The following account of Pheidole navigans is from Sarnat et al. (2015).
Pheidole navigans is a small, short-limbed, reddish brown species that belongs to the P. flavens complex. See discussion under corresponding section of P. flavens complex for how to distinguish this species from introduced Pheidole outside the complex. Within the complex, minor workers are impossible to distinguish based on known characters. Major workers can be separated from those of P. flavens by the combination of predominantly longitudinal rugae on the posterolateral lobes, the more distinct and narrow antennal scrobe bordered mesially by strong, unbroken frontal carina, and the more continuously glossy scrobe depression.
Although the type locality of Pheidole navigans is Germany, the species was originally described by Forel from specimens intercepted during quarantine inspection of orchids originating from Veracruz, Mexico. We revise this name from synonymy and elevate it to species rank so that it can be applied to a putative species that has recently established in the southeastern United States and Hawaii. This ant has most often been referred to as P. moerens since it was first reported from Alabama nearly fifty years ago by M.R. Smith (1967). However, the examination of type specimen photographs (MCZ-ENT00009137) suggests that these introduced populations are heterospecific with P. moerens Wheeler.
Whether the introduced populations are actually conspecific with P. navigans Forel will require a thorough revision of this taxonomically vexing species complex. Of all the type material we have examined, however, that of P. navigans bears the closest resemblance in gross morphology. Thus we propose P. navigans Forel be used in place of P. moerens for referring to the aforementioned introduced populations. Future systematic study of this species should also examine P. floridana subsp. aechmeae (currently synonymized under P. flavens, but also recorded from Veracruz, Mexico) and P. flavens var. mediorubra Santschi (described from Loreto, Argentina and currently treated as a synonym of P. alacris Santschi).
The major workers of Pheidole navigans differ from those of P. moerens in the following respects. They exhibit a distinct and narrow antennal scrobe capable of receiving the entire antennal scape in repose. The scrobe is bordered by a strong, unbroken frontal carina mesially, and the depression is marked by a continuous smooth surface entirely (or nearly entirely) uninterrupted by rugulae. The rugulae of the frons extend to approximately an eye’s length distance from the posterior head margin. The anterior portion of the promesonotum is crossed by long and distinct transverse striae.
The examined major workers of P. navigans from Alabama (CASENT0106664) and Venezuela (CASENT0248831), along with those from Florida and Hawaii, and a specimen imaged from Paraguay (CASENT0178020), share a notably consistent morphology for being spread across such as wide range. The characteristics shared among these majors include the following. Frontal carinae strongly produced, forming the mesad border of a shallow but well-demarcated antennal scrobe capable of accommodating the entire scape in repose. Antennal scrobe weakly foveolate. Cephalic carinulae mostly longitudinal with very little reticulation posterior to the eye. Cephalic carinulae extending up to, but not beyond the medial excision (‘V’) of the posterior head margin. Promesonotal dome with a relatively low profile, mesonotal declivity short and relatively gradual. In dorsal view, promesonotum weakly punctate, anterior portion with distinct transverse carinulae. Although we tentatively treat the specimen from California (CASENT0005742) as P. navigans, it differs morphologically from the aforementioned specimens and bears closer resemblance to P. exigua var. tuberculata Mayr (currently synonymized under P. flavens).
The similarity of these northern hemisphere specimens to the one from Paraguay raises the possibility that these putatively conspecific populations originated in South America. Indeed, the Paraguay specimen was collected in the Reserva Natural del Bosque Mbaracayú near the Río Paraná—a region infamous for serving as a cradle of ant invasion (Suarez and Tsutsui 2008).
In Florida, Naves (1985) reported Pheidole navigans (as P. moerens) nesting under boards, at base of oak trees and fence posts, along roots, under palm leaves, inside wall crevices, and rarely in the ground. The chambers are built with small soil or debris particles and have small openings. Most nuptial flights occur in July. The species was found to practice dependent nest founding, but became monogynous before the first brood was reared. Mature colonies can support over 100 majors and over 500 workers. They feed on seeds and scavenge and prey on small dead or live arthropods, and forage very close to the nesting sites. Deyrup (2000) also provided observations of this species (as P. moerens) from Florida, adding that it occurs in both disturbed areas and mesic or moist woods, also nests in hollow twigs, nuts and in leaf litter, and is occasionally arboreal.
The species most often referred to as P. moerens in the southeastern United States, and treated here as P. navigans, has been expanding its range since it was first reported in Alabama in 1967. However, this species is not considered a major pest and is only occasionally reported to enter houses (Deyrup et al. 2000). In Louisiana P. navigans is considered a pest (Dash and Hooper-Bùi 2008). Pheidole navigans could become more regionally and possibly globally widespread in the future.
Diagnosis among introduced Pheidole. Color reddish brown. Major | HW 0.84–0.88, HL 0.88–0.91, SL 0.46–0.48, CI 95–99, SI 53–56 (n=4). Head subquadrate. Longitudinal carinae of the frons extend to approximately an eye’s length distance from the posterior head margin. Rugae of posterolateral lobes predominantly longitudinal. Posterior head margin always free of distinct rugae or rugoreticulum. Microsculpture of posterolateral lobes glossy to weakly punctate. Antennal scrobe distinct and narrow, shallow but capable of receiving the entire antennal scape in repose; bordered by strong, unbroken frontal carina mesially; depression marked by a continuous smooth surface entirely (or nearly entirely) uninterrupted by rugulae. Hypostoma with stout median and submedian teeth. Promesonotal dorsum with distinct transverse striae. Promesonotum in profile forming a single dome, lacking a distinct mound or prominence on the posterior slope. Promesonotum not strongly transverse with strongly projecting sides in dorsal view. Postpetiole not swollen relative to petiole. Postpetiole relatively narrow in dorsal view; distinctly less than 2x petiolar width. Gaster with entire first tergite glossy. Minor | HW 0.40–0.45, HL 0.45–0.50, SL 0.40–0.44, CI 86–92, SI 96–102 (n=8). Head covered in punctate microsculpture, giving it a dull appearance. Antennal scapes reach or weakly surpass posterior head margin; if they do it is usually by a distance less than eye length. Antennal scapes with standing hairs present. Promesonotum in profile forming a single dome, lacking a distinct mound or prominence on the posterior slope. Hairs on mesosoma fine and flexuous, not arranged in pairs. Pronotal humeri not angular. Postpetiole not swollen relative to petiole. Postpetiole relatively narrow; distinctly less than 2x petiolar width in dorsal view. Gaster with entire first tergite glossy.
The precise native range of P. navigans is unknown, but it is certainly of Neotropical origin. The record of the species from the Paraná region of South America suggests it could be South America. We tentatively treat both known South American records (Paraguay and Venezuela) as native, and the Mexican record as introduced, but other scenarios are equally possible. Pheidole navigans was first reported as introduced in the United States by M.R. Smith (1967) under the name P. moerens. The name P. moerens has since been applied to North American records from Alabama (Glancey et al. 1976; Smith 1967), California (Garrison 1996; Martínez 1997), Florida (Deyrup et al. 1988; Deyrup et al. 2000; Wojcik et al. 1975), Louisiana (Dash and Hooper-Bùi 2008), Mississippi (MacGown and Hill 2010), North Carolina (Guénard et al. 2012) and Texas (Wilson 2003). We tentatively treat all of these records as P. navigans, but the California and Texas records could also belong to another species in the flavens complex. In the Pacific, P. navigans is established in Hawaii (Gruner et al. 2003). We cannot confirm whether the P. moerens records from Cocos Island (Solomon and Mikheyev 2005) or the indoor records from a butterfly house in the northwestern United States (collection code KRW26Feb99) refers to P. navigans or another member of the flavens complex.