Subfamily MYRMICINAE
Tribe ATTINI
Pheidole navigans Forel, 1901
(=Pheidole moerens Wheeler, 1908, auct.)

Navigating big-headed ant

Uploaded, October 25, 2016
Author: Joe A. MacGown

Pheidole navigans, full face view of a major worker (Hancock Co., MS) (photo by Joe A. MacGown & James Lewis)

Pheidole navigans, lateral view of a minor worker (Hancock Co., MS) (photo by Joe A. MacGown & James Lewis)

Pheidole navigans, dorsal view of a minor worker (Hancock Co., MS) (photo by Joe A. MacGown & James Lewis)

Pheidole navigans, full face view of a major worker (Lauderdale Co., MS) (photo by Joe A. MacGown & James Lewis)

Pheidole navigans, lateral view of a major worker (Lauderdale Co., MS) (photo by Joe A. MacGown & James Lewis)

Pheidole navigans, lateral view of a major worker (Lauderdale Co., MS) (photo by Joe A. MacGown & James Lewis)

Pheidole navigans, hypostomal teeth of a major worker (Lauderdale Co., MS) (photo by Joe A. MacGown )

Pheidole navigans, dorsal view of petiole and postpetiole of a major worker (Lauderdale Co., MS) (photo by Joe A. MacGown )

Pheidole navigans, full face view of a male (Hinds Co., MS) (photo by Joe A. MacGown & James Lewis)

Pheidole navigans, full face view of a male (Hinds Co., MS) (photo by Joe A. MacGown & James Lewis)

Pheidole navigans, lateral view of a male (Hinds Co., MS) (photo by Joe A. MacGown & James Lewis)

Pheidole navigans, dorsal view of a male (Hinds Co., MS) (Hinds by Joe A. MacGown & James Lewis)

Pheidole navigans, lateral view of a dealate queen (Forrest Co., MS) (photo by Joe A. MacGown & James Lewis)

Pheidole navigans, lateral view of a dealate queen (Forrest Co., MS) (photo by Joe A. MacGown & James Lewis)

Introduction
The genus Pheidole has been described as 'hyperdiverse" by Wilson (2003) in large part by the abundance and diversity of the genus, especially in the Neotropics. Workers in this genus have discrete major and minor castes, with the major workers having greatly enlarged heads. In the southeastern US, 21 named species are known to occur and at least one undescribed species is known from the region. In the Southeast, members of this genus can be found in all terrestrial habitats from fine sandy localities bordering creeks to hard-pan dirt in open area beside roadways to inside rotting wood in natural forested habitats. Both major and minor workers have 12-merous antennae (including scape and pedicel) that terminates in a 3-merous club; waist with both petiole and postpetiole, and atrophied sting. Many species possess propodeal spines, although a few do not. Majors are distinctive with their greatly enlarged heads (proportional to the body), which may be as wide or as long as the mesosomal length, and by the large mostly edentate mandible. Minor workers are much smaller than majors and often have dramatic physical differences. Queens are similar to major workers, except the heads are not proportionally enlarged as compared to the body, eyes are proportionally larger to the head than with workers, ocelli are present, overall are typically larger than both minor and major workers, and have enlarged thoracic regions to accommodate wings. Males are quite different in appearance from both workers and queens being somewhat wasp-like with beaded 13-merous antennae, greatly enlarged eyes (eye length may be half of more of the head length), reduced mandibles, enlarged ocelli that bulge from posterior margin of head, and with proportionally large wings.

Pheidole navigans (Forel) (Myrmicinae), the navigating big-headed ant, is a small, reddish brown species thought to be native to the Neotropics (Sarnat et al. 2015). This species has become widespread and abundant along the Coastal Plain from southeastern Texas to eastern South Carolina. In Mississippi, this species is now well established in southern third of the state. Until a recent paper on invasive Pheidole species of the world was published by Sarnat et al. in 2015, specimens of this species from the United States and the Caribbean were identified as P. moerens Wheeler. However, after examination of type specimens, Sarnat et al. (2015) removed P. navigans from synonomy with P. flavens, and after examining southeastern US specimens, they proposed that US and Caribbean specimens previously identified as P. moerens were actually P. navigans.

Taxonomic History (Bolton 2016)
Pheidole navigans Forel, 1901: 79 (s.w.) GERMANY (Intercepted In Quarantine From Veracruz, Mexico). Neotropic. Junior synonym of Pheidole flavens: Wilson, 2003: 419. Removed from synonymy, elevated to species rank: Sarnat, Fischer, Guénard & Economo, 2015. 57.

Diagnosis
Workers of P. navigans are dimorphic with major and minor castes. Both worker castes are small, reddish brown to dark brown, have 12-segmented antennae with a 3-segmented club, promesonotum elevated above propodeum, prododeum with small spines, waist with petiole and postpetiole, and sting is atrophied. Minor workers are dark reddish brown and have dense foveolate sculpture on the face, mesosoma, and waist. They are similar to the related P. bilimeki, P. flavens, and P. floridana, but differ by having more striae on the face and usually having darker coloration. Major workers can be separated from  P. bilimeki and P. floridana by their  darker color (dark brown usually), having transverse striations on the pronotum (near the humeral area), striations on the head extending more near the occiput, and at least some rugoreticulation on the face between the eyes and frontal carina. Additionally, P. navigans seems to prefer nesting in rotting wood, whereas, P. bilimeki and P. floridana typically nest in sandy soil. Pheidole navigans majors are distinguished from P. flavens by the broader smooth space of the occiput and feebler intercarinular foveolation on the head.

Identification
Minor Worker: Small (HW 0.44–0.47 mm, HL 0.49–0.51 mm, SL 0.42–0.43  mm, EL 0.08–0.10 mm, MeSL 0.67–0.74 mm, SPL 0.08–0.10 mm, PetW 0.13–0.14 mm, PpW 0.18–0.22 mm, PronW 0.42–0.45 mm) (MEM specimens) (n = 5). Overall reddish brown to dark brown with gaster sometimes darker brown, legs orangish brown. Head slightly longer than wide; foveolate, with rugulae present from anterior margin of head (including clypeus) to just posterior of eyes (occasionally extending almost to posterior margin of head on sides of head); and with limited, loose rugoreticulation between eye and frontal carina; scattered semi erect to erect, flexuous, somewhat thickened setae scattered over the entire surface of head and mandibles, similar setae present on antennal scapes; eyes relatively small (6-8 ommatidia in longest view), located about mid point of the head at edges (in full face view), ocelli lacking: antennae 12-segmented, with 3-segmented club, scape barely surpassing posterior margin of head; mandibles triangular, with 6-7 small teeth along inner border and longer apical tooth. Mesosoma with dense foveolate sculpture; mesosomal dorsum with scattered flexuous setae of varying lengths, more numerous on promesonotum; promesonotum fused, in profile dorsum appearing as a continuous arced shape; metanotal groove prominent; propodeum, angular, distinctly set below level of promesonotum; propodeal spines present, triangular, slightly upturned. Waist two-segmented: Petiole about twice as long as wide (dorsal view), petiole pedunculate, node raised, conical in lateral view, almost circular in dorsal view; sides of petiole foveolate, dorsum shiny. Postpetiole about as wide as long (dorsal); appearing circular in both lateral and dorsal views; some foveolate sculpture present posterolaterally, remainder shiny. Scattered elongate, flexuous setae present on both waist segments. Gaster smooth and shiny, lacking obvious sculpture; with scattered elongate, flexuous setae that are directed posteriorly present; sting not present, atrophied.

Major Worker: Small, larger than minor worker (HW 0.80–0.84 mm, HL 0.86–0.91 mm, SL 0.44–0.46mm, EL 0.11–0.13mm, MeSL 0.52–0.55mm, SPL 0.05–0.06 mm, PetW 0.08–0.09 mm, PpW 0.11–0.13 mm, PronW 0.28–0.31 mm) (MEM specimens) (n = 5). Head, mesosoma, waist, and antennae dark reddish brown; gaster darker brown to somewhat infuscated at base to darker brownish black posteriorly; legs lighter, orangish brown. Head greatly enlarged, larger than mesosoma, almost as wide as long; posterior margin of head deeply concave, corners of head strongly rounded; posterior corners of head shiny, lacking sculpture, remainder of head with strong longitudinal rugae and some rugureticulation between eye and frontal carina, spaces between mostly shiny; clypeus and mandibles mostly lacking sculpture; frontal triangle prominent, shiny; entire head with scattered, but regular short, erect setae, mostly directly anteriorly and away from head on and near margins, but setae located on central portion of head directed toward midline; eyes small, located below midline of head; ocelli lacking; antennae 12-segmented, with 3-segmented club; scapes short, about half of head length; mandible triangular, enlarged, edentate except for apical tooth; venrally, five hypostomal teeth present, mid tooth wide and short, inner and outer teeth stout triangular;. Mesosoma sides and dorsum of foveolate, transverse carina present on dorsum of pronotum, a few weak carina present on mesopleura and sides of propodeum; scattered erect setae present on promesonotal dorsum; with distinct promesonotal groove present; strong metanotal groove present; propodeum set distinctly below level of promesonotum; propodeal declivity concave, propodeal spines strong, somewhat finger-like, directed upward and slightly posteriorly; propodeal spiracle circular. Waist two-segmented, with scattered elongate, flexuous setae present on both waist segments: petiole about twice as long as wide (dorsal view), pedunculate, node raised, and conical in lateral view, somewhat rectangular in dorsal view; sides of petiole foveolate, dorsum shiny; postpetiole wider than long in dorsal view, node elliptical, some foveolate sculpture present posterolaterally, remainder shiny. Gaster smooth and shiny, lacking obvious sculpture; with scattered elongate, flexuous setae that are directed posteriorly present; sting absent.

Queen: Small, larger then major worker (HW 0.81–0.90 mm, HL 0.74–0.83 mm, SL 0.45–0.49 mm, EL 0.24–0.27 mm, MeSL 1.16–1.22 mm, SPL 0.08–0.11 mm, PetW 0.23–0.25 mm, PpW 0.37–0.40 mm, PronW 0.71–0.75 mm, FWL 3.80–4.20 mm) (MEM specimens) (n = 5). Head (including mandibles and antennae), mesosoma, and waist dark reddish brown; gaster dark brownish black; legs orangish brown; and wings hyaline with dark stigma. Head slightly wider than long, widest posteriorly before gradually diverging anteriorly, posterior margin broadly concave; entire head in full face and lateral view with strong longitudinal rugae present, with transverse rugae on posterior region of head and between eye and frontal carina forming a rugoreticulate pattern, spaces between rugae shiny in mid region of head (full face view) and foveolate laterally from the approximately the edge of the frontal carina to margins of the head; entire head with numerous scattered erect setae with setae on sides of head directed posteriorly away from the head and setae on central region of head directed toward midline of head; scapes with numerous erect setae; eyes large, length of eye less than the distance between eye and posterior edge of head; ocelli conspicuous; frontal triangle obvious, with rugae from head extending through it; antennae 12-segmented, with 3-segmented club; scapes short, length about 2/3 the head length; mandible large, triangulare edentate except apical tooth. Mesosoma rounded recangular, dorsally flattened; overal shiny, mostly lacking sculpture dorsally, pronotum and propodeum with rugulate sculpture, mesopleura mostly lacking sculpture, smooth; entire mesosoma with numerous short, semi erect curved setae that are directed posteriorly. Wings translucent with light brown tint and light brown venation; forewing venation of typical myrmicine type with pterostigma and closed costal, basal, subbasal, discal, marginal, and two submarginal cells closed; hindwing lacking jugal lobe and with costal, basal, and subbasal cells. Waist two segmented, with scattered elongate, flexuous setae present on both segments: petiole about twice as long as wide (dorsal view), pedunculate, in lateral view node raised and conical, somewhat rectangular in dorsal view, sides of petiole foveolate, dorsum shiny; postpetiole wider than long in dorsal view, almost twice as long as petiole, node elliptical (dorsal view), some foveolate sculpture present posterolaterally, remainder shiny. Gaster smooth and shiny, lacking obvious sculpture; with scattered elongate, flexuous setae that are directed posteriorly present; lacking sting.

Male: Small (HW 0.59–0.60 mm, HL 0.47–0.50 mm, SL 0.12 mm, EL 0.29–0.32 mm, MeSL 1.12–1.16 mm, PetW 0.15–0.16 mm, PpW 0.20–0.23 mm, PronW 0.65–0.68 mm, FWL 2.80–3.04 mm) (MEM specimens) (n = 3). Head dark brown; mesosoma, waist and gaster light brown; antennae and legs pale yellow brown; and wings hyaline. Head longer than wide, full face view head widest posteriorly measuring from outer edge of eye to eye, somewhat hexagonal in shape (excluding eyes); clypeal area with some longitudinal carinae, remainder of head with strong, tightly woven rugoreticulation; numerous erect setae of varying lengths present on head; scapes with short, semi erect setae, funiculus dense pubescent; mesosoma, waist, and gaster mostly shiny, lacking sculpture, with numerous short, semi erect setae present; eyes huge, more than half the length of head, placed on lower half of head; ocelli large and protruding; antennae 13-segmented, beaded, scapes short and rectangular, pedicel globular. Mesosoma rounded rectangular; shiny with limited sculpture; with numerous, short, erect setae; pronotum somewhat trapzoidal; mesoscutum broadly rounded anteriorly and overhanging pronotum; mesoscutellum with a declivious face that is distinctly elevated above propodeum; propodeal declivity irregular. Wings pale translucent hyaline with veins light brown; forewing venation of typical myrmicine type with pterostigma and costal, basal, subbasal, discal, and two submarginal cells closed; hindwing lacking jugal lobe and with costal, basal, and subbasal cells closed. Waist two-segmented, with limited sculputure, numerous erect setae present: petiole pedunculate, elongate, node barely raised; postpetiole short, rounded, somewhat circular. Gaster shiny, with numerous erect setate present.

Biology
This species is a common species in the Gulf Coast region being found from western Texas to eastern Georgia. We (MEM researchers) routinely find this species nesting in natural wooded habitats in rotting wood, especially logs found laying on the ground, or in the soil just under rotting wood, but also in suburban landscapes under boards, under bark of dead and live trees, at bases of oak trees and fence posts, along roots, under palm leaves, and inside wall crevices. The MEM has found this species in the following habitats and microhabitats: live oak and palmetto litter, slash pine litter, magnolia litter, leaf litter in hind dunes, chenier woodland, coastal savanna, mixed pine/hardwood forest near estuary, longleaf pine forest, pine oak forest near coastal dunes, longleaf pine/palmetto scrub, relic dune scrub, oak-pine sandhill, foredunes, sandy xeric scrub, longleaf pine/hardwood forest, a mixed oak/pine forest, hardwood forest, oak-hickory forest, riparian hardwood forest, open grassy area by highway, long roadside, under mulch at plant nursery, in house (swarming), along railroad tracks, and on palm trees at nursery. In general, the diet of  P. navigans appears to be seeds, small arthropods, and scavenged human food (in urban areas). This species is attracted to hotdog bait, cookie bait, brown sugar/yeast bait, peanut butter, and other baits.

Colony sizes range range from small (a few hundred workers) to relatively large with thousands of workers with about five times as many minor workers as major workers. The individual chambers within a colony are small and usually are built with small soil or debris particles and have small openings. Colonies are monogynous, although several queens may start founding a nest. Before the first brood emerges, the dominant female typically kills the other queens. As with many other introduced species, P. navigans reproductives have a broad flight period, and the MEM has flight records from early May to late October. I have frequently collected P. navigans nesting side by side with the large introduced ponerine ant, Odontomachus haematodus. Unlike many of our native species of Pheidole, it is not unusual to see major workers of P. navigans foraging with minor workers.

Pest Status
According to Deyrup, et al. (2000), this species rarely enters homes. However, workers are readily attracted to both sugary and protein baits, and could pose a nuisance threat in areas with high populations. Additionally, the MEM has received numerous complaints of large numbers of winged reproductives swarming in homes causing a nuisance. Some homeowners were concerned that the dark colored females were swarming termites. As with other members of the genus, the sting is atrophied, and therefore this species does not pose a stinging threat to humans.

It seems likely that dense populations of this species have some effect on native insects that serve as prey, but the diet of P. navigans has not been investigated in detail. Deyrup, et al. (2000) reported that this species occurs throughout Florida in a wide variety of habitats, and that it may have already partially replaced P. dentigula and P. bilimeki (=floridana auct.). Because colonies are often in rotten wood, in leaf litter, and in hollow twigs and nuts on the ground or occasionally arboreal, so P. navigans may compete for nest sites with native species in the genera Aphaenogaster, Camponotus, Nylanderia, Solenopsis, Hypoponera, Strumigenys, Brachymyrmex, and other species

Distribution
Native Range: Neotropics, possibly South America (Sarnat et al. 2015).

Oceania: Hawaii (AntWeb.org, Sarnat et al. 2015).
Nearctic: United States (AntWeb.org, MEM, Sarnat et al. 2015).

U.S. Distribution: AL, CA, FL, GA, HI, LA, MS, NC, SC, TX, WA (AntWeb.org and MEM).   
Southeastern U.S. Distribution: AL, FL, GA, LA, MS, NC, SC (AntWeb.org and MEM).

Acknowledgments
Funding for the ant work being done by the MEM in Alabama and Mississippi is from several sources including the National Institute of Food and Agriculture, United States Department of Agriculture, under Project No. MIS-012040, the Mississippi Agricultural and Forestry Experiment Station at Mississippi State University, with support from State Project MIS-311080, NSF Grants BSR-9024810 and DFB-9200856, the Tombigbee National Forest (U.S. Forest Service), the Noxubee Wildlife Refuge, Mississippi Natural Heritage Program Research Grant, USDA Forest Service Agreement No. 08-99-07-CCS-010, the William H. Cross Expedition Fund, and primarily by the USDA-ARS Areawide Management of Imported Fire Ant Project. Additionally, special cooperation has been provided by State Parks, National Forests, National Wildlife Refuges, the Natchez Trace Parkway, and from various private landowners in both Alabama and Mississippi.

Literature Cited

Bolton, B. 2016.  Bolton World Catalog Ants. Available online: http://www.antweb.org/world.jsp. Accessed 9 February 2016.

Forel, A. 1901. Formiciden des Naturhistorischen Museums zu Hamburg. Neue Calyptomyrmex-, Dacryon-, Podomyrma- und Echinopla-Arten. Mitteilungen aus dem Naturhistorischen Museum in Hamburg 18:43-82.

Naves, M. A. 1985. A monograph of the genus Pheidole in Florida, USA (Hymenoptera: Formicidae). Insecta Mundi 1:53-90.

Sarnat, E.M.; Fischer, G.; Guénard, B.; Economo, E.P. 2015. Introduced Pheidole of the world: taxonomy, biology and distribution. ZooKeys 533:1-109.

Wheeler, W. M. 1908. The ants of Porto Rico and the Virgin Islands. Bulletin of the American Museum of Natural History 24:117-158. 

Wilson, E. O. 2003. Pheidole in the New World. A dominant, hyperdiverse ant genus. Cambridge, Mass.: Harvard University Press, [ix] + 794 pp.

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