Pheidole obtusospinosa

Stefan Cover (unpublished field notes), who has collected obtusospinosa many times in southern Arizona, has found it consistently in woodland, variously composed of different combinations of pine, oak, and juniper. It typically nests under rocks, although Cover found one colony beneath a cow pat and another 2.5 m from the ground in the dead branch of a standing oak tree (Quercus arizonica). Creighton (1958) reports that colonies are much smaller than those of the closely related Pheidole hirtula, and that in southern Arizona nuptial flights occur in early July. (Wilson 2003)

Identification
This is a polymorphic species, which has the usual minors, intermediate sized workers (which look like major workers of other species) and very large major workers. The scape is flattened at the base, but the flattened area is not as wide as the width of the scape near the apex. It extends about 2/3 the length of the head in the largest workers, but may extend nearly to the posterior lateral lobes in the smallest majors. The entire dorsal surface of the head is sculptured, with granulose sculpture on the front faces of the posterior lateral lobes. The entire dorsum of the head is dull. The mesonotum projects above the level of the pronotum and propodeum. The dorsal surface of the gaster is finely punctate. (Mackay and Mackay 2002)

See the description in the nomenclature section.

Distribution
Known from the mountains of southern Arizona at 300–1900 m, New Mexico (very common in Clanton Draw in the Coronado National Forest, Hidalgo Co.), and from Nayarit to Nuevo Leon in Mexico. (Wilson 2003)

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

Habitat
Creosotebush scrub, grasslands, oak-pinyon-pine forests, pinyon-juniper communities, Chihuahua pine, up to 2100 m elevation. (Mackay and Mackay 2002)

Biology
Mackay and Mackay (2002) - This species nests under stones, often the largest stones in an area, or nests in the soil with several separate entrances. It is common in sandy soils, including dunes, but also occurs in areas of rocky loam. Brood is found in nests in March. They are preyed on by Neivamyrmex leonardi, and make no attempt to defend the nest and only escape and rescue brood. Foragers are both diurnal and nocturnal, and visit extrafloral nectaries.

Huang (2010) reported how supermajors of this species served an important role in effectively repelling a Neivamyrmex texanus raid. The observed P. obtusospinosa colony nested at the base of a living oak tree and had a triangular-shaped nest entrance (base: ~ 12mm height: ~ 9mm) partially bordered by hard tree bark and the softer soil ground surface. The nest was in an oak, sycamore, and juniper forest in Gardner Canyon in Tucson, Arizona (31º42.56’N and 110º42.58’W; Elevation: 1618 meters). When a raid occurred in the early afternoon, the super majors effectively blocked the nest entrance using their heads. "After failing to penetrate the nest entrance of the P. obtusospinosa colony, some of the army ants turned away and swarmed around the base of the oak tree, possibly trying to find another entrance into the colony. As the number of army ants at the nest entrance dwindled, the P. obtusospinosa super majors broke their head-blockade formation and stormed out of the nest. One group of P. obtusospinosa majors (large and small) attacked the army ants circling around the tree base from behind, while another group attacked the front line of the incoming army ant reinforcements by heading straight into the army ant foraging trail, occasionally dragging their abdomens on the ground. These actions of the second group of P. obtusospinosa majors resulted in the disorientation of army ant reinforcements at the front end of the trail. The major sign of army ant disorientation was their change from moving in an initially straight path to moving in various, random, directions. Without reinforcements, the group of army ants circling around the tree trunk was left behind. As this isolated group of army ants returned their attention toward the original nest entrance, both groups of P. obtusospinosa majors retreated into the nest, and the super majors resumed their head-blockade formation at the entrance. Even after the P. obtusospinosa majors had abandoned the front line of the army ant trail, army ant reinforcements continued to show signs of disorientation. After 30 to 45 min of switching at least three times between the defensive head-blockade formation and the dual offensive attacks, the P. obtusospinosa colony drove away the raiding army ants."

Huang suggested the effectiveness of the super majors in this Pheidole species may an important reason for their existence. It also supports this being a general hypothesis to explain the presence of super majors in other trimorphic Pheidole species, all of which are found in areas where army ants are present.

Castes
Huang (2010) - Minor workers have an extremely narrow size range (head width = 0.5mm to 0.7mm) and are discretely separated from major workers. The major worker head width distribution was bimodal and ranged from 1.1 mm to 2.4 mm with smaller major workers present in colonies approximately three times as frequently as larger major workers. Results of the cluster analysis (assuming two modes) suggested that small majors range from 1.1 mm to 1.7 mm in head width while larger majors range from 1.7 mm to 2.4 mm. Figure 1b shows that there is no major change in overall head shape when comparing small and larger majors, despite an increase in absolute head size.

Worker
Minor

Nomenclature

 *  obtusospinosa. Pheidole obtusospinosa Pergande, 1896: 889 (s.) MEXICO. Junior synonym of subdentata Pergande: Wheeler, W.M. 1914b: 50; hence first available replacement name for Pheidole subdentata Pergande, 1896: 888 [Junior secondary homonym of Oecophthora subdentata Mayr, 1853a: 145.]; designated by Bolton, 1995b: 326. See also: Wilson, 2003: 587.
 * subdentata. Pheidole subdentata Pergande, 1896: 888 (w.) MEXICO. [Junior secondary homonym of subdentata Mayr, above.] Subspecies of vasliti: Wheeler, W.M. 1914b: 50. Revived status as species: Creighton, 1958: 211. Senior synonym of obtusospinosa: Wheeler, W.M. 1914b: 50; of arizonica: Creighton, 1958: 211. Replacement name: obtusospinosa Pergande, 1896: 889; first available replacement name for subdentata Pergande: Bolton, 1995b: 326.
 * arizonica. Pheidole arizonica Santschi, 1911d: 3, fig. (s.) U.S.A. Subspecies of vasliti: Creighton, 1950a: 192. Junior synonym of subdentata: Creighton, 1958: 211; Gregg, 1959: 31.

Description
From Wilson (2003): DIAGNOSIS A large trimorphic species placed in the pilifera group because of the 2-toothed hypostoma of the major but with other traits conforming to the fallax group. Very close to Pheidole hirtula, distinguished most readily in the supermajor, as illustrated, by the elongate foveae of the rear half of the dorsum of the head, with the interspaces densely foveolate and opaque. The tangled taxonomic history of this species and the true status of Pheidole vaslitii, previously associated with it but now revealed as a junior synonym or sibling species of Pheidole hyatti (q.v.), have been presented by Ward (2000).

MEASUREMENTS (mm) Supermajor: HW 2.50, HL 2.36, SL 1.12, EL 0.26, PW 1.06. Major: HW 1.44, HL 1.46, SL 1.12, EL 0.24, PW 0.72. Minor: HW 0.62, HL 0.80, SL 0.98, EL 0.16, PW 0.44.

COLOR All castes: yellowish to reddish brown.



'''Figure. Upper: major, with heads of major (left) and supermajor (right). Lower: minor. ARIZONA: Sunnyside Canyon, Huachuca Mts., Cochise Co. (Stefan Cover). Scale bars = 1 mm.'''

Type Material
MEXICO. Tepic, Nayarit, col. Eisen and Vaslit, - as reported in Wilson (2003)

Etymology
L obtusospinosa, with blunt thorns, referring to the propodeal spines. (Wilson 2003)