Myrmecocystus navajo

A nocturnal foraging species that forms nests that can contain more than 1000 individuals.

Identification
A member of the mexicanus group of the Myrmecoystus subgenus Myrmecocystus .

Key to Myrmecocystus subgenus Myrmecocystus species.

Worker - Dorsum of propodeum evenly convex, with abundant erect hairs; appressed pubescence sparse on head, thorax and gaster; hind tibia with few or no erect hairs on outer face. Female - Penultimate maxillary palpal segment broadest in middle, femora without erect hairs on dorsal face; scape with abundant erect hairs. Male- HW less than 0.8 mm; apical margin of forewing without fringe hairs; scape with scattered erect hairs. (Snelling 1976)

Distribution
United States, Mexico. Eastern Colorado south to western Texas, westward to southern Utah, Nevada, California and northern Sonora.

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

Habitat
Ranging from Creosote bush Scrub in the Lower Sonoran to Upper Sonoran Chaparral; it seems to be most common in Oak-Juniper and Pinon-Juniper Woodlands. The elevational range of navajo is poorly known: in New Mexico it ranges from 4000' to 6100'; in Nevada from 2200' to 6500' and in California from about 700' to 4500'. West of central Arizona the distribution is sporadic, probably limited to suitable habitat elevations in the mountain ranges.

Biology
Snelling (1976) - Gregg (1963) found this species in Short Grass Prairie of the Upper Sonoran of Colorado. The type material came from an area of Saltbush-Greasewood Desert near Albuquerque, New Mexico. Creighton (1950), based on information from Wheeler (1908), noted that this "...species makes obscure nests in sandy soil. It does not make a crater but spreads the excavated soil out into a disc." According to Wheeler, too, the colonies are small, of about 100-150 workers.

Some data on foraging activity were reported by Fautin (1946) who studied the species in White Valley, western Utah. Here, the ant was present in the Shadscale and Tetradymia Communities. Fautin found that the sensitivity of the workers to heat seemed to be a factor regulating emergence from the nest: "During April they became active just before dark and the time of their activity was progressively delayed until a later hour of the night as the summer temperatures became higher. By midsummer they failed to emerge until near midnight."

The ants appeared at the entrance of the nest when air temperature dropped below 88°F. They were found to leave the mound to begin foraging when air temperature was within a reported range of 52.2°F to 70.5°F, the average being 63.5°F. Fautin found that when ants were exposed to direct sun and an air temperature of 94°F they died within 1-10 min. Unfortunately, no readings were taken of soil surface temperatures which might have yielded more precise data than air temperature can provide.

The foraging activities are nocturnal, as usual in this subgenus, and the species solicits aphids and pseudococcids for honeydew as well as foraging nectar directly from plants. Workers may be attracted to sweet baits. Dead arthropods are also collected. Wheeler (1908) reported that he found no repletes in the twenty-two nests which he examined in the vicinity of Albuquerque. Creighton (1950) surmised that repletes undoubtedly are developed. A part of the series from Ft. Stockton, Texas, consists of repletes, and I took repletes from a nest studied at Texas Canyon, Arizona. Although it is possible that the colonies examined by Wheeler may all have been young colonies, I think it more likely that excavation was not complete. Mature colonies number over 1000 individuals, not 100-150.

Wheeler also commented on the lack of a tumulus about the nest entrance. It may be that the nests which he studied were without tumuli due to wind and/or rain action. All the nests which I have seen possess well developed crateriform tumuli composed of coarse grains of sand or fine pebbles.

Little is known of the activities of the sexual forms. Alate females have been taken most commonly at lights. One collection of females made near Scissors Crossing, Calif., on 4 Aug. 1974 bears the note "crepuscular following heavy rain." The enlarged ocelli of the female suggest that mating flights regularly take place during evening or at night.

New Mexico
Mackay and Mackay (2002) - Chihuahuan Desert, sagebrush and grasslands up to oak and pinyon juniper woodlands. These ants nest in the soil with the entrance surrounded by a low mound of soil. Wheeler (1908) reports that the colonies are small, 100 - 150 workers, although this estimate is probably low as was suggested by Snelling (1976). Larvae occurred in nests in March. Foraging occurs primarily at night, with the workers tending Homoptera and feeding on flower nectar. Dead arthropods are also collected.

Nomenclature

 *  navajo. Myrmecocystus mexicanus subsp. navajo Wheeler, W.M. 1908d: 360, fig. 11 (w.q.) U.S.A. Snelling, R.R. 1976: 126 (m.). Raised to species: Creighton, 1950a: 449.

Snelling (1976) - Superficially this species looks like a diminutive Myrmecocystus mexicanus. From that species the worker is separable by smaller size, sparse cephalic pubescence and lack of erect hairs on the extensor surfaces of the femur and tibia. The female differs from that of mexicanus chiefly in smaller size and lack of erect hairs on the extensor surfaces of the femur and tibia, the male separable largely by its smaller size.

The node of the petiole of navajo workers is usually thinner and with fewer erect hairs than that of mexicanus, as noted by Creighton (1950) and Gregg (1963). The differences, however, require comparison and will not always hold up in material from California where some workers of mexicanus have narrower and less pilose petiolar nodes. Creighton also commented on the high placement of the eye of navajo, its upper border coincident with the occipital angles. This is not always true, even in material from New Mexico. Also, this characteristic occurs in some samples of mexicanus.

Populations from Nevada and California differ from those of the eastern parts of the range of the species. In the female and worker castes of these western samples the pubescence of head, thorax and gaster is very sparse and the integument is quite shiny. Specimens from the eastern parts of the range are more pubescent and therefore appear less shiny. Erect hairs are also a little more abundant, especially on the thorax, in the samples from Colorado, New Mexico and Arizona. The few samples available from Utah seem to be of an intermediate character. Some are fully as pubescent as the eastern samples; others are more like those of Nevada and California. Many, however, cannot be assigned to one or the other. Since both extremes, as well as the intermediates, may be present in the same nest sample, it is safe to assume that we are dealing with a single, variable species. This reduction in vestiture. in western samples parallels the situation in the related mexicanus

Worker
Snelling (1976) - Measurements. HL 0.83-1.68 (1.20); HW 0.62-1.57 (1.00); SL 1.00-1.80 (1.45); WL 1.2-2.5 (1.9); PW 0.43-1.05 (0.70).

Head: Distinctly longer than broad in smallest workers, as long as broad in largest, CI 70-100 (81), distinctly shorter than scape, SI 105-138 (121); in frontal view, broadest a little below eyes, sides barely convex, slightly narrowed toward mandibular bases. Occiput, in frontal view, flattened, or slightly convex in small workers, without lateral angles. Eye large, 1.09- 1.57 X first flagellomere; upper margin coincident with occipital margin; OMD 0.92-1.58 (1.25) x EL. Mandible novemdentate.

Thorax: Slender, PW 0.33-0.43 (0.36) x WL; propodeum higher than long, basal face in profile, evenly convex and shorter than posterior face.

Petiole: Thick in profile, summit rounded, without sharp crest; from behind, summit evenly rounded, without median notch; from above, less than twice wider than long.

Vestiture: Erect hairs present on all cephalic surfaces; longest occipital hairs less than 0.5 x MOD. Thoracic dorsum with numerous erect hairs, longest pronotal hairs about equal to those of occiput, mesonotal hairs shorter; 10 or more erect hairs on propodeum; petiolar scale with six or more short erect hairs; first two terga with numerous erect discal hairs, these as long as those of mesonotum. Scape usually with numerous erect to subdecumbent hairs; tibiae with not more than three erect hairs beyond basal third; femora without erect hairs on dorsal face.

Pubescence very sparse or absent from all surfaces in some small workers, a little more abundant on pleura and propodeum.

Integument: Head polished, or nearly so; punctures fine, very widely scattered, even near mandibular bases. Thoracic dorsum strongly shiny, with scattered obscure fine punctures; pleura and propodeum lightly shagreened, hence a little duller. Terga strongly shiny, very lightly shagreened and with scattered fine punctures.

Color: Light brownish yellow, front of head and thoracic dorsum a little darker, legs paler; mandibular margins ferruginous to brownish.

Queen
Snelling (1976) - Measurements. HL 1.90-2.02; HW 1.87-1.95; SL 1.73-1.80; EL 0.60-0.63; OMD 0.66-0.73; WL 3.9; PW 2.1-2.2.

Head: Longer than broad, CI 96-98; a little longer than scape, SI 90-91. In frontal view, head broadest at level of lower eye margin, sides straight, slightly convergent below. Occiput, in frontal view, flat, broadly rounded at sides. Eye large, about 1.4 x length of first flagellomere; EL 0.86-0.90 x OMD. OOD 2.6-2.8 x OD; IOD 1.8-2.0 x OD. Penultimate segment of maxillary palp broadest in middle, narrowed at each end.

Thorax: Robust, 0.53-0.56 x WL. In profile, posterior half of mesoscutum and anterior four-fifths of scutellum flat, forming a single plane; posterior one-fifth of scutellum sharply sloping.

Petiole: In profile, compressed, crest sharp; from behind, crest with narrow, shallow median notch; from above, about twice wider than long.

Vestiture: Cephalic pilosity about as described for worker, but erect hairs less abundant on malar area, those of occiput shorter, longest hairs distinctly less than 0.5 x MOD. Pronotum with fine erect hairs on anterior margin and scattered short, erect, coarser hairs on neck and sides; scutum with sparse, short, erect hairs; scutellum with sparse erect hairs, a few about half as long as minimum eye diameter; pleura and propodeum with scattered short erect hairs, about equal to those of scutum. Petiole with sparse erect hairs, shorter than those of scutum, on crest and sides, a few finer hairs on front face. First two terga with short, sparse erect hairs on discs; hairs progressively longer and more abundant on succeeding segments. Scape with abundant erect hairs; fore femur without erect hairs on dorsal and inner faces; these abundant on outer and ventral faces; hind tibia with abundant decumbent to subdecumbent hairs; forewing without fringe hairs; hindwing with a few fringe hairs along basal half of posterior margin.

Pubescence short, fine, sparse on head, but denser and partially decumbent on malar area; very sparse on thoracic dorsum, denser on pleurae and propodeum; short and fine on first three terga, sufficiently dense to impart sheen.

Integument: Cephalic integument, shiny, weakly shagreened between punctures; clypeal punctures coarse, separated by 1-2 puncture diameters, closest along midline; frontal lobes finely and densely punctate and with scattered coarse punctures; frons and occiput a little more coarsely and less closely punctate; malar area with coarse, elongate punctures, interspaces duller than elsewhere on head.

Pronotum moderately shiny between dense, very fine punctures; parapsis with abundant punctures equal to those of frons, a little sparser near parapside, with scattered coarse punctures, shiny; discal area of mesoscutum shinier, with punctures distinctly more separated, a broad central portion sparsely and more finely punctate; scutellum polished, with sparse very fine punctures; pleura moderately shiny, with dense fine punctures. Propodeum slightly shiny, with fine, obscure punctures and strongly shagreened. First three terga shiny, finely and sparsely punctate.

Color: Yellowish, dorsum more brownish; legs and scapes paler. Wings transparent, whitish, veins and stigma reddish.

Male
Snelling (1976) - Measurements. HL 0.83; HW 0.73; SL 0.96; EL 0.36; WL 1.7; PW 1.1.

Head: Sides distinctly convergent toward mandibular bases; head distinctly longer than broad, CI 88; shorter than scape, SI 86. OMD 9.50 x EL. Anterior ocellus smaller than lateral ocelli; IOD 2.33 x OD; OOD 1.66 x OD. Mandible with preapical notch and two small teeth basad of notch.

Thorax: Robust, PW 0.64 x WL. Basal face of propodeum narrow and broadly rounded onto posterior face.

Petiole: In profile, distinctly higher than long, sharply cuneate; in frontal view, sides distinctly convergent above, with narrow median notch; from above, about twice wider than long.

Vestiture: Sparse erect hairs present on head, shortest on frontal lobes; very fine short hairs on pronotal neck; mesoscutal and scutellar hairs short; pleura with scattered short hairs; propodeum with a few erect hairs at sides and base; scape with numerous erect hairs; fore femur with erect hairs on lower and outer faces only; hind tibia with decumbent hairs on outer face; first two terga with scattered fine erect discal hairs. Fringe hairs present on basal one-half of posterior margin of hind wing only.

Pubescence sparse on head and thorax, a little denser on frons and propodeum; longer and denser on first two terga.

Integument: Slightly shiny, densely shagreened, with a few obvious punctures on scutum and pleurae.

Color: Uniformly light brownish, appendages yellowish. Wings whitish hyaline, veins and stigma light brownish.

Type Material
Snelling (1976) - NEW MEXICO. Albuquerque, May 1905 (W. M. Wheeler). Lectotype worker, agreeing with above description, parenthetical measurements and type data, by present designation, in. Lectoparatypes in AMNH, and.