Pogonomyrmex imberbiculus

This species nests under stones or directly in the soil, sometimes nest entrances are surrounded by a small mound (approximately 4 cms in diameter). Rarely they will nest under stones. Nests are located in sandy soils, to coarse rocky gravel. Nests appear to be small, perhaps 50 to a few hundred workers. Brood is found in nests in August. This is primarily a single foraging, seed harvester, dead insect collector, apparently specializing on grass seeds. Workers forage individually. It is very docile and easy to study. Flights occur in mid July. (Mackay and Mackay 2002)

Identification
This is a member of the “Ephebomyrmex” group, and is very easily recognized as the clypeus is bent as a projecting flange anterior to the insertions of the antennae. This is very easy to see by looking from the top of the head towards the mandibles. This is the only North American species of Pogonomyrmex with this characteristic. (Mackay and Mackay 2002)

Distribution
United States: southwestern Oklahoma, western Texas, southwestern Colorado, southern New Mexico, southern Arizona, Navajo Reservation, southern Nevada, southeastern California. Mexico: Sonora, Chihuahua, Durango, Coahuila

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

Habitat
Chihuahuan Desert in grasslands, open weedy areas, creosotebush scrub, up into juniper forests and oak forests. (Mackay and Mackay 2002)

Biology
Cole (1968): Wheeler (1917b, p. 178) witnessed a mating flight of Pogonomyrmex imberbiculus at 10 a.m. on July 12, and he must have collected the sexes at that time, for he states that "the black males were flying rapidly to and fro about the branches of a few small lote bushes (Zizyphus lycioides) in the open desert" near Deming, New Mexico, and "The winged females . . . were found running on the ground near the lote bushes." Smith (1943, p. 296) gave a very brief diagnosis of the male based upon specimens in the U. S. National Museum. Creighton (1956, p. 62) reports having found callow males and females in a nest in the Davis Mountains of Texas (May 26).

For an interesting and informative account of the habits of imberbiculus, the reader is referred to Creighton's (1956) paper. Nests have been found at elevations from 640 to 7,000 feet. Creighton (p. 61) remarks that "on rare occasions these ants will nest beneath stones but in most cases they build their nests without any covering object." My field data do not support this conclusion. In a total of 39 nests established in Texas, New Mexico, Arizona, and Nevada, 21 were beneath stones, 17 were in open areas without covering materials, and one was at the base of a clump of dry grass. When a crater was present, it was shallow and only 3 inches or less in diameter.

The workers are extremely docile; they show no defensive behavior when their nest is disturbed. They forage singly at a slow and rather steady gait. Seeds make up a considerable part of their diet, which also includes dead arthropods and especially insects.

Nevada, Wheeler and Wheeler (1986) - We have 5 records from 5 localities all in the Hot Desert, but we have data on only 3 colonies: 2 were under stones, 1 had a 5-cm crater.

Wheeler's original description of this species (Wheeler 1902a) contained what was, at the time, a remarkable observation of ant larvae feeding directly on seeds. Wheeler had maintained a colony in an artificial nest and reported the following:

During the first few days of their captivity the ants were fed on house flies. These were not only eaten with avidity by the adult Pogonomyrmex, but cut into pieces and fed to the larvae in the same manner as I have described for the Ponerinae and some Myrmicinae (1900 and 1900a). On one occasion nearly every larva in the nest could be seen munching a small piece of house fly. But a still more remarkable method of feeding was adopted after a few days, when the supply of insect food was exhausted. Then the ants were seen to bring seeds from their granary, crack them open with their strong mandibles, and, after consuming some of the softer portions themselves, to distribute the remainder among their larvae. The latter could be seen under the lens cutting away with their mandibles and devouring the softer starchy portions of the seeds. The hard and useless hulls were afterwards carried away by the ants and placed on the refuse heap. These observations show that the larvae of certain ants are not only able to subsist on solid food, but even on food of a vegetable nature. The adaptation of what were probably once exclusively carnivorous ants to a vegetable diet, although not yet complete, is, nevertheless, so far advanced that the larva already participates in the peculiar feeding habits of the adult insect. P. imberbiculus seems not to possess the power of feeding one another or their larvae by regurgitation. At any rate they were not seen to make use of this method in the artificial nests.

Nesting
Pogonomyrmex imberbiculus will nest in the open, under stones and at the base of grass clumps. Nests are relatively shallow and are comprised of a few broad and shallow surface chambers that are attached to a number of galleries deeper underground. The workers are quite docile and do not defend their nest when disturbed by a curious myrmecologist. Workers will act aggressively towards other Pogonomyrmex imberbiculus that are dropped near their nest entrance but are not generally aggressive to neighboring colonies' workers. Nests can be found in high densities, up to 0.14 nests per square meter. Colonies are small, containing less than 100 individuals.

Foraging
Foraging is conducted individually, with seeds and dead insects making up the majority of the material collected. Foragers range up to 3m from the nest with the average foraging trip being 0.9 m (Schilder et al. 2004). The daily pattern of foraging activity matches that of numerous other Pogonomyrmex: a morning foraging period, a respite during the hottest part of midday and a second foraging period in the afternoon and early evening.

Castes
Two types of queens are known to occur in Pogonomyrmex imberbiculus, typical winged queens and a wingless intermorph that is intermediate between a worker and a winged queen (Hienze et al. 1992). Both queen forms are capable of heading a colony although nests with an intermorph typically contain more than one inseminated intermorph-queens.


 * Worker

Nomenclature

 *  imberbiculus. Pogonomyrmex imberbiculus Wheeler, W.M. 1902a: 87, figs. 1, 2 (w.) U.S.A. Creighton, 1957a: 63 (q.); Cole, 1968: 171 (m.); Wheeler, G.C. & Wheeler, J. 1972b: 237 (l.); Taber, Cokendolpher & Francke, 1988: 51 (k.); Heinze, Hölldobler & Cover, 1992: 267 (ergatoid q.). Combination in P. (Ephebomyrmex): Wheeler, W.M. 1902c: 390. Senior synonym of townsendi: Creighton, 1957a: 56.
 * townsendi. Pogonomyrmex (Ephebomyrmex) townsendi Wheeler, W.M. 1909e: 80 (w.) MEXICO. Subspecies of imberbiculus: Creighton, 1950a: 132. Junior synonym of imberbiculus: Creighton, 1957a: 56.

Worker
Cole (1968) - HL 1.04-1.21 mm, HW 0.97-1.21 mm, CI 92.4-102.9. SL 0.71-0.87 mm, SI 68.6-76.3, EL 0.24-0.34 mm, EW 0.15-0.22 mm, OI 21.8-28.1, WL 0.26-1.48 mm, PNL 0.24-0.39 mm, PNW 0.27-0.37 mm, PPL 0.31-0.37 mm, PPW 0.44-0.56 mm.

Mandible as shown in Pl. III, Fig. 21; all teeth well developed and well separated; apical tooth very broad, rather thin, acute apically, and distinctly larger than subapical tooth which is also broad, rather thin, and acute apicalIy; first basal tooth shorter than subapical, much longer and broader than either second or third basal; penultimate basal tooth very small, slightly offset from the straight to broadly concave basal mandibular margin; ultimate basal loolh absent. Base of antennal scape as illustrated in Pl. I, Fig. 5; base of shaft very strongly curved, the inferior declivity of the basal enlargement meeting it at a strong but well-rounded angle which forms a prominent concavity; shaft rather strongly constricted but not flattened along the bend. Basal enlargement well developed, both lobes prominent; superior lobe with a broad, blunt, weakly concave apex, its declivity steep, meeting the shaft at a pronounced, well-rounded angle; basal flange rather weak, its edge narrow; lip well developed, just distad of which is a prominent narrow notch which sets off distinctly the inferior declivity from the lip; point absent; lateral surface of basal enlargement not impressed.

Eye very large, notably longer than wide, the distance between its anterior margin and mandibular insertion no more than about one-half the greatest eye length. Anterior margin of clypeal lobe, in front of each antennal fossa, with a prominent projection directed forward. Thoracic, petiolar. and postpetiolar contours as shown in Pl. V, Fig. 10. Epinotal spines short, blunt, laterally compressed, joined posteriorly by a prominent transverse keel. Dorsum of petiolar and postpetiolar nodes as shown in Pl. VII, Fig. 27. Venter of petiolar peduncle with a prominent lobe. In lateral view, anterior declivity of petiolar node rather short, straight, steep, only slightly more than one-half the length of the less steep posterior declivity.

Head, thorax, and dorsum of petiolar node coarsely rugo-reticulose, the interspaces shining. I n terrugal spaces of head densely and very finely punctale. Dorsum of postpetiolar node faintly shagreened, subopaque; with a few irregular, largely lateral, longitudinal rugae.

Hairs golden brown; abundant on head, thorax, petiole, postpetiole, and gaster; coarse, stiff, rather short, everywhere very uneven; on thorax longest on pronotal humeri, sparse on venter of petiole; numerous and more delicate on antennae, erect and suberect on scape, subdecumbent on flagellum; short, erect, blunt, and numerous on eyes; abundant, erect, suberect, and subdecumbent on legs.

Queen
Cole (1968) - HL 1.11-1.28 mm, HW 1.12-1.22 mm, CI 95.1-99.2, SL 0.87-0.88 mm, SI 72.1-77.6, EL 0.27-0.29 mm, EW 0.20-0.22 mm, OI 22.7-23.8, WL 1.53-1.58 mm, PNL 0.34-0.44 mm, PNW 0.36-0.39 mm, PPL 0.37-0.39 mm, PPW 0.54-0.58 mm.

Basically similar to the worker. Thoracic, petiolar, and postpetiolar contours as shown in Pl. IX, Fig. 5. Scutum, in lateral view, rather thick, its anterior declivity steep and meeting the scutal dorsum at a rounded obtuse angle; scutal dorsum rather flat and only indistinctly separated from the scutellum which scarcely rises above it. Metanotum not strongly separated from the adjoining epinotum which, with the mctanotum, forms a rather even contour. Petiolar node, in lateral view, with its apex blunt, rather flat, and lacking a nipple; its anterior declivity steep and meeting the peduncle at nearly a right angle; its less steep posterior declivity broadly and evenly convex.

Macropterous; dorsum of basal one-third of wing membrane with dense, long, delicate hairs, remainder of membrane with notably shorter hairs scattered rather evenly over the surface. Margin of fore wing with a prominent fringe of closely spaced, rather long, delicate, whitish, silky hairs of approximately equal length beginning at the stigma and extending to a point on the anal margin opposite that of the stigma, the hairs being very short and decumbent from the stigma to the outer wing margin, then becoming progressively a little longer and more erect; entire anal margin of hind wing with a fringe of rather long, closely spaced, erect, delicate, silky hairs of approximately equal length.

Frons and genae rather strongly, longitudinally rugose, the interspaces shining and bearing prominent transverse foveae; clypeus with a few weaker, longitudinal rugae, the interspaces shining and without perceptible sculpture; vertex and occiput strongly rugo•reticulose, the interspaces strongly shining and without sculpture. Frontal area strongly shining; bearing one, median, longitudinal ruga. Pronotal collar with fine, irregular, transverse striae; sides og thorax strongly rugo•reticulose, the interspaces smooth and shining; scutal dorsum and the scutellum with moderately strong, irregular, closely spaced rugae, the interspaces bearing minute, very sparse, scattered punctures, remainder of scutum rugo-reticulose; base of epinotum strongly reticulose, the interspaces smooth and shining; infraspinal facet faintly, irregularly punctate; petiolar node strongly rugo-reticulose, the interspaces smooth and strongly shining; postpetiolar node irregularly, longitudinally rugulose, the interspaces densely and finely punctate and rather shining, its ventral lobe shagreened. Gaster strongly shining, with small piligerous punctures.

Color a rather uniform, deep, ferrugineous red.

Male
Cole (1968) - HL 0.85-0.94 mm, HW 0.94-0.99 mm, CI 105.3-110.6, SL 0.22-0.27 mm, SI 22.2-28.4, EL 0.39-0.48 mm, EW 0.26-0.29 mm, OI 45.9-54.5, WL 1.62-1.77 mm, PNL 0.34-0.36 mm, PNW 0.31-0.34 mm, PPL 0.39-0.46 mm, PPW 0.46-0.48 mm.

Viewed from in front, head strongly narrowed between lower margins of eyes and mandibular insertions, the two constricted portions straight and subparallel, not convergent anteriorly; transverse occipital keel prominent, frequently somewhat depressed mesally; frontal area weakly depressed, subovoid. Mandible as shown in Pl. VIII, Fig. 18; blade narrow, not notably broader distally than proximally; apical margin broadly convex; basal margin long, straight, meeting the mandibular base at a strong and rather sharp angle; masticatory margin subtransverse, bearing a broad, subacute apical tooth, a shorter subapical tooth, and 2 or 3 irregular basal teeth; penultimate tooth much longer than the small, frequently offset basalmost tooth. In general habitus, the mandible is somewhat like that of the maricopa complex, but in dentition it is markedly different. In repose, tips of mandibles do not overlap. the subtransverse masticatory borders contiguous. Antennal scape less than one-half as long as the first two flagellar segments combined, broader than widest part of first flagellar segment which is strongly attenuated and notably narrower basally than apically; flagellum very long, more than twice the maximal head width, the apical segment notably less than twice the length of the penultimate segment. Eye very large, notably elongate, strongly protruding from side of head, its anterior margin separated [rom the mandibular insertion by a distance of less than one-third the maximum eye length.

Thoracic, petiolar, and postpetiolar contours as shown in Pl. IX, Fig. 8. Mayrian furrows strongly developed. Dorsum of petiolar and postpetiolar nodes as shown in Pl. XII, Fig. 17. Entire petiolar node, in lateral view, elongate, strongly and evenly convex; the anterior declivity meeting the peduncle at a prominent obtuse angle. Epinotum with a pair of short, broad, blunt, laterally compressed angles; its declivity very steep and notably shorter than the less steep base.

Contour of paramere as shown in Pl. X, Fig. 16; apical parameral lobe as in Pl. XI, Fig. 18. Digitus of volsella (Pl. XII, Fig. 8) a long, free, rather robust lobe, its apical portion much broadened and subspatulate in terminal view, its apex broadly oval in outline. Aedeagus (Pl. X, Fig. 19) compressed laterally; the denticles on its ventral margin very fine. visible only with high magnification.

Cephalic rugae numerous, prominent, rather coarse, wavy, longitudinal, reticulose around the eyes; on median lobe of clypeus finer, sparse, irregular, confined chiefly to the lateral portions. Rugae on pronotum prominent, numerous, coarse, irregular, tending to form reticulations; on scutum strong, wavy, transverse; on scutellum somewhat weaker, longitudinal; on epinotum prominently reticulose. Sides of petiole with a few, irregular, longitudinal rugae. Petiolar and postpetiolar nodes finely shagreened, shining. Interrugal spaces of head and thorax shining; antennal scapes smooth and shining; legs and gaster smooth and highly polished.

Hairs on both dorsum and venter of head and on thorax, petiole, and postpetiole abundant, long, slender, both straight and flexuous, golden brown. Eyes with numerous, prominent, scattered, rather long, erect, straight hairs. Hairs on antennal scape rather long and numerous, confined chiefly to anterior surface; those on legs dense, long, slender, suberect. Venter of petiole with sparse, long, erect hairs. Hairs on dorsum of first gastric segment numerous and long, those on other gastric segments confined chiefly to the posterior border. Flagellum densely pubescent; hairs absent. Pilosity of wing membrane as in the female.

Body rather uniform, deep brownish black; appendages a little lighter. Head, thorax, petiole, post petiole, and appendages subopaque to opaque; gaster somewhat shining.

Described, for the first time, from seven males collected with associated females and workers from nests at Ft. Davis, Texas, on June 10, 1902 (W. M. Wheeler leg.) and on June 16, 1915 (F. M. Gaige leg.).

Etymology
Morphological. L. imberbis beardless + L. oculus diminutive. This species does not possess a prominent psammophore or, as Creighton (1956) described "...do not have full beards, they... have goatees."