Pseudomyrmex arcanus

This species is found at low to medium elevations in western Mexico, from Baja California to Oaxaca. Recorded elevations vary from 5 m to 1,280 m (mean 474 m; n = 19). Habitats in which P. arcanus has been collected include coastal sand dunes, pasture, tropical dry forest, and mixed tropical/ temperate mesic forest. Nests have been taken in a dead stalk of a woody Asteraceae (?Viguiera), dead stem of Cassia, dead twig of thorny vine, dead twig of vine, and dead twigs of unidentiﬁed woody plants. The records of ‘Pseudomyrmex apache’ cited in Ward (1985: 231) from Baja California refer to this species.

Identification
Small to medium-sized species (HW 0.82–0.97) with moderately elongate head (CI 0.80–0.85) and eyes (see REL and REL2 values); frontal carinae separated by less than basal scape width; metanotal groove present but weakly impressed; in profile, dorsal face of propodeum rounding obtusely into declivitous face, the two faces subequal in length; petiole relatively slender (PLI 0.49–0.57, PL/HL 0.50–0.56), in proﬁle the convex anterodorsal face of petiole ascending gradually to summit in the posterior half of node, then rounding into more rapidly descending posterior face (Fig. 5); profemur slender; hind leg relatively short (LHT/HL 0.68–0.72). Head sublucid, densely punctulate-coriarious to coriarious-imbricate. Standing pilosity sparse, absent from propodeum and mesonotum (MSC 2–5). Uniformly light yellow-brown to orange-brown.

This species is characterized by a combination of light yellow-orange coloration, without additional maculation; moderately elongate eyes; weakly impressed metanotal groove (in the worker); and slender petiole. The workers of P. arcanus can be distinguished from those of other orange-yellow species in the P. elongatulus group as follows.
 * Pseudomyrmex elongatulus has a better-developed metanotal groove, more elongate eyes (REL 0.47–0.53), petiole (PL/HL 0.57–0.61) and legs (LHT/HL 0.74–0.79), and some degree of infuscation on the gaster.
 * Pseudomyrmex exoratus has a much more elongate head (CI 0.67–0.69) and a robust profemur (FI 0.46–0.49).
 * Pseudomyrmex fasciatus essentially lacks a metanotal groove (as seen in in proﬁle) and has conspicuous transverse maculation on the gaster.
 * Pseudomyrmex apache has shorter eyes (on average), a more angular propodeum, and a more robust petiole, and the tip of the gaster is frequently darkened.

Although they are not one another’s closest relatives (Fig. 1), P. apache and P. arcanus can be difficult to distinguish, the differences in propodeal and petiole shape being rather subtle. Bivariate plots of some measurements reveal two separate clouds of points (Fig. 3), but increased samples sizes might show some overlap.

Distribution based on Regional Taxon Lists
Neotropical Region: Mexico.

Nomenclature

 * . Pseudomyrmex arcanus Ward & Branstetter, 2022: 18, figs. 5, 20 (w.) MEXICO (Michoacán).

Type Material

 * Holotype Worker. MEXICO Michoacán: 20 km S Uruapán, 880 m, 19° 14′ N 102° 03′ W, 28 Dec 1987, ex dead twig of vine, tropical dry forest, P. S. Ward PSW09293 (UNAM) (CASENT0863536).
 * Paratypes: series of workers, same data as holotype (CASC, CZUG, IEXA, MCZC, PSWC, UCDC, USNM).