Trachymyrmex desertorum, (Wheeler)

Rabeling, Ch., Cover, S. P., Johnson, R. A. & Mueller, U. G., 2007, A review of the North American species of the fungus-gardening ant genus Trachymyrmex (Hymenoptera: Formicidae)., Zootaxa 1664, pp. 1-53 : 10-11

T. desertorum (Wheeler)

Atta (Trachymyrmex) desertorum Wheeler , 1911: 98. Syntype workers, Carnegie Desert Botanical Laboratory, Tucson , Arizona, U.S.A. ( MCZC, AMNH) [examined]

Trachymyrmex desertorum (Wheeler) ; Creighton 1950: 321 [Combination in Trachymyrmex ]

Diagnosis

Worker: HL 0.8-1.12, HW 0.8-1.28, CI 100-114, SL 0.8-1.16, SI 91-100, ML 1.08-1.76. A mediumsized, relatively robust species (HL 0.8-1.12, HW 0.8-1.28) with relatively short antennal scapes (SI 91-100) that surpass the posterior corners of the head by at least their maximum diameter. Head broader than long in most workers, broad as long in some small workers (CI 100-114), gradually tapering anteriorly behind the eyes, more strongly tapering between the eyes and mandibular insertions. Posterior margin of head slightly to moderately concave. Preocular carinae short, traversing about half the distance between the eye and the frontal carinae. In full-face view, frontal lobes simple, rounded or subtriangular, more or less symmetrical in shape (anterior side sometimes slightly longer than posterior). Anterolateral promesonotal teeth short, thick, usually pointed, not blunt. Propodeal teeth sharply pointed, shorter than the distance between their bases. Dorsal surface of body moderately tuberculate, but tuberculi are generally small and their setae short and strongly recurved. Side of mesosoma sparsely tuberculate, tuberculi very small, scarcely visible. Color variable, ranging from brownish yellow to medium reddish-brown.

Queen: HL 1.2-1.25, HW 1.35-1.4, CI 89-113, SL 1.15, SI 82-85, ML 2.0-2.05. Generally as in worker diagnosis, except with typical caste-specific structures related to wing-bearing, and head with minute ocelli. Dorsolateral pronotal teeth prominent, broadly triangular, sharply pointed. Ventrolateral pronotal teeth short, triangular, and more or less pointed. Mesoscutum longitudinally rugulose, minutely tuberculate, setae abundant, short, straight, and suberect.

Male: HL 0.95, HW 1.0, CI 105, SL 0.9, SI 90, ML 2.05. In dorsal view, dorsolateral pronotal teeth short, sharp, and broadly triangular. Ventrolateral pronotal teeth small, more or less triangular. Irregular rugulae present on all surfaces of pronotum; mesoscutum covered with coarse, longitudinal, slightly reticulate rugulae. Antennal scrobe granulate, with at least several small transverse rugulae distributed over anterior 3/4. First gastric tergite minutely tuberculate, with abundant short, weakly recurved, decumbent seta.

Discussion

Trachymyrmex desertorum is broadly sympatric throughout much of southern Arizona with the similar T carinatus and T pomonae (see distribution maps), but it generally occurs at lower elevations in true desert habitats, rather than in mid elevation woodlands or forests. Females can be distinguished from those of T. carinatus by head shape (square to longer than broad in T. carinatus ), short preocular carinae that do not closely approach the frontal carinae (closely approaching the frontal carinae in T. carinatus ), and shorter antennal scapes. It may be distinguished from T. pomonae by its slightly asymmetric frontal lobes (lobes strongly asymmetric in T. pomonae ). In the field this ant is most likely to be confused with small workers of Acromyrmex versicolor Pergande , which is common in many of the same habitats. In Acromyrmex the head is cordate, the mesosoma is spinose, rather than tuberculate, and the frontal lobes have two short laterallydirected teeth. None of these characters is present in T. desertorum .

Etymology

Wheeler collected the T. desertorum types a few hundred meters from the Carnegie Desert Botanical Laboratory in Tucson, Arizona, on the bank of a dry arroyo that skirts Tumamoc hill in the "…feeble shade of the Parkinsonia and Acacia trees in the very hard, pebbly, desert soil…" (Wheeler 1910, p.100). This typical Sonoran Desert habitat no doubt inspired the species name.

Biology

Trachymyrmex desertorum is a Sonoran Desert species occurring at 530-840 m elevation in central and southern Arizona and the Mexican State of Sonora. A Trachymyrmex record from Willacy County, Texas (coll. Creighton 8-XI-1951) is erroneously cited as T. desertorum in the literature (Wheeler & Wheeler 1985; O 'Keefe et al. 2000). The specimen belongs to T. turrifex and is currently deposited in the Jeanette Wheeler Collection at the University of Arizona. Therefore, there is no evidence that T. desertorum occurs in west Texas.

Typically, T. desertorum occurs in Sonoran Desert habitats with palo verde ( Parkinsonia spp.), creosote bush ( Larrea tridentata ), Acacia , jojoba ( Simmondsia chinensis ), honey mesquite ( Prosopis glandulosa ), and saguaro ( Carnegiea gigantea ). Nests are often in the shade under palo verde or mesquite trees, and are sometimes common in small washes. Foragers have been observed to collect green leaflets and fresh flower petals, but they have not been observed climbing plants and cutting live vegetation (C. Rabeling, personal observation). Nest craters are moderate in size (10-20 cm in diameter), conical in shape, and can be confused with the small craters of incipient A. versicolor nests. Trachymyrmex desertorum nests in very rocky soil, such that the limited excavatable space between the boulders often results in amorphously shaped fungus chambers. Nests contain 1 to 3 chambers that are placed up to 120 cm below the surface. Mating flights occur near dawn on mornings following summer rains. The single flight observed to date (J. Weser, pers. comm.) occurred on the same day as a mating flight of A. versicolor .

Additional material examined: U.S.A.: Arizona, Apache County: Santa Catalina Mtns. (WS Creighton) , Cochise County: Chiricahua Mtns. Southwestern Research Station (HV Weems, Jr.) , Dragoon (WS Creighton) ; Gila County: 6.3mi NNW Jct. Salt River on Rt. 288 (RA Johnson, UG Mueller, C Rabeling, A Rodrigues, SP Cover) ; Maricopa County: McDowell Mountain Park (RA Johnson) ; Pima County: Avra Valley (JH Hunt) , Baboquivari Mtns. (WS Creighton) , Oro Valley near First Ave. and Tangerine (RA Johnson) , Tucson Tumamoc Hill (RA Johnson) , MEXICO: Sonora, 10mi S Sonoyta (WS Creighton) .