Trachymyrmex smithi

Identification
Trachymyrmex smithi might be confused with Mycetomoellerius jamaicensis due to its large size and dark coloration, but the species are allopatric; M. jamaicensis is only known from southwest Florida, the Florida Keys, and the Caribbean, whereas T. smithi occurs in the deserts of western Texas, New Mexico, and the State of Coahuila in northern Mexico. In addition, the frontal and preocular carina of T. smithi do not form a well developed antennal scrobe that extends back to the preoccipital margin as in M. jamaicensis, and the frontal lobes are triangular in T. smithi, not rounded, as in M. jamacensis. (Rabeling et al. 2007)

Distribution
Trachymyrmex smithi is a Chihuahuan Desert species that occurs in southwest Texas, south-central New Mexico, and the Mexican states of Chihuahua and Coahuila. (Rabeling et al. 2007)

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

Habitat
Inhabits creosote bush bajadas (alluvial fans), mesquite/Yucca grassland playas (dry lakebeds) and mesquite coppice dune habitats at elevations of 1100–1500 m. (Rabeling et al. 2007)

Biology
From Rabeling et al. (2007): Nests are often in the shade of creosote bush or Mormon tea (Ephedra trifurca). Older nests may have large nest mounds (~30 cm diameter) with conspicuous middens consisting of dried leaves and exhausted fungus substrate. The subterranean nests of T. smithi are the largest of all Trachymyrmex species occurring in the US. Older nests consist of more than 20–30 chambers (Johnson et al. 2006; Rabeling & Mueller, unpublished data) of which 50–60% contain fungus gardens in the summer. Near El Paso, Texas, the fungus gardens are nourished with entire mesquite leaflets (Johnson et al. 2006; Rabeling, unpublished data), and resemble the fragile fungus gardens of grass-cutting Acromyrmex species in South America. Colonies can be very populous; Johnson et al. (2006) report up to 786 workers and 6 dealate queens in one nest, and Schuhmacher and Whitford (1974) estimate 1250 workers for one colony based on mark-recapture experiments. Colony activity and the number of fungus gardens decrease from November through May (Schuhmacher & Whitford 1976). Two nests, which we partially excavated during winter, had 26 chambers per colony, reaching down to 180 and 130 cm depth, respectively. None of the chambers contained a fungus garden in December, suggesting that T. smithi moves its gardens to deeper layers in winter. During springtime the excavation of two adjacent colonies showed, that in April the ants already moved the fungus garden to shallower nest chambers. Numerous fungus gardens were encountered hanging from the chambers’ ceilings in 25–130 cm depth. From the partially excavated winter colonies, 212 and 362 workers were collected (Rabeling & Mueller, unpublished data). Trachymyrmex smithi forages mostly at night during the summer months, to avoid soil temperatures exceeding 50°C during the day (Whitford 1978).

Nomenclature

 *  smithi. Trachymyrmex smithi Buren, 1944b: 5 (w.) MEXICO. Senior synonym of neomexicanus: Rabeling, Cover, et al. 2007: 20.
 * neomexicanus. Trachymyrmex smithi subsp. neomexicanus Cole, 1952a: 159, fig. 1 (w.) U.S.A. Cole, 1953f: 300 (q.m.). Junior synonym of smithi: Rabeling, Cover, et al. 2007: 20.

from Rabeling et al. (2007): Buren (1944) described T. smithi from Mexico (La Rosa, Coahuila) and Cole (1952) later based the subspecies T. smithi neomexicanus on workers from the United States (Las Cruces, New Mexico). Cole separated neomexicanus from smithi because it possessed larger and less tuberculate spines, a more concave posterior margin of the postpetiole, larger body size, darker color, and more abundant gray “granulation” on the integument. For a Trachymyrmex, T. smithi workers show considerable size variation and all characters, except color and the presence of granulation, vary proportionally to size. The morphological differences cited by Cole for neomexicanus fall well within the range of variation shown by this widely distributed species. Likewise, black, reddish-brown and intermediate color morphs are distributed over the species entire range, including the type locality (C. Rabeling personal observation). Molecular evidence also supports our contention that in T. smithi we are dealing with a single variable species. The short branch lengths in the molecular phylogenetic analysis (see below) show that the sequence diversity is minimal and similar for both smithi and neomexicanus. The more abundant gray granulation of neomexicanus mentioned by Cole (1952) is most likely caused by actinomycete bacteria of the genus Pseudonocardia (Cafaro & Currie 2005), which grow on the ant’s exoskeleton. Actinomycete load on the worker’s body surface varies among individuals of the same nest and is affected by worker age, characteristic activity (foraging versus garden-tending), or the health of the fungus garden. This coating is therefore of no taxonomic importance. To remove the sometimes confusing actinomycete coating, specimens can be washed with acidic acid (vinegar).

Worker
Diagnosis from Rabeling et al. (2007): HL 0.94–1.25, HW 1.0–1.375, CI 100–111, SL 0.86–1.19, SI 84–89, ML 1.25–1.69. A large, dark colored species. Head trapezoidal, almost cordate; always broader than long (HW > HL) even in the smallest workers, widest at midpoint between the eye and the posterior corner, and strongly tapering anteriorly. Posterior margin of head moderately concave, more so in larger workers. Antennal scapes short, surpassing the posterior corner of the head by its maximum diameter or less. In full-face view, frontal carinae extending almost to the posterior corners, but weakening before they reach the vertex. Preocular carinae variably developed, traversing approximately half the distance between eye and frontal carina, never touching the frontal carinae. Antennal scrobes weakly developed. In full-face view, frontal lobes small, broadly triangular, usually asymmetrical, with anterior side longer than the posterior. In dorsal view, anterolateral promesonotal tooth thick, sharply pointed, projecting horizontally, not vertically. Anterior median promesonotal tubercles short, vertical, toothlike in frontal or posterior view. Propodeal teeth strongly divergent, spinelike and longer than the distance separating their bases. Vertex of head and gaster strongly tuberculate, remainder of body moderately tuberculate, tuberculi small, tubercular setae weakly to strongly recurved; tuberculi on sides of mesosoma miniscule and sparse. Texture of entire body surface coarse, sandpaperlike. Trachymyrmex smithi displays considerable color variation, ranging from grayish black or blackish brown to rarely dark red or reddish-brown.

Queen
Diagnosis from Rabeling et al. (2007): HL 1.2, HW 1.35–1.4, CI 113–114, SL 1.05–1.1, SI 78–79, ML 1.9–2.0. As in worker diagnosis but with typical caste-specific mesosomal morphology related to wing-bearing and head with small ocelli. Dorsolateral pronotal teeth well developed, tuberculate and sharply triangulate in dorsal view. Ventrolateral pronotal teeth short, triangular, not tuberculate and pointed. Mesoscutum longitudinally rugulose, not tuberculate. Pronotal sides, mesopleura and propodeum with only a few minute tuberculi, if any. Setae abundant, short, straight and suberect. Dorsum of mesosoma, petiole, postpetiole and gaster distinctly bicolored.

Male
Diagnosis from Rabeling et al. (2007): HL 0.81–0.84, HW 0.84–0.87, CI 100–107, SL 0.93–0.99, SI 107–118, ML 1.9–2.05. A large male with relatively long appendages and antennal scapes. Preocular carina a distinctive vertical ridge as it passes the eye and curves towards the midline, remaining strongly developed until the posteriormost portion of the “scrobe.” Ocelli moderately large, slightly elevated above the remainder of the head in side view. Dorsolateral pronotal teeth very short, indistinct, or absent. Ventrolateral pronotal teeth short, triangular. Mesoscutum with weakly reticulate longitudunal rugluae, interrugal spaces granulate. First gastric tergite minutely tuberculate, with numerous, short, decumbent or suberect recurved setae.

Type Material
From Rabeling et al. (2007):

Holotype worker, Paratype workers , , ; Holotype worker , Paratype workers , , ,. Examined.

To our great surprise we encountered two holotypes of T. smithi Buren; one deposited in the LACM and the other in the USNM collection. Most likely, the USNM specimen was mislabeled and actually represents a paratype, because Buren (1944, p. 6) stated that the single holotype would remain in his personal collection and paratypes would be deposited in the National Museum and his personal collection. Since the LACM accessioned the W. F. Buren collection in 1983, we here designate the holotype of T. smithi as the specimen deposited at the LACM.

Type Locality Information
La Rosa, Coahuila, Mexico; 6 mi N Las Cruces along Hwy US 85, New Mexico, U.S.A. (Rabeling et al. 2007).

Etymology
Buren (1944) named this species after Marion R. Smith, myrmecologist and curator of Hymenoptera at the National Museum of Natural History in Washington, DC for many years during the mid twentieth century. (Rabeling et al. 2007).