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: 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)

Wheeler and Wheeler (1986):

The mounds of P. occidentalis are built of two types of material: (1) the soil which the ants have excavated in the construction of the subterranean portion of the nest and (2) the fine gravel collected from the surrounding soil surface and placed on the mound. Where gravel is not available a wide variety of substitutes is used. In North Dakota we (Wheeler and Wheeler, 1963) found gypsum, selenite, lignite, petrified wood, empty snail shells, plant debris, and pellets of soil. Paleontologists have collected teeth of small extinct mammals from the surface of mounds. It is even rumored that early prospectors found small gold nuggets, but we were never so lucky. The function of the gravel coating of the mound is probably to prevent erosion by strong winds and torrential rains. Because of the large volume of dead air space (chambers and galleries) the mound also serves as a regulator of temperature.

The mounds (in North Dakota) were generally conoidal. Of the nests for which measurements were recorded the mound varied from 30 cm in diameter and 5 cm in height to 135 cm in diameter and 25 cm in height; average 61 and 14 cm (Wheeler and Wheeler, 1963:114).

Surrounding the mound was a bare area or clearing, which (in North Dakota) reached 3 m in diameter; the average was 1 1/2 m. According to Cole (1932a:140) this was "due to the destruction, by the worker ants, of the vegetation closely surrounding the mounds. The plants are literally 'chewed-down', bit by bit, from the apex to the base." He concluded that the clearing protected the mound from grass fires. We would suggest also that the bare area surrounding the mound makes the sunlight more effective earlier in the day. It must be kept in mind that P. occidentalis occupies a range of cool nights (even in summer) and hot days. Earlier rising enables the workers to get their day's work done before the midday sun in summer heats the ground to lethal temperatures.

In warm weather in North Dakota the workers of P. occidentalis stayed inside the nest during the hottest part of the day, but the entrance remained open. At night the workers retreated into the nest and closed the entrances. Any ant that did not get back in time had to stay outside all night. Workers and the queen overwinter; there is no brood at this time, nor are winged (sexual) forms present. The workers cluster in masses and in a comatose condition, but they revive quickly when warmed; many will survive freezing temperatures overnight. The greatest depth at which ants were found in winter in Wyoming was 277 cm (Lavigne, 1969).

Lavigne reported (1969: 1167) that the actual counts of workers in the 33 nests he excavated varied from 412 to 8,796. There was no direct relation between the size of the mound and the population of the nest. This might be due in part to the fact that small colonies have been known to move into larger abandoned nests.

We had samples of seeds taken from 16 nests in North Dakota. The number of species we found in a nest at the time of collection ranged from 1 to 9. The seeds collected by all the colonies studied were from 30 species of plants (Wheeler and Wheeler, 1963: 116).

The enemies of this ant are lizards and birds. Several species of rodents have been reported robbing the seeds stored in the chambers of the mound.

The myrmecophilous beetle Araeoschizus armatus Horn (Coleoptera: Tenebrionidae), was found in a colony.

Cole et al. (2001) - We examined the spatial pattern of the ant Myrmecocystus mexicanus and although intraspecific dispersion is highly uniform, colonies were significantly associated with reproductively mature nests of the harvester ant Pogonomyrmex occidentalis. Colonies of M. mexicanus were more likely to be found within 3m of P. occidentalis and less likely to be found as far as 10m away. The protein component of the diet of M. mexicanus is almost exclusively dead or moribund workers of P. occidentalis. M. mexicanus appears to associate with one of its consistent food sources.

Castes

 * Worker

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.

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

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.