Trachymyrmex septentrionalis

In suitable habitats, typically semi-open vegetation types occurring on sandy soil, this ant can be locally abundant. In such areas, the conspicuous crescent-shaped soil mound found around their ground nest entrances occur in high densities.

Identification
Trachymyrmex septentrionalis is disturbingly similar to the allopatric Trachymyrmex carinatus, but separable by the characters given in the key and by their non-overlapping distributions; carinatus occurs only in the desert southwest, septentrionalis occurs in Texas and the states north and east of there. Molecular evidence suggests the similarity is convergent. It is likely that collections identified in the literature as T. septentrionalis from Durango, Mexico may actually represent T. carinatus (Rojas-Fernandez 1994). Solely considering the publication date, Oecodoma virginiana Buckley (1867) would have seniority over McCook’s (1881) Atta septentrionalis. But since Buckley’s species description is insufficiently detailed to either recognize T. septentrionalis or distinguish between septentrionalis and its congeners, Wheeler (1902) proposed Oocodoma virginiana Buckley to be a junior synonym of Atta septentrionalis McCook. We agree with Wheeler and continue using the species name septentrionalis. Unfortunately, the locality of both types is unknown and we could not examine them. (Rabeling et al. 2007)

Distribution
Trachymyrmex septentrionalis has the most extensive distribution of all attine ants in the US. It has been collected from Texas to Florida and as far north as central Illinois, southern Ohio and Long Island, New York. (Rabeling et al. 2007)

Distribution based on Regional Taxon Lists
Nearctic Region: United States. Neotropical Region: Mexico.

Habitat
Like most fungus-growing ants, the distribution of T. septentrionalis is locally patchy. In part, this may be a consequence of the distribution of suitable soils. In the northern part of its range, T. septentrionalis occurs exclusively on pure sand soils in open habitats and open woodlands of the Pine Barrens. In the southern US, T. septentrionalis is abundant in a wide variety of similar oak and pine dominated habitat types, all characterized by very sandy soils and light (if any) shade. It has also occasionally been found nesting in sandy clay soils in well-developed forests with considerable shade, particularly in the southern part of its range. (Rabeling et al. 2007)

Biology
From Rabeling et al. (2007): During periods of high nest excavation activity in spring and fall, the crescent shaped mounds of T. septentrionalis are distinctive and conspicuous (Tschinkel & Bhatkar 1974). Older colonies may have several chambers, connected by one to few tunnels and inhabit a few hundred individuals (Weber 1972). As the northernmost distributed species of Trachymyrmex, colonies of T. septentrionalis hibernate during the winter and the fungus garden assumes a dormant condition. The length of hibernation varies considerably with respect to latitude. The northernmost populations in New York and New Jersey have short active periods from early May to September, whereas Florida populations remain active during the winter, as long as temperatures exceed 18oC (Weber 1972). The army ant Neivamyrmex nigrescens and the ectatommine ant Gnamptogenys hartmani are known to raid colonies of T. septentrionalis (Cole 1939).

Nomenclature

 * virginiana. Oecodoma virginiana Buckley, 1867: 346 (w.) U.S.A. Junior synonym of septentrionalis: Wheeler, W.M. 1902f: 29.
 *  septentrionalis. Atta septentrionalis McCook, 1881a: 362 (w.) U.S.A. Wheeler, W.M. 1907c: 707 (q.m.); Wheeler, G.C. 1949: 673 (l.). Combination in Atta (Trachymyrmex): Forel, 1912e: 182; in Trachymyrmex: Gallardo, 1916b: 242. Senior synonym of virginiana: Wheeler, W.M. 1902f: 29; of vertebrata: Creighton, 1950a: 323; of obscurior, seminole: Weber, 1958b: 53. See also: Wheeler, W.M. 1911g: 246; Smith, D.R. 1979: 1411; Petralia & Vinson, 1980: 383; Rabeling, Cover, et al. 2007: 17.
 * obscurior. Atta (Trachymyrmex) septentrionalis var. obscurior Wheeler, W.M. 1907c: 709 (w.q.) U.S.A. Wheeler, W.M. 1911g: 246 (q.). Combination in Trachymyrmex: Creighton, 1950a: 324. Subspecies of septentrionalis: Wheeler, W.M. 1911g: 247. Material of the unavailable names crystallina, irrorata referred here by Creighton, 1950a: 324. Junior synonym of septentrionalis: Weber, 1958b: 53.
 * vertebrata. Atta (Trachymyrmex) septentrionalis var. vertebrata Wheeler, W.M. 1911g: 246 (w.q.) U.S.A. Junior synonym of septentrionalis: Creighton, 1950a: 323.
 * seminole. Trachymyrmex septentrionalis subsp. seminole Creighton, 1950a: 324 (w.q.m.) U.S.A. [First available use of Atta (Trachymyrmex) septentrionalis subsp. obscurior var. seminole Wheeler, W.M. 1911g: 247; unavailable name.] Junior synonym of septentrionalis: Weber, 1958b: 53.

Worker
Diagnosis from Rabeling et al. (2007): HL 0.84–0.96, HW 0.88–1.0, CI 104–105, SL 0.84–0.96, SI 96, ML 1.12–1.32. A relatively small (HL 0.84–0.96, HW 0.88–1.0), conspicuously tuberculate species with normally proportioned legs and antennal scapes (SI 96). Head slightly broader than long (CI 104–105), sides subparallel behind the eyes, gradually tapering anteriorly between the eyes and the mandibular insertion. Posterior margin moderately concave. Preocular carinae long, traversing 2/3 to 3⁄4 of the distance between the eye and the frontal carina but not nearly touching the frontal carinae. In full-face view, frontal lobes rounded or subtriangular, usually somewhat asymmetric, with the anterior side longer than the posterior. Anterolateral promesonotal teeth well- developed, thin, sharply pointed in dorsal view, projecting somewhat upward as well as horizontally. Anterior median pronotal tubercles upright, toothlike in posterior view. Propodeal teeth usually longer than the distance separating their bases, often spinelike, pointed. Posterior margin of head, mesonotal dorsum, dorsal surfaces of petiole, postpetiole and gaster conspicuously tuberculate, tuberculi largest particularly on posterior corners of head and on the first gastric tergite. Color brownish yellow to medium reddish-brown.

Queen
Diagnosis from Rabeling et al. (2007): HL 1.05–1.2, HW 1.1–1.25, CI 104–107, SL 1.05–1.15, SI 96–100, ML 1.55–1.85. As in worker diagnosis, but with caste-specific morphology the mesosoma related to wing-bearing. Dorsolateral pronotal teeth variable in size, but sharply triangulate in dorsal view, often blunt in anterior view. Ventrolateral pronotal teeth variable, but generally lobelike. Mesoscutum coarsely, longitudinally rugulose, finely tuberculate, tuberculi with short, sharply recurved hairs. First gastric tergite with numerous small tubercles bearing short, recurved, decumbent, recurved setae.

Male
Diagnosis from Rabeling et al. (2007): HL 0.75, HW 0.8–0.9, CI 100–107, SL 0.9–1.1, SI 113–122, ML 1.5–1.85. In frontal view, posterior corners of the head angulate, angles formed by short, tuberculate ridges best seen in dorsal view; ocelli small and inconspicuous. Dorsoventral pronotal teeth, short, sharp, triangulate in dorsal view; ventrolateral teeth short, triangular or lobelike. Sculpture of mesoscutum coarse, irregular, rugulose, sometimes weakly longitudinal. Tuberculi generally absent (miniscule, if present) on first gastric tergite.

Etymology
McCook (1881) described the worker of T. septentrionalis based on material collected from Island Heights in the New Jersey Pine Barrens. The species name septentrionalis refers to the seven plowing oxen, the brightest stars of the Great Bear constellation, which dominate the skies of the northern hemisphere. Thus, septentrionalis loosely translates to “northern” in the context of North American fungus-growing ants. (Rabeling et al. 2007).