Leptalea: Erichson, 1839: 309; in Pseudomyrma: Roger, 1862c: 289; in P. (Clavanoda): Enzmann, E.V. 1944: 61; in Pseudomyrmex: Kusnezov, 1953e: 214. Senior synonym of longinoda: Brown, 1949a: 43; Kempf, 1961a: 370; of variabilis: Ward, 1989: 439; of bicolor, canescens, dimidiata, glabriventris, mexicana, pilosula, sericata, velifera, volatilis and material of the unavailable name guayaquilensis referred here: Ward, 1993: 155; of atrinoda: Wild, 2007b: 55. Current subspecies: nominal plus argentinus. See also: Ward, 1999b: 521.
 * bicolor. Pseudomyrma bicolor Guérin-Méneville, 1844a: 427 (w.) COLOMBIA. Combination in Pseudomyrmex: Kempf, 1958f: 434. Junior synonym of gracilis: Roger, 1862c: 289; Mayr, 1863: 452. Revived from synonymy as subspecies of gracilis: Emery, 1890b: 59; Santschi, 1925d: 223; Wheeler, W.M. 1942: 166. Junior synonym of gracilis: Ward, 1993: 155.
 * sericata. Pseudomyrma sericata Smith, F. 1855c: 159 (w.) BRAZIL. Gallardo, 1932a: 50 (m.). Combination in Pseudomyrmex: Kempf, 1958f: 434. Status as species: Forel, 1912g: 19. Subspecies of gracilis: Emery, 1890b: 60; Santschi, 1916e: 370; Gallardo, 1932a: 48. Junior synonym of gracilis: Ward, 1993: 155.
 * dimidiata. Pseudomyrma dimidiata Roger, 1863a: 177 (w.) COLOMBIA. Combination in Pseudomyrmex: Kempf, 1958f: 434. Subspecies of gracilis: Mayr, 1870a: 406; Forel, 1912g: 19. Junior synonym of gracilis: Ward, 1993: 155.
 * mexicana. Pseudomyrma mexicana Roger, 1863a: 178 (w.) MEXICO. Wheeler, G.C. & Wheeler, J. 1956: 385 (l.); Petralia & Vinson, 1980: 381 (l.). Combination in Leptalea: Smith, M.R. 1951a: 788; in Pseudomyrmex: Kusnezov, 1953e: 214. Subspecies of gracilis: Mayr, 1870a: 409; Emery, 1890b: 45; Forel, 1907e: 7; Creighton, 1950a: 80. Revived status as species: Whitcomb, Denmark, et al. 1972: 31. Junior synonym of gracilis: Ward, 1993: 155. See also: Ward, 1985b: 225.
 * canescens. Pseudomyrma canescens Smith, F. 1877b: 66 (q.) BRAZIL. Combination in Pseudomyrmex: Kempf, 1958f: 434. Junior synonym of gracilis: Ward, 1993: 155.
 * pilosula. Pseudomyrma pilosula Smith, F. 1877b: 62 (w.) BARBADOS. Combination in Pseudomyrmex: Kempf, 1958f: 435. Junior synonym of gracilis: Ward, 1993: 155.
 * variabilis. Pseudomyrma variabilis Smith, F. 1877b: 62 (w.) BARBADOS. Combination in Pseudomyrmex: Kempf, 1958f: 435. Junior synonym of gracilis: Ward, 1989: 439.
 * volatilis. Pseudomyrma volatilis Smith, F. 1877b: 65 (m.) MEXICO. Combination in Pseudomyrmex: Kempf, 1958f: 435. Junior synonym of gracilis: Ward, 1993: 155.
 * glabriventris. Pseudomyrma gracilis var. glabriventris Santschi, 1922b: 345 (w.) BOLIVIA. Combination in Pseudomyrmex: Kempf, 1958f: 434. Junior synonym of gracilis: Ward, 1993: 156.
 * velifera. Pseudomyrma gracilis var. velifera Stitz, 1933: 68 (q.) GUATEMALA. Combination in Pseudomyrmex: Kempf, 1958f: 434. Junior synonym of gracilis: Ward, 1993: 156.
 * atrinoda. Pseudomyrma gracilis var. atrinoda Santschi, 1934c: 26 (w.q.) BRAZIL. Combination in Pseudomyrmex: Kempf, 1958f: 434. Junior synonym of gracilis: Wild, 2007b: 55.
 * longinoda. Pseudomyrma gracilis var. longinoda Enzmann, E.V. 1944: 87 (w.) PERU. Junior synonym of gracilis: Brown, 1949a: 43; Kempf, 1961a: 370.

Type Material
Ward (1993):
 * Pseudomyrmex gracilis Fabricius, 1804: 405. Lectotype worker, Essequibo, Guyana [Examined].
 * Pseudomyrma bicolor Guerin 1844:427. Syntype queen (unique?), Colombia [Examined].
 * Pseudomyrma sericata F. Smith 1855:159. Holotype (unique syntype) worker, Brazil [Examined].
 * Pseudomyrma dimidiata Roger 1863a:177. Syntype workers, Colombia (not in or ) [Not examined].
 * Pseudomyrma mexicana Roger 1863a:178. Syntype workers, Mexico (not in MNHN or ZMHB) [Not examined].
 * Pseudomyrma variabilis F. Smith 1877:62. Lectotype worker, Barbadoes (BMNH) [Examined].
 * Pseudomyrma pilosula F. Smith 1877:62. Two syntype workers, Barbadoes (BMNH) [Examined]. One syntype here designated LECTOTYPE.
 * Pseudomyrma volatilis F. Smith 1877:65. Holotype (unique syntype) male, Mexico (BMNH) [Examined].
 * Pseudomyrma canescens F. Smith 1877:66. Holotype (unique syntype) queen, Abydos, Brazil (BMNH) [Examined].
 * Pseudomyrma gracilis var. glabriventris Santschi 1922:345. Syntype workers, Izozo, Bolivia (Lizer & Deletang) (NHMB) [Examined].
 * Pseudomyrma gracilis mexicana var. guayaquilensis Forel 1907:7. Worker, Guayaquil, Ecuador (Buchwald) [Examined].
 * Pseudomyrma gracilis var. velifera Stitz 1933:68. Holotype queen, Champerico, Guatemala (Paessler) (not in ; Weidner 1972) [Not examined].
 * Pseudomyrma gracilis var. longinoda Enzmann 1945:87. Syntype worker, Peru [Examined].

Ward (1993):

The P. gracilis complex presents one of the more taxonomically challenging problems in the genus Pseudomyrmex and the above treatment is by no means a final solution. The worker- and queen-based forms, newly synonymized under P. gracilis, fall within the bounds of the preceding diagnosis, but it is quite possible that my concept of this species will prove to be too broad. The types of P. dimidiatus, P. mexicanus and P. veliferus could not be located. They are judged to be junior synonyms on the basis of the original descriptions. The unique male holotype of P. volatilis is clearly a member of the P. gracilis group based on size (HW 1.48), mandibular dentition, pilosity, petiole shape, and shape of the parameres. In comparison with males of gracilis group species known to occur in Mexico, namely P. gracilis, P. major (see below), P. nigropilosus and P. opaciceps, the type specimen agrees best with P. gracilis.

The concept of P. gracilis adopted above encompasses an impressive amount of phenotypic variability. Collections from single regions often give the impression that this variation is distributed bimodally or multimodally, as more or less discrete morphs. For example, nest samples from Costa Rica can be segregated on the basis of worker morphology into (i) a large (HW > 1.80), usually lighter-colored form (with orange mesosoma, petiole, and postpetiole, and black head and gaster), (ii) a smaller, bicolored, usually more heavily infuscated form, and (iii) an all-black form of variable size. The first two are typically found in open or xeric habitats while the third is more common in closed forest, suggesting some ecotypic differentiation. Yet when large enough sample sizes are obtained all degrees of intermediacy in size and color are encountered, and the variation in color (less so size) can be seen among individuals (workers and alate queens) from the same nest. Thus, if there are ecotypes they do not appear to be reproductively isolated.

Left unresolved after the establishment of the above synonymy is the relationship of P. gracilis to the following nominal taxa: Pseudomyrmex alternans (Santschi), Pseudomyrma gracilis atrinoda (Santschi), Pseudomyrmex gracilis argentinus (Santschi) and Pseudomyrmex santschii (Enzmann). But the following deserves recognition as a distinct species: Pseudomyrmex major (Forel 1899:91), stat. nov. (syntype worker, Pinos Altos, Chihuahua, Mexico (Buchan-Hepburn) (BMNH) (examined); original combination: Pseudomyrma gracilis var. major). Workers of Pseudomyrmex major can be distinguished from those of P. gracilis by their emarginated median clypeal lobe, less distinct anterior peduncle of the petiole, and larger average size. Males of P. major have broadened fore-tarsal segments. P. major is confined to western Mexico, where it occurs sympatrically with P. gracilis without showing signs of intergradation.

Ward (1989):

The lectotype worker of P. gracilis, although lacking a head, seems to correspond rather well to the concept of P. gracilis which has become prevalent in publications. The mesosoma, postpetiole, and gaster are dark brown to black, the petiole a contrasting light castaneous brown; fine, silvery (not black) pilosity covers most of the body, and the associated piligerous punctures subdue the lustre of the integument; the petiole is narrow and slender, with a long anterior peduncle; and the pronotum is margined laterally but not sharply so. I do not attach much taxonomic significance to the light-colored petiole. The P. gracilis lectotype worker agrees well with material from Kartabo, Guyana (leg. Wheeler) in which there is variable infuscation of the petiole. A second worker in the P. gracilis "type series" in ZMUC, with a red "TYPE" label, but no locality or identification label, is in fact not conspecific (it is a worker of Pseudomyrmex maculatus (F. Smith) and should be excluded from consideration as type.

There is a bewildering and variable array of forms, variously described as subspecies or "varieties" of P. gracilis, which require detailed taxonomic study. I suspect that most of these will prove to be synonyms of a single polytypic species (P. gracilis), but at this stage there is insufficient information about the intra- and inter-specific components of this variation. One unambiguous synonymy can be established here: the lectotype worker of P. variabilis (F. Smith) in BMNH agrees very closely with that of P. gracilis, the only substantial difference being that the P. variabilis petiole is black. I have designated a lectotype of P. variabilis because a second worker glued to the same card (and bearing therefore the same type label as P. variabilis) is that of a different species - P. maculatus (F. Smith)! I have printed a lectotype label for P. variabilis and marked the card shared by the two specimens in such a way that the P. maculatus worker is clearly excluded as a type specimen of P. variabilis''.

Description
Ward (1993) - With the traits of the gracilis group and the following more specific features. Head broad, about as wide as long (CI 0.95-1.08); anterior margin of median clypeal lobe straight to broadly convex, rounded laterally; pronotum dorsolaterally marginate but not sharply so; in lateral view mesonotum more steeply inclined than basal face of propodeum; petiole long and slender (PU 0.46-0.57) with a distinct anterior peduncle; head densely punctulate with a subopaque to sublucid (not matte) appearance; standing pilosity abundant, fine, predominantly pale silvery-white (not black).

Size and color extremely variable (HW 1.39-2.07), varying from unicolorous black (appendages lighter) to unicolorous orange-brown, with many intermediate and bicolored combinations. In populations from Mesoamerica the gaster is typically black, or if paler (orange-brown) then it is usually accompanied by a similar light coloration of the mesosoma (and sometimes also the head).

Ward (1999) - Measurements (n=74). HL 1.38–1.99, HW 1.39–2.07, MFC 0.033–0.079, LHT1.10–1.84, CI 0.95–1.08, REL 0.54–0.65, REL2 0.54–0.65, FCI 0.02–0.04, FI 0.36–0.44, PLI 0.46–0.57, PWI 0.38–0.54.

With the traits of the P. gracilis group (i.e. palp formula 6,4; masticatory margin of mandible with 7–10 teeth; median clypeal lobe laterally rounded; eyes large and elongate; standing pilosity common on mesosoma dorsum and on external faces of tibiae; pronotum dorsolaterally submarginate) and the following more specific features. Head broad, about as wide as long (CI 0.95–1.08); anterior margin of median clypeal lobe straight to broadly convex; in lateral view mesonotum more steeply inclined than dorsal face of propodeum; petiole long and slender (PLI 0.46–0.57) with a well developed anterior peduncle; head and mesosoma densely and finely punctulate-coriarious to coriarious-imbricate, subopaque to sublucid (not matte) in appearance; standing pilosity abundant, fine, predominantly pale silvery-white (not black). Size and colour extremely variable (HW 1.39–2.07), varying from unicolorous black (appendages lighter) to unicolorous orange-brown, with many intermediate and bicoloured combinations.