Pheidole passivaeferox

This species occurs in lower rainforest. It is a specialized plant ant, nesting in domatia of several species of myrmecophytic Piper (Risch et al. 1977, Letourneau 1983, Tepe et al. 2004).

Identification
Minor: face smooth and shining, with variably developed faint patches of foveolation at margins; head rounded behind; promesonotal groove absent; pronotum mostly smooth and shining, with patches of weak foveolation on humeri and ventral margins; promesonotal dorsum grading to foveolate posteriorly; katepisternum foveolate; propodeal spines relatively thin and spiniform, about one third length of posterior face of propodeum; gaster smooth and shining; abundant, flexuous, almost woolly setae on mesosomal dorsum, tibiae, and gastral dorsum; color yellow brown.

Major: inner hypostomal teeth distinct, closely spaced; scape base terete; frontal carinae produced anteriorly as elevated, triangular teeth, most visible in profile; head elongate rectangular; scrobes shallowly impressed; face heavily sculptured, mostly reticulate rugose, rugae becoming more transversely oriented posteriorly; propodeal spines spiniform, about one fourth length of posterior face of propodeum; gastral dorsum smooth and shiny; abundant erect setae on face, sides of head, mesosomal dorsum, tibiae, and gastral dorsum.

Measurements, minor worker: HW 0.51, HL 0.58, SL 0.40, EL 0.12, WL 0.73, PSL 0.04, PTW 0.12, PPW 0.16, CI 88, SI 79, PSLI 9, PPI 132 (n=7).

Measurements, major worker: HW 0.80, HL 1.08, SL 0.43, EL 0.14, WL 1.04, PSL 0.09, PTW 0.18, PPW 0.26, CI 74, SI 54, PSLI 11, PPI 141 (n=5).

Measurements, queen: HW 0.71, HL 0.90, SL 0.46, EL 0.24, WL 1.29, PSL 0.07, PTW 0.20, PPW 0.32, CI 80, SI 65, PSLI 10, PPI 164 (n=2).

Previously thought to be a single species, Pheidole bicornis, morphological discontinuities suggest a clade of three related species are involved. The types of P. bicornis, which are from far southwestern Panama, and almost all the collections from the adjacent Pacific lowlands of Costa Rica have major workers with relatively broad heads (CI 80-83; n=2). All the collections from the Atlantic lowlands of Costa Rica and Nicaragua have noticeably more narrow heads (CI 72-76; n=5) and are here considered to be P. passivaeferox. Both of these species have the minor workers with a smooth, shiny face. Disjunctions of insect clades across the mountains of Costa Rica are repeatedly being shown to be relatively old, with varying degrees of morphological divergence and at a level of genetic and morphological separation that warrants species status.

An unexpected discovery was a third species, Pheidole bicornisculpta, from an ant Piper on the Pacific slope of Costa Rica, very near populations of true P. bicornis. This species has the major worker smaller overall (HW 0.92 vs. > 1.00), intermediate with respect to head shape (CI 78), and with a striking difference in the head shape and sculpture of the minor worker. The face of the minor is fully foveolate, overlain with faint rugulae. Additional differentiating characters are in the species description.

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

Biology
One of the myrmecophytic systems that has evolved in the Neotropics involves the plant genus Piper and specialized Pheidole inhabitants. A set of multiple Piper species have modified clasping petioles and stem pith that favors nesting by Pheidole. These Piper species occur in lowland rainforests from parts of Nicaragua, across all of Costa Rica, and into parts of adjacent Panama. The evolution of the plants involved has received attention (Tepe et al. 2004), as have the ecological relationships of ants and plants (Letourneau 1983), but the taxonomy of the ants has been neglected. It is now known that three species are involved, Pheidole bicornis, Pheidole bicornisculpta and P. passivaeferox.

Queen
Images from AntWeb

Nomenclature

 *  passivaeferox. Pheidole passivaeferox Longino, 2019: 52, fig. 25 (w.q.) COSTA RICA.

Type Material

 * HOLOTYPE: 1 major worker, Costa Rica, Heredia: La Selva Biological Station, 10.43333 -84.01667 ± 2 km, 50 m, 21-Jul-1986, lowland rainforest, nest in Piper coenocladum (J. Longino, JTL1429) [MUCR, unique specimen identifier INBIOCRI002279624].
 * PARATYPES: major, minor workers: same data as holotype [CAS, DZUP, JTLC, MCZC, MUCR, USNM].

Etymology
In reference to the "passive aggressive" nature of host plant defense by this specialized plant ant (Letourneau 1983).

References based on Global Ant Biodiversity Informatics

 * Longino J. T. 2019. Pheidole (Hymenoptera, Formicidae) of Middle American wet forest. Zootaxa 4599: 1-126