Trachymyrmex arizonensis

This species is common in its preferred habitats within its range. They can often be located by finding a messy soil crater around their nest entrance, along with a distinctive yellowish-gray colored external refuse midden located nearby.

Identification
Trachymyrmex arizonensis is often sympatric in central and southern Arizona with the slightly smaller Trachymyrmex carinatus and rarely sympatric with the larger Trachymyrmex nogalensis. It is easily distinguished from all other North American Trachymyrmex by the unusual shape of the frontal lobes in both workers and queens. (Rabeling et al. 2007)

Distribution
From Rabeling et al. (2007): Trachymyrmex arizonensis is typically found at mid elevations (1000–2000 m) in mountainous areas within the Chihuahuan and Sonoran Deserts in central and southern Arizona, western New Mexico, and the Mexican states of Chihuahua and Sonora. The species has also been reported from western Texas. Weber identified a single specimen of T. arizonensis from the Chisos Mountains (Van Pelt 1983). It is also reported from west Texas by O’Keefe et al. (2000), but as we have not been able to verify these records, the presence of T. arizonensis in western Texas remains uncertain.

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

Habitat
Trachymyrmex arizonensis occurs in a variety of habitats including arid Ocotillo- and Acacia-dominated scrub in mountain foothills, oak-juniperpine woodlands, and relatively mesic mid elevation creek valley forests. (Rabeling et al. 2007)

Biology
From Rabeling et al. (2007):

Nests are found under rocks or logs or in open soil, frequently in areas that are partly or lightly shaded. A sloppy crater of excavated soil and a diagnostic yellowish-gray external refuse midden is often present near the nest entrance. Trachymyrmex arizonensis and Trachymyrmex smithi are the only US species of Trachymyrmex that routinely have conspicuous external refuse middens near their nest entrances. Other species occasionally accumulate a small refuse pile close to the nest, but these are usually ephemeral.

Colony-founding queens of T. arizonensis are frequently found under rocks. Older colonies often have 3–5 fungus garden chambers and may contain well over 1000 workers (R.A. Johnson pers. obs.; see also Wheeler 1911).

Other Ants
From Rabeling et al. (2007): Trachymyrmex arizonensis is associated with Strumigenys arizonica, a tiny dacetine ant that has been found only within or adjacent to T. arizonensis nests (Ward 1988; see also Yéo et al. 2006). Most species in the genus Strumigenys are specialist predators on Collembola and strongly prefer relatively mesic habitats. We suspect that S. arizonica benefits from the controlled, moist microenvironment the Trachymyrmex provide for their fungal symbiont and feeds on the numerous collembolans that live in the chambers and refuse piles of the Trachymyrmex colony (Johnson & Cover, unpublished data).

In the mountains of southern Arizona, two army ant species, Neivamyrmex nigrescens and Neivamyrmex rugulosus, prey on T. arizonensis (Miranda et al. 1980, LaPolla et al. 2002). In Tamaulipas, Mexico, N. texanus was observed raiding a colony of T. saussurei (Rabeling & Sanchez-Peña, unpublished data). Based on these few observations, army ants seem to be important predators of at least some Trachymyrmex species, and their raids may result in a significant brood loss and partial destruction of the fungus garden (LaPolla et al. 2002).

Nomenclature

 *  arizonensis. Atta (Trachymyrmex) arizonensis Wheeler, W.M. 1907c: 710, pl. 49, figs. 9, 10 (q.m.) U.S.A. Wheeler, W.M. 1911e: 93 (w.). Combination in Trachymyrmex: Gallardo, 1916b: 242. See also: Rabeling, Cover, et al. 2007: 7.

Worker
From Rabeling et al. (2007): HL 0.88–1.20, HW 0.88–1.28, CI 96–107, SL 0.92–1.4, SI 103–113, ML 1.28–1.8. Large species (HL 0.88–1.20, HW 0.88–1.28) with relatively long legs and antennae (SI 103–113). Head as long as broad or slightly longer than broad (CI 96–107), gradually tapering anteriorly, widest at midpoint between eye and posterior margin. Frontal lobes well developed and strongly asymmetric, with a long, curving anterior margin that meets the much shorter posterior margin to form an acute angle. A broad notch is formed by the frontal lobe and the posterior continuation of the frontal carinae (Figure 1B). Preocular carinae sharply curving mesially and nearly always distinctly separated from the frontal carinae. Anterolateral promesonotal teeth often sharp, spinelike, directed laterally, not upwards. Propodeal teeth thin, spinelike, strongly divergent in dorsal view, shorter than the distance between their bases. Head, mesosoma and petiole moderately tuberculate, postpetiole and first gastric tergite strongly tuberulate. Color brownish yellow to medium reddish brown.

Queen
From Rabeling et al. (2007): HL 1.19–1.38, HW 1.19–1.38, CI 100, SL 1.25–1.31, SI 96–105, ML 1.88–2.13. As in worker diagnosis, but mesosoma with caste-specific morphology related to wing-bearing and head with minute ocelli. Dorsolateral pronotal teeth large, robust, and tuberculate; ventrolateral pronotal teeth large, blunt, and lacking tuberculi.

Male
From Rabeling et al. (2007): HL 0.98, HW 0.88, CI 93, SL 1.06, SI 121, ML 2.0–2.06. Legs and antennal scapes relatively long. Dorsolateral and ventrolateral pronotal teeth well-developed. Mesoscutum longer than broad, sculpture variable but longitudinal rugulae always present. First gastric tergite with “bumpy” surface. 1–3 toothlike tubercles present on each posterior corner of head and frontal lobes bluntly triangular, more or less symmetrical.

Etymology
Since Wheeler (1907, 1911) collected both the type series and subsequently the workers of Trachymyrmex arizonensis in southeast Arizona, the collection locality clearly motivated the species name. (Rabeling et al. 2007)

Additional References

 * La Polla, J. S., Mueller, U. G., Seid, M. & Cover, S. P. (2002) Predation by the army ant Neivamyrmex rugulosus' on the fungus-growing ant Trachymyrmex arizonensis''. Insectes Sociaux, 49, 251–256.


 * Mirenda, J. T., Eakins, D.G., Gravelle, K. & Topoff, H. (1980) Predatory behavior and prey selection by army ants in a desert-grassland habitat. Behavioral Ecology and Sociobiology, 7, 119–127.


 * O’Keefe, S. T., Cook J. L., Dudek T., Wunneburger D. F., Guzman M. D., Coulson R. N. & Vinson S. B. (2000). The distribution of Texas ants. Southwestern Entomologist, 22 (Supplement), 1–93.


 * Rabeling, C., S. P. Cover, R. A. Johnson, and U. G. Mueller. 2007. A review of the North American species of the fungus-gardening ant genus Trachymyrmex (Hymenoptera : Formicidae). Zootaxa.1-53.


 * Van Pelt, A. F. 1983. Ants of the Chisos Mountains, Texas (Hymenoptera: Formicidae). Southwestern Naturalist. 28:137-142.


 * Ward, P. S. (1988) Mesic elements in the western Nearctic ant fauna: taxonomic and biological notes on Amblyopone, Proceratium and Smithistruma (Hymenoptera: Formicidae). Journal of the Kansas Entomological Society, 61, 102–124.


 * Yéo, K., Molet, M. & Peeters, C. (2006) When David and Goliath share a home: compound nesting of Pyramica and Platythyrea ants. Insectes Sociaux, 53, 435–438.