Pseudomyrmex penetrator

P. penetrator is associated with Tachigali, making its nests within and foraging upon plants of this genus.

Distribution
Widely distributed in the east/central Amazon basin and adjacent Guianas.

Distribution based on Regional Taxon Lists
Neotropical Region: Brazil, Guyana, Suriname, Venezuela.

Biology
Fonseca (1993) provides information on the biology of this ant (under the name P. nigrescens).

Nomenclature

 *  penetrator. Pseudomyrma penetrator Smith, F. 1877b: 66 (q.) BRAZIL. Combination in Pseudomyrmex: Kempf, 1958: 435. Junior synonym of concolor: Kempf, 1967c: 5. Revived from synonymy and senior synonym of coronata Wheeler (and its junior synonym coronata Enzmann), nigrescens: Ward, 1999b: 499.
 * nigrescens. Pseudomyrma latinoda var. nigrescens Forel, 1904c: 38 (w.) BRAZIL. Combination in Pseudomyrmex: Kempf, 1961a: 400. Raised to species: Ward, 1990: 485. Junior synonym of penetrator: Ward, 1999b: 500.
 * coronata. Pseudomyrma latinoda var. coronata Wheeler, W.M. 1942: 167 (w.m.) GUYANA. Combination in Pseudomyrmex: Kempf, 1961a: 400. Senior synonym of coronata Enzmann: Brown, 1949a: 42. Subspecies of concolor: Ward, 1989: 436. Raised to species: Ward, 1990: 484. Junior synonym of penetrator: Ward, 1999b: 500.
 * coronata. Pseudomyrma latinoda var. coronata Enzmann, E.V. 1944: 88 (w.) GUYANA. [Unresolved junior primary homonym of coronata Wheeler, above.] Junior synonym of coronata Wheeler: Brown, 1949a: 42.

Type Material
Ward (1999):
 * Pseudomyrmex penetrator Smith, F. 1877b: 66. Syntype alate queen, “St. Paul” [Sao Paulo de Olivenca], Amazonas, Brazil [Examined].
 * Pseudomyrma latinoda var. nigrescens Forel, 1904a:38. Syntype workers, Para, Brazil (Goldi) [Examined].
 * Pseudomyrma latinoda var. coronata Wheeler, 1942:167. Syntype workers, Kamakusa, Guyana (H. O. Lang) (MCZC) [Examined]. One worker here designated as Lectotype.
 * Pseudomyrma latinoda var. coronata Enzmann, 1944:88. Syntype workers, Mouth of Merume, Guyana (H. O. Lang) (MCZC) [Examined]. One worker here designated as Lectotype.

Ward (1999) - This is a ‘dark form’ very closely related to, and possibly conspecific with, Pseudomyrmex concolor. The circumstances are complicated. In a number of localities the two color forms occur sympatrically and appear to retain their distinctness. Studies in the Manaus region even indicate that the two morphs are ecological differentiated, with P. penetrator (= P. nigrescens) occupying older Tachigali plants than P. concolor and achieving larger colony sizes (Fonseca, 1993). Yet elsewhere, the morphological distinctions are blurred, and one collection from 100 km E Humaita in southeastern Amazonas (leg. F. Rickson) contains both dark and light workers within a single nest series. The queen of this nest series is mostly dark brown, but has a contrastingly light head and patches of orange-brown coloration on the pronotum, mesoscutum, and anepisternum. There are also morphologically intermediate individuals in collections made by Benson, Brandao, and others in the Serra do Carajas, Para. In the Carajas samples some workers and queens appear to be typical dark-bodied P. penetrator, but others are more problematic: these have a lighter orange-brown coloration on all or parts of the mesosoma, petiole and postpetiole, and in a few workers the light coloration even extends to the fourth abdominal (first gastric) segment. Finally, the colour differences between the type queens of P. penetrator and P. concolor, both from Sao Paulo de Olivenca in western Amazonia, are less pronounced than in populations from eastern Amazonia and the Guianas, although this might be due to a loss of colour difference with time.

Similar patterns of colour variation, involving light- and dark-coloured individuals, occur in the related species Pseudomyrmex insuavis and Pseudomyrmex malignus, but in those cases the variation is clearly intraspecific. The situation in P. concolor/P. penetrator is more intriguing, and suggests reproductive isolation between the forms in some regions but not others. Besides the breakdown in colour as a diagnostic character in peripheral (southeastern) populations, another observation indicating gene flow between populations of P. concolor and P. penetrator is the occurrence of concordant geographical variation in morphology. For example, the type queens of P. concolor and P. penetrator, from the same locality in western Amazonia, are both exceptionally large (HW 1.39) with elongate heads (CI 0.79). Conversely, queens of P. concolor and P. penetrator from Guyana and Venezuela tend to have broader (CI 0.84–0.87) and less shiny heads (owing to the greater development of fine striolate-reticulate sculpture on the frons) than their eastern Amazonian counterparts (CI 0.79–0.86). Of course, an alternative explanation for such concordant variation is similar selection pressures on queen morphology in two sibling species. The situation calls for a detailed genetic analysis. I could find no consistent differences between P. concolor and P. penetrator in the form of the male genitalia, and this contrasts with the typical situation in the viduus group in which the male genitalia provide species-diagnostic characters.

Worker
Ward (1999) - Measurements (n=24). HL 0.97–1.23, HW 0.89–1.13, MFC 0.093–0.136, LHT 0.70–0.93, CI 0.86–0.97, REL 0.36–0.42, REL2 0.38–0.44, FCI 0.09–0.13, SI 0.39–0.43, FLI 0.97–1.44, FI 0.39–0.46, PLI 0.77–0.92, PWI 0.75–0.89, PPWI 1.48–1.82.

Very similar to Pseudomyrmex concolor (q.v.) in size, shape, sculpture and pilosity (MSC 28–53; HTC 8–16), differing primarily in colour. Gaster, and usually also mesosoma, petiole and postpetiole, dark brown; head concolorous brown or a contrasting orange-brown, with variable amounts of infuscation (especially around the ocellar triangle). Antennae, tarsi and (sometimes) tibiae lighter yellow-brown. Petiole tending to be perhaps lower (compare PLI values), with a smaller subpetiolar process, and postpetiole tending to be broader (compare PPWI values), but these differences, if real, are slight and not diagnostic.