Pogonomyrmex occidentalis

Its conspicuous nests and abundance make this an iconic ant of the southwestern United States. This species constructs conical pebble mounds with internal chambers and galleries, with basal entrances, and with peripheral clearings. The colonies are very populous and the workers pugnacious. Pogonomyrmex occidentalis is an active harvester which feeds upon and stores great quantities of seeds.

Identification
The position of the basalmost mandibular tooth separates Pogonomyrmex occidentalis from Pogonomyrmex salinus. This tooth is distinctly offset from the basal mandibular margin with which it makes a prominent rounded angle; in salinus the tooth is not at all offset, but instead makes a straight angle with the basal mandibular margin.

Although rather closely related, Pogonomyrmex maricopa and Pogonomyrmex occidentalis are readily distinguishable by a number of strongly contrasting morphological and behavioral characteristic. P. maricopa nests normally only in sand or loose sandy soils whereas occidentalis occupies soils of a notably tighter texture and generally of a gravelly type. Narrow edaphic amplitudes of these species seem to be the rule.

Distribution
United States: Kansas, Arkansas, northern Texas (Panhandle), Oklahoma, southwestern North Dakota, western South Dakota and Nebraska, Wyoming (except northwestern pan), Colorado, extreme southeastern Idaho, central and northern New Mexico. Utah (except extreme northwest), Arizona, Nevada, east central California.

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

Habitat
Never found in completely arid habitats, most common in higher elevation grasslands, sagebrush sites, oak forests, pinyon juniper forests into pine forests - but always in clearings. (Mackay and Mackay 2002)

Biology
This is a common and, due to their large gravel mounds, conspicuous apecies. Foragers are active during the day, retreating into the nest or into protected sites during the hot part of the day. During the night the ants often block the nest entrance with pebbles. It feeds on seeds and dead insects. It is a very aggressive species and can inflict numerous painful stings in a surprisingly short period of time. Mounds play a role in thermoregulation of the nest. Surfaces exposed to the sun are warmer. The nests are usually covered with pebbles, snail shells or even Indian beads and fossil mammal teeth. The function of the gravel is unknown, but may protect the mound from erosion or may mark the entrance hole (in species with no mound or smaller mounds) to make it easier to find. Monomorium minimum may be found in nests as is the scarabaeid beetle Cremastocheilus saucius. (Mackay and Mackay 2002)

Nomenclature

 *  occidentalis. Myrmica occidentalis Cresson, 1865: 426 (w.q.) U.S.A. Cole, 1968: 94 (m.); Wheeler, G.C. & Wheeler, J. 1953a: 110 (l.); Taber, Cokendolpher & Francke, 1988: 51 (k.); Taber & Francke, 1986: 274 (gynandromorph). Combination in Pogonomyrmex: Cresson, 1887: 22. Senior synonym of opaciceps, seminigra: Mayr, 1886d: 449; Forel, 1886b: xlii; of ruthveni: Olsen, 1934: 507; of utahensis: Cole, 1968: 95. See also: Smith, D.R. 1979: 1355; Taber & Francke, 1986: 274; Shattuck, 1987: 172.
 * seminigra. Myrmica seminigra Cresson, 1865: 427 (m.) U.S.A. Junior synonym of occidentalis: Mayr, 1886d: 449; Forel, 1886b: xlii.
 * opaciceps. Pogonomyrmex opaciceps Mayr, 1870b: 971 (diagnosis in key) (w.) U.S.A. Junior synonym of occidentalis: Mayr, 1886d: 449; Forel, 1886b: xlii.
 * ruthveni. Pogonomyrmex occidentalis subsp. ruthveni Gaige, 1914: 93 (w.q.m.) U.S.A. Junior synonym of occidentalis: Olsen, 1934: 507.
 * utahensis. Pogonomyrmex occidentalis var. utahensis Olsen, 1934: 509 (w.q.m.) U.S.A. Subspecies of occidentalis: Cole, 1942: 365. Junior synonym of comanche: Creighton, 1950a: 128; of occidentalis: Cole, 1968: 95.

Worker
Cole (1968) - HL 1.52-2.09 mm, HW 1.44-2.l3 mm, CI 94.7-101.9, SL 1.10-1.52 mm, SI 71.4-86.8, EL 0.38-0.53 mm, EW 0.19-0.30 mm, OI 25.0-25.4, WL 1.56-2.28 mm, PNL 0.34-0.53 mm, PNW 0.38-0.53 mm, PPL 0.38-0.57 mm, PPW 0.53-0.80 mm.

Mandible as shown in Pl. III, Fig. 1; basalmost tooth larger than penultimate, notably offset from the basal mandibular margin with which it makes a prominent rounded angle; basal mandibular margin straight, comparatively short. Base of antennal scape as in Pl. IV, Fig. 1; broad and pronounced; superior declivity long, not steep; superior lobe broadly truncate; basal flange prominent, evenly and rather strongly curved distad over apex of superior lobe; lip pronounced, broad, strongly curved distad; point weak or absent; longitudinal peripheral carina weakly developed, the depression it borders very shallow.

Contour of thorax, petiole, and postpetiole, in lateral view, as shown in Pl. VI. Fig. 1; surface between apex of anterior mesonotal declivity and base of epinotal spines notably flattened; mesonotum usually distinctly elevated, but sometimes only slightly, above level of pronotum, the promesonotal contour not smooth and even. Epinotal armature generally a pair of long, slender, tapered, pointed spines, but sometimes spines are short or of medium length, more robust, even or u neven in length. Venter of petiolar peduncle without a prominent process. Contour of petiole and postpetiole, viewed dorsally, as in Pl. VII, Fig. 1.

Interrugal spaces of head densely and coarsely punctate and opaque, the punctures producing a beaded appearance; interrugal spaces of thorax densely punctate and opaque, the punctures not obscuring the rugose surface; dorsum of petiolar node generally rather coarsely and irregularly rugose, densely and finely punctate, opaque; rugae of dorsum of postpetiolar node usually sparse, fine, with a transverse trend, confined largely to posterior portion; dorsum of petiolar and postpetiolar nodes sometimes only densely punctate. Base of gastric dorsum with light to moderate shagreening which does not dull the shining surface. Body color light to medium ferrugineous red.

Queen
Cole (1968) - HL 1.98-2.09 mm, HW 2.17-2.32 mm, CI 109.6-111.0, SL 1.41-1.52 mm, SI 65.0-65.5, EL 0.46-0.49 mm, EW 0.27-0.30 mm, OI 23.2-23.4, WL 2.77-2.89 mm, PNL 0.46-0.49 mm, PNW 0.68-0.76 mm, PPL 0.53-0.61 mm, PPW 1.03-1.10 mm.

Largely with the characters of the worker. Contours of thorax, petiole, and post petiole, in lateral view, as shown in Pl. IX, Fig. 1. Epinotum armed with a pair of short, acute spines.

Male
Cole (1968) - HL 1.22-1.44 mm, HW 1.25-1.56 mm, CI 102.5-108.9, SL 0.61-0.76 mm, SI 48.7-48.8, EL 0.53-0.61 mm, EW 0.30-0.34 mm, OI 42.4-43.4, WL 2.28-2.66 mm, PNL 0.38-0.46 mm, PNW 0.49-0.61 mm, PPL 0.49-0.61 mm, PPW 0.7.-0.87 mm.

Head, in full front view, with vertex strongly acute. Mandible as shown in Pl. VIII, Fig. 2; blade broad, wider distally than basally; provided generally with 6 or 7 well-developed teeth; basal most tooth notably larger than penultimate, distinctly offset from the broadly and weakly convex basal mandibular margin with which it makes a pronounced rounded angle, curved somewhat inwardly; masticatory border rather strongly oblique; apical margin straight proximally, weakly convex distally. Antennal scape about one-half again as long as combined lengths of first two flagellar segments; apical segment of flagellum approximately twice the length of penultimate segment, the latter notably longer than broad.

Epinotum armed with a pair of tubercles or weak to strong, broad, blunt or sharp angles. Paramere as shown in Pl. X, Fig. 1, and Pl. XI, Fig. 1; apex of terminal lobe distinctly flattened. Head and thorax deep reddish or blackish brown; petiole, postpetiole, and gaster generally considerably lighter.

Type Material
Type locality: Colorado Territory. A.N.S.P.