Pseudomyrmex inquilinus

This species is an inquiline in the nests of an undescribed species of (species PSW-64). A nest was found with its host and multiple parasitic queens. One individual was riding on top of the host queen.

Identification
Ward (1996) - The queen of this species is immediately distinguishable from all known congeners by any one of the following features: the concave and edentate masticatory margin of the mandibles, the reduced palp formula (3,2), the shape of the petiole (especially the lack of a posterior face), shape of the postpetiole (short, broad and deep), and the lack of standing pilosity on nearly all parts of the body except the mouthparts and apex. Also distinctive are the edentate basal margin of the mandible, the lack of a transverse truncation on the median clypeal lobe, the well separated frontal carinae, the slender forefemur, the short legs, and the long basal face of the propodeum. The male is diagnosable by the mandible shape, palp formula, lack of a posterior face on the petiole, highly reduced standing pilosity, and the male genitalia (see description).

Based on worker and queen morphology, as well as male genitalia, the host species is easily recognizable as a member of the Pseudomyrmex pallidus group. Within this group it belongs to a taxonomically vexing assemblage of species that may be termed the Pseudomyrmex flavidulus complex. Members of this complex are characterized by the following combination of character states: male paramere with large posterodorsal lobe preceded anteriorly by a much smaller finger-like lobe; abdominal tergite IV of worker and queen densely pubescent; head relatively elongate (worker CI ≤ 0.90); worker profemur relatively slender (worker FI ≤ 0.47); and worker and queen predominantly orange or orange-brown in colour. There are several species within the P. flavidulus complex, but the limits of intra- and interspecific variation are not yet clear. For the moment the host species is referred to using a code number, Pseudomyrmex sp. PSW-64. As currently interpreted this species is known only from Catamarca and Tucumiin provinces in Argentina, although a related (and possibly conspecific) form occurs in Bolivia. The flavidulus complex as a whole is widely distributed and common in the Neotropics, from Costa Rica to Argentina.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina.

Biology
Ward (1996) - The host species was found nesting in dead stems of Baccharis salicifolia and a second Baccharis species, probably Baccharis angulata, in pastured riparian grassland next to the Rio Santa Maria. The Baccharis species are dominant in this community; other plants include Salix humboldtiana, Prosopis, Juncus, and various grasses. The town of Santa Maria has an average annual rainfall of 182 mm and is located in the Argentinian desert biome known as monte (Morello, 1958).

Pseudomyrmex inquilinus queens were recovered from two out of six nests of the host species. The contents of the two parasitized nests are summarized in Table 2. Both nests contained a dealate host queen and sexual pupae of the host species. Thus P. inquilinus was not inhibiting sexual reproduction in its host.

Brief observations on behaviour were made in the field during the collection of nest no. 12844. Most of the P. inquilinus queens were close to the host queen. One was riding on the back of the host queen gaster, holding on with her mandibles around the anterior peduncle of the petiole. (The concave masticatory margins of the P. inquilinus mandibles make them well-suited for this task.) As the nest was broken open Pseudomyrmex host workers were seen encountering P. inquilinus queens and no antagonistic behaviour ensued. The single parasite male was in the upper part of the twig nest, where most of the host males were concentrated, while the nine parasite queens and the host queen occurred in the lower portions of the nest. The entire host nest occupied a hollow section of the dead twig about 0.5 m in length (out of a total length of about 1 m).

At the type locality (Santa Maria) and at neighbouring sites near Amaichii del Valle (Tucumiin Province) two species of Pseudomyrmex were found to be common inhabitants of dead Baccharis stems, both members of the P. pallidus group: P. sp. PSW-64 (six nests collected, two parasitized by P. inquilinus) and Pseudomyrmex rufiventris (five nests sampled, none parasitized). The sample sizes are too small to draw any firm conclusions about host specificity. During a 9-day period of collecting Pseudomyrmex in northern Argentina (in Cordoba, Catamarca and Tucumiin provinces) a total of 24 nests of P. rufiventris was sampled and none was parasitized. Both the parasite and its only known host (P. sp. PSW-64) were found only in the vicinity of Santa Maria.

Nomenclature

 *  inquilinus. Pseudomyrmex inquilinus Ward, 1996: 255, figs. 1-4, 11, 12, 21, 22 (q.m.) ARGENTINA.

Queen
(n = 7). HW 0.63-0.65, HL 0.74-0.78, EL 0.36-0.38, DPW 0.29-0.33, PPW 0.41-0.46, LHT 0.47-0.50, CI 0.83-0.87, 01 0.60-0.62, REL 0.49-0.50, REL2 0.57-0.59, FCI 0.091-0.115, SI 0.40-0.45, FI 0.37-0.40, PLI 1.12-1.27, PWI 1.08-1.20, PPWI 1.65.-1.83.

Of diminutive size for the genus Pseudomyrmex (HW < 0.68, LHT < 0.52). Mandible relatively broad (MD2/MD3 =0.55), with basal and external margins subparallel (MD1/MD2 =0.96); basal and masticatory margins distinctly differentiated, the former lacking teeth or denticles, except for an acute apicobasal tooth; masticatory margin strongly concave and essentially edentate, at best very weakly crenulate, with a small denticle, discernable only at high magnification, at about two thirds of the distance between the apicobasal tooth and the apical tooth. Closed mandibles with a gap (0.03-0.04 mm wide) between the midpoints of their masticatory margins as a consequence of the aforementioned concavity. Palp formula 3,2. Median clypeal lobe broadly rounded, and lacking a distinctive transverse truncation on the upper surface (typical of nearly all other Pseudomyrmex- see Ward, 1990). Frontal carinae well separated, the distance between them subequal to the maximum scape width (FCI = 0.10). Head moderately elongate (CI 0.83-0.87), with weakly convex sides and a straight to shallowly concave posterior margin, in frontal view. Evidently fully winged: all known specimens dealate, with a full complement of thoracic sclerites (and queen pupa seen to have wings). Metapleural gland op<!ning and bulla reduced, inconspicuous. Legs relatively short, the profemur in particular small and slender (FL 0.44---D.46, FI 0.37-0.40). Meso- and metatibiae each with a pair of apical spurs, the posterior one of each pair conspicuously pectinate. In dorsal view propodeum subtrapezoidal, the sides converging posteriorly; in lateral view, the basal (dorsal) face of propodeum with a long weakly declining surface, followed by a shorter steeper portion, which is nevertheless well differentiated from the short, vertical declivitous face of the propodeum. Petiole short, broad, apedunculate, submarginate laterally, and with a large conspicuous ventral keel that protrudes posteroventrally. Dorsal face of petiole fiat, with a slight median depression; posterior face absent, owing to an expanded helcium. Postpetiole short and broad, with a bulging ventral protrusion that is thick and wide, not keel-like. Remainder of gaster about the same length as the mesosoma.

Mandible sublucid, with very weak, almost obsolete, transverse striolae. Head moderately shiny, with numerous punctulae, about 0.01 mm in diameter or less, separated by one to several diameters, on a finely coriarious background scupture. Mesoscutum and mesoscutellum similarly sculptured, punctures more scattered; in contrast, pronotum, propodeum and petiole densely coriarious-reticulate, and subopaque, especially the propodeum; episternum and sides of pronotum less strongly sculptured. Postpetiole and gaster sublucid, with scattered, very fine punctures on a weakly coriarious background.

Standing pilosity very scarce, essentially absent from the scapes (except apex), head capsule (above the clypeus), mesosoma, legs, petiole, postpetiole and fourth abdominal (first gastric) tergite. A single short seta present consistently on each side of the mesoscutum, near the lateral margin. Gastric pilosity beginning on the fifth abdominal tergite (or on the posterior margin of the fourth), and becoming long and dense near the apex. Very fine, short, submicroscopic pubescence present on much of the body but scarcely visible at normal magnification (50-100X). Body orange-brown, mesoscutellum and metanotum with variable darker infuscation; mandibles, clypeus and gula pale yellow-brown.

Male
Mandibles similar to those of queen; masticatory margin edentate and with same peculiar concave edge, although not as pronounced. Palp formula 3,2 as in queen. Head broad (HW 0.65, CI 0.96), and posterior margin very rounded. Eyes and ocelli not strongly protruding from head capsule. Scape short, subequal in length to second funicular segment. Total length of antenna about 1.4 mm. Propodeum, petiole and postpetiole similar to those of queen, but less extreme in shape: propodeal faces less distinct, petiole and postpetiole less broad, and petiole less strongly margined. Subgenital plate much wider than long, its posterior margin shallowly and broadly concave. Pygidium with a truncate posterior margin, not recurved anteroventrally. Paramere as in figures; mesal dorsoventral lobe developed as an oblique, rounded, dorsomesal protrusion, joined through a low saddle to a posterodorsal ridge, the surface between the two concave and directed posterodorsomesally. Aedeagal plate simple, somewhat circular in outline, outer margin entire except for a weak medial incision and one or two adjacent, very small denticulae. Outer surface of aedeagus with a broad oblique impression (anterodorsal to posteroventral), bordered on either side by corresponding weak ridges, the anterior ridge situated about midway between the posterior margin and the lateral apodeme, the posterior ridge merging with the posterior margin above the level of the aforementioned incision. Body sculpture paralleling that of queen, but integument a little less shiny; subopaque, shagreened areas on the head between the antennal insertions and in the ocellar triangle; mesoscutum and mesoscutellum with coarser sculpture than in queen. Standing pilosity very scarce, distributed as in the queen. Pubescence slightly more noticeable than in queen, but still rather inconspicuous. Dark brown, with paler, transverse yellow-brown bands on the posterior margins of the gastric segments; mandibles, fronto-clypeal complex, gula and forecoxa constrastingly pale, almost white; antennae and legs light brown.

Type Material
Holotype queen. Argentina, Catamarca: Santa Maria, 2000 m, 26°42'S, 66°03'W, 2 February 1995; P. S. Ward ace. no. 12845; in nest of host species, Pseudomyrmex sp. PSW-64 (Ward ace. no. 12844), in dead twig of Baccharis sp. HW 0.63, HL 0.74, EL 0.36, LHT 0.48. . Paratypes. Series of (i) eight queens and one male from same host nest as the holotype (ii) two queens from a second nest, same host species, locality and date. 

References based on Global Ant Biodiversity Informatics

 * Brandao C. R. F., F. A. Esteves, and L. P. Prado. 2010. A catalogue of the Pseudomyrmecinae ant type specimens (Hymenoptera, Formicidae) deposited in the Museu de Zoologia da Universidade de Sao paulo, Brazil. Papeis Avulsos de Zoologia 50(45): 693-699.
 * Ward P. S. 1996. A new workerless social parasite in the ant genus Pseudomyrmex (Hymenoptera: Formicidae), with a discussion of the origin of social parasitism in ants. Syst. Entomol. 21: 253-263.