Pseudomyrmex gracilis ( Fabricius 1804 )

Ward, Philip S., 1993, Systematic studies on Pseudomyrmex acacia-ants (Hymenoptera: Formicidae: Pseudomyrmecinae), Journal of Hymenoptera Research 2, pp. 117-168 : 155-157

Pseudomyrmex gracilis ( Fabricius 1804)

(Fig. 6)

Formica gracilis Fabricius 1804:405 . Lectotype worker, Essequibo , Guyana ( ZMUC) [Examined] ,

Pseudomyrma bicolor Guerin 1844:427 . Syntype queen (unique?), Colombia ( ZSMC) [Examined] Syn. nov.

Pseudomyrma sericata F. Smith 1855:159 . Holotype (unique syntype) worker, Brazil ( BMNH) [Examined] Syn. nov.

Pseudomyrma dimidiata Roger 1863a:177 . Syntype workers, Colombia (not in MNHN or ZMHB) [Not examined] Syn. nov.

Pseudomyrma mexicana Roger 1863a: 178 . Syntype workers, Mexico (not in MNHN or ZMHB) [Not examined] Syn. nov.

Pseudomyrma variabilis F. Smith 1877:62 . Lectotype worker, Barbadoes ( BMNH) [Examined] [Synonymy by Ward 1989:439] .

Pseudomyrma pilosula F. Smith 1877:62 . Two syntype workers, Barbadoes ( BMNH) [Examined], One syntype here designated LECTOTYPE. Syn. nov.

Pseudomyrma volatilis F. Smith 1877:65 . Holotype (unique syntype) male, Mexico ( BMNH) [Examined] Syn. nov.

Pseudomyrma canescens F. Smith 1877:66 . Holotype (unique syntype) queen, Abydos , Brazil ( BMNH) [Examined] Syn. nov.

Pseudomyrma gracilis var. glabriventris Santschi 1922:345 . Syntype workers, Izozo , Bolivia (Lizer & Deletang) ( NHMB) [Examined] Syn. nov.

Pseudomyrma gracilis mexicana var. guayaquilensis Forel 1907:7 . Worker, Guayaquil , Ecuador (Buchwald) ( MHNG) [Examined] Unavailable infrasubspecific name .

Pseudomyrma gracilis var. velifera Stitz 1933:68 . Holotype queen, Champerico , Guatemala (Paessler) (not in ZMUH; Weidner 1972) [Not examined] Syn. nov.

Pseudomyrma gracilis var. longinoda Enzmann 1945:87 . Syntype worker, Peru ( MCZC) [Examined] [Synonymy by Brown 1949:43], Pseudomyrmex gracilis (Fabricius) ; Kusnezov 1953:214 .

Worker diagnosis. - With the traits of the gracilis group (see couplet 6 of the key; p. 130) and the following more specific features. Head broad, about as wide as long (Cl 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 (PLI 0.46-0.57) with a distinct anterior peduncle (Figs. 6,53); 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).

Taxonomic comments. - 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: P. alternans (Santschi) , P. gracilis atrinodus (Santschi) , P. gracilis argentinus (Santschi) and P. 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-Hepbum) ( BMNH) (examined); original combination: Pseudomyrma gracilis var. major ). Workers of P. major can be distinguished from those of P. gracilis by their emarginate 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.

Distribution and biology. - Befitting its wide distribution (southern United States to Argentina and Brazil) 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 (PLI ≈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).