Trachymyrmex arizonensis

T. arizonensis (Wheeler)

Atta (Trachymyrmex) arizonensis Wheeler, 1907: 710. Syntype queen and males, Palmerlee, Cochise County, Arizona, U.S.A. (MCZC, USNM) [examined]

Atta (Trachymyrmex) arizonensis Wheeler; Wheeler 1911: 93 [description of worker]

Trachymyrmex arizonensis (Wheeler); Gallardo 1916: 242 [Combination in Trachymyrmex]

Diagnosis

Worker: 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: 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: 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.

Discussion

Trachymyrmex arizonensis is often sympatric in central and southern Arizona with the slightly smaller T. carinatus and rarely sympatric with the larger T. nogalensis. It is easily distinguished from all other North American Trachymyrmex by the unusual shape of the frontal lobes in both workers and queens (Figure 1B).

Etymology

Since Wheeler (1907, 1911) collected both the type series and subsequently the workers of T. arizonensis in southeast Arizona, the collection locality clearly motivated the species name.

Biology

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 (Figure 22A). 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.

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. 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 T. 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).

Trachymyrmex arizonensis is associated with Pyramica arizonica (Ward), 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 Pyramica are specialist predators on Collembola and strongly prefer relatively mesic habitats. We suspect that P. 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 N. 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).

Additional material examined: U.S.A.: Arizona, Cochise County: 1.6km NW Portal (RA Johnson), 5.5km W Portal (C Rabeling), 8.1mi SE Sunnyside (RR Snelling), Chiricahua Mtns. Southwestern Research Station (G Alpert, WS Creighton, RA Johnson, J LaPolla, RA Mendez, UG Mueller, C Rabeling & SP Cover), Dragoon (WM Wheeler), Huachuca Mtns. Sunnyside Canyon (SP Cover), Huachuca Mtns. Miller Canyon (WM Wheeler), Huachuca Mtns. Hunters Canyon (WM Wheeler); Coconino County: Hualpais Mtns. (DJ & JN Knull); Gila County: 12.1mi NE Globe (RA Johnson), Hwy 288 at 14.8mi N Salt River (RA Johnson, SP Cover), Sierra Ancha Pocket Creek (RA Johnson, C Strehl); Graham County: Graham Mtns, Post Canyon (WM Wheeler); Pima County: Baboquivari Mtns. (WS Creighton), Santa Catalina Mtns. Old Mt. Lemmon Rd. (RA Johnson), Tucson Sabino Canyon (DJ & JN Knull); Pinal County: USFS Rd. 287 at Pinto Creek (RA Johnson); Santa Cruz County: 1.5mi W Ruby on USFS Rd. 39 (RA Johnson), 1mi E Atascosa Lookout (RA Johnson), 3.8mi SE Jct FSR139 on FSR58 (SP Cover), Pajarito Mtns. 0.1mi W Jct. FSR 4181 on FSR39 (SP Cover), Pajarito Mtns. Sycamore Canyon (RA Johnson), Pajarito Mtns. Yanks Canyon (RA Johnson), Tumacacori Mtns. (DJ & KN Knull), 1.1mi W San Raphael Valley (RA Johnson), Pena Blanca Canyon (RA Johnson), Willow spring canyon (RA Johnson); New Mexico, Grant County: 60km E Silver City (W Mackay); Texas, Brewster County: Big Bend National Park (A van Pelt); MEXICO: Chihuahua: Hwy. 16 at 44 mi E Yecora (RA Johnson); Sonora: Sierra Mazatan (RA Johnson), without locality information (V Roth).