Pseudomyrmex nimbus

Pseudomyrmex nimbus is known only from several cloud forest sites in Costa Rica, at elevations of 730 m to 1,200 m. The following are Jack Longino’s field notes on the type series (JTL2694): ‘Workers abundant on small Inga sapling. I dissected the entire tree, finding 3 nests excavated in live stems (there was also stem-borer damage here and there, so the ants could have been invading pre-existing cavities). I never found reproductives. Either the queen was little different from workers and escaped, or the colony was queenless, or there was more of the colony off the Inga tree.’ Another nest series (JTL/5Jul84/1017) was collected ‘inside branch of still live but recently felled Inga’ and included workers, alate queens, and a male. A third collection by Longino (JTL0900) was taken in a live Cecropia insignis sapling. Thus, the available data suggests that this species is a live-stem nester, a habit not seen in the majority of Pseudomyrmex species (Ward 1991). There is no evidence that P. nimbus keeps trophobionts (hemipterans) in these nests.

Identification
Relatively large species (HW 1.03–1.20, LHT 0.96–1.09); head only moderately elongate (CI 0.85–0.89) and with rounded posterolateral corners in full-face view (Fig. 16); eyes relatively large (REL 0.48–0.51); frontal carinae separated by less than basal scape width; metanotal groove strongly impressed, conspicuous in profile; dorsal face of propodeum generally convex in profile, rounding into declivitous face, the two subequal in length; petiole slender, elongate-triangular in profile (PLI 0.44-0.53, PL/ HL 0.58–0.60, PL/LHT 0.71–0.79); petiole with slight anterior peduncle, in profile the anterodorsal face flat to convex, ascending gradually to summit in posterior third of node, then rounding into more steeply descending posterior face; profemur relatively slender (FI 0.41–0.45); hind leg relatively long (LHT/HL 0.76–0.82). Head subopaque, densely punctulate-coriarious, the punctures becoming less dense on vertex (separated by their diameters or more). Standing pilosity sparse, absent from propodeum and mesonotum (MSC 2–5). Head, dorsum of propodeum, metafemur, and gaster dark brown, other parts of body with variably lighter coloration (medium brown to yellow-brown).

Pseudomyrmex nimbus can be recognized by the combination of large size (worker HW 1.03–1.20; queen HW 1.15–1.18), well-developed metanotal groove (in worker), and predominantly dark coloration. The relatively long legs (worker LHT/HL 0.76–0.82, queen LHT/HL 0.70–0.72) and elongate eyes (worker REL 0.48–0.51; queen REL 0.48) are also notable. This species is in the P. elongatulus complex (within the P. elongatulus group), and is a distant sister to the trio of Pseudomyrmex exoratus, Pseudomyrmex elongatulus, and Pseudomyrmex ereptor (Fig. 1).

Distribution based on Regional Taxon Lists
Neotropical Region: Costa Rica.

Nomenclature

 * . Pseudomyrmex nimbus Ward & Branstetter, 2022: 25, figs. 16, 26 (w.) COSTA RICA.

Type Material

 * Worker. COSTA RICA Alajuela: Río Peñas Blancas, 800 m, 10° 19′ N, 84° 43′W, 22–25 May 1990, ex live stems Inga sapling, wet forest clearing, J. Longino JTL2694 (MUCR) (CASENT0863541).
 * Paratypes: series of workers, same data as holotype (CASC, JTLC, LACM, MCZC, PSWC, UCDC, UNAM, USNM).