Myrmecocystus yuma

Identification
A member of the subgenus Eremnocystus.

Key to Myrmecocystus subgenus Eremnocystus species.

Worker - Scape without conspicuous erect hairs; propodeum with cluster of fully erect hairs at juncture of basal and posterior faces; erect pronotal hairs with blunt apices; hind femur without erect hairs on flexor surface and few or none on extensor surface; petiolar scale not strongly compressed. Female - Scape with few or no erect hairs; mesoscutum largely irnpunctate in median area, the scattered punctures much coarser than those of the densely and uniformly punctate scutellum; frontal lobe finely, densely and uniformly punctate.

Although there is some variation in the profile of the petiolar scale, that of Myrmecocystus yuma is definitely thicker than that of Myrmecocystus lugubris. A notch is usually not present on the crest of the scale in yuma. When a notch is present, it is weak. The petiolar scale of lugubris rarely has a weak notch, more often a very distinct one.

Of far greater utility is the fact that the dorsum of the propodeum of yuma always has six or more fully erect hairs, while none are present here in lugubris. Both possess a number of fully erect hairs on the pronotum and mesonotum. The punctures of the frontal lobes are finer and closer in yuma than in lugubris, while in the latter species the discal hairs of the first tergum are shorter and stiffer. (Snelling 1976)

Distribution
United States, Mexico. Mojave and Colorado Deserts of southern Nevada, southern California, western Arizona, northwestern Sonora and northeastern Baja California.

This taxon was described from U.S.A.

Biology
Snelling (1976) - Wheeler (1912) observed that nests at the type locality were surmounted by craters three to four inches in diameter and that remains of dead insects were abundant around the edges of the craters. From the latter he surmised the ant to be predatory. Nests are usually, if not always, located in sand, either coarse or fine. Craters seem not to exceed 10.5 cm in diameter. Bits of twigs and leaves are sometimes found about the margins of the tumulus, as noted by Wheeler, but whether these were placed there by the ants or wind-blown is not known.

Little is known of the foraging behavior of this ant. Wheeler believed the species to be predatory. Since I have observed workers visiting flowers and soliciting both aphids and pseudococcids for honeydew, it would appear that the food gathering habits do not differ materially from those of other Eremnocystus. I have observed workers foraging both during early morning and late afternoon.

All collections of this species have been made in Creosote bush and Creosote bush-Bur sage Desert and at elevations ranging from sea level to 2000 feet.

Nomenclature

 *  yuma. Myrmecocystus yuma Wheeler, W.M. 1912d: 176 (w.) U.S.A. Snelling, R.R. 1976: 110 (q.). Junior synonym of lugubris: Creighton, 1956: 1. Revived from synonymy: Snelling, in Hunt & Snelling, 1975: 23.

Snelling (1976) - Creighton (1956) synonymized this species with Myrmecocystus lugubris based on a study of type material of both forms and of samples which he collected in Arizona and California in 1952. He carefully examined the separatory characters utilized by Wheeler (1912) and Creighton (1950) and found them worthless. While I agree that the traditional method of separation is ineffectual, I cannot agree that the two ants are conspecific. Had Creighton had available to him females from the colonies he collected he doubtless would have realized that he had four species rather than one. With the Creighton material before me, it is evident that, in addition to lugubris and yuma, he also had samples of Myrmecocystus creightoni and Myrmecocystus tenuinodis. The recognition of these two latter species, prompted by female characteristics, enabled me to separate once again lugubris and yuma.

Worker
Snelling (1976) - Measurements. HL 0.76-1.00 (0.83); HW 0.73-1.00 (0.83); SL 0.80-1.03 (1.00); WL 1.00-1.33 (1.20); PW 0.50-0.66 (0.57).

Head: Little broader than long or as broad as long, CI 92-100 (l00); a little shorter than scape, SI 103-120; in frontal view, sides slightly convex and a little narrowed toward mandibular insertions; occiput, in frontal view, evenly and gently convex from side to side, without lateral angulations. Eye small, barely longer than first flagellomere; OMD 1.37-1.57 (1.47) X EL.

Thorax: Moderately robust, PW 0.43-0.53 (0.47) x WL. Mesonotum, in profile, nearly flat, without posterior angulation or abrupt convexity. Propodeum higher than long, basal face broadly rounded onto posterior face.

Petiole: Scale erect, thick in profile, a little more than twice higher than thick, summit rounded; crest, in frontal view, flat, sometimes with a very weak, angular, median notch; from above, about twice wider than thick.

Vestiture: Cephalic pubescence scattered, a little more evident, but still quite sparse, on frontal lobes and occiput; thoracic pubescence sparse, a little denser on propodeum; gastric sheen evident on first two terga, dilute sheen often present on third tergum of larger workers; remaining terga with scattered pubescence or none.

Malar area normally without erect hairs, occasionally one or two present near mandibular base; clypeus, frontal lobes and occiput with sparse erect hairs, longest occipital hairs more than 0.5 x MOD. Pronotum and mesonotum each with ten or more standing hairs, metanotum with fewer than six short erect hairs; basal face and side of propodeum with scattered erect hairs. Crest of petiole with a few erect hairs. Terga with sparse erect hairs, progressively longer on succeeding segments. Scape without erect hairs except near apex, elsewhere with a few very fine reclinate hairs; inner face of fore femur and dorsal face of hind femur without erect hairs; hind tibia without fully erect hairs, but a variable number of fine, decumbent hairs.

Integument: Clypeus polished and shiny, with scattered fine punctures; frontal lobes lightly shagreened and shiny, with sparse, fine punctures; frons and occiput shiny, obscurely shagreened; malar area mostly shiny, a little duller and more distinctly shagreened near mandibular base. Thorax moderately shiny, distinctly shagreened, more distinctly so on the slightly shiny propodeum. First tergum slightly shiny, closely shagreened; second tergum shinier, more weakly shagreened; third tergum moderately shiny and shagreened in large workers, shiny and very weakly shagreened or polished in smaller.

Color: Light to dark brownish, gaster usually a little darker than head and thorax; appendages lighter; mandible, sides of clypeus and malar area near mandibular base, yellowish.

Queen
Snelling (1976) - Measurements. HL 1.40; HW 1.53; SL 1.23; EL 0.40; OMO 0.53; WL 3.0; PW 1.73.

Head: Broader than long, CI 109; in frontal view, malar margin straight, very slightly convergent toward mandibular base, occiput gently and evenly convex in frontal view, without evident lateral angles; distinctly longer than scape, Sl 88. Eye 1.5 x first flagellomere; OMO 1.33 x EL. Lateral ocelli slightly larger than anterior ocellus; IOD 4.5 x OD; OOD 6.0 x OD. Mandible septendentate. Penultimate maxillary palpal segment broader near middle than at either base or apex; fourth segment nearly parallel-sided, preapical width about one-fourth greater than basal width.

Thorax: Robust, PW 0.57 X WL. Posterior half of mesoscutum flattened and continuous with anterior portion of scutellum, posterior half of scutellum more strongly sloping. Metanotum distinctly protuberant. Propodeum with narrow basal face.

Petiole: In profile, distinctly narrowed above, crest narrow; in frontal view, sides a little convergent above, median emargination narrow, deep, angulate; from above, about three times wider than thick.

Vestiture: Cephalic pubescence thin; scattered on clypeus; sparse on frons; denser, but still sparse, on frontal lobes; sparse on malar area; short and sparse on occiput. Thoracic pubescence thin, densest on pronotum, sides of mesoscutum, pleura and propodeum. First four terga with pronounced sheen.

Malar area with a few scattered short erect hairs and two or three longer decumbent hairs near mandible; clypeus with sparse erect and scattered suberect shorter hairs; frontal lobes with sparse erect hairs; longest occipital hairs more than 0.5 x MOD. Scutum with scattered erect hairs, longest equal to about 0.5 x MOD; scutellum with scattered hairs, some about twice as long as longest scutal hairs; pleura with a few erect hairs, especially above; metanotum with a few erect hairs; propodeum with a few erect hairs above and on sides; petiolar scale with erect hairs on crest and sides. All terga with sparse, short erect hairs, progressively longer on succeeding segments. Antennal scape with numerous decumbent to suberect short hairs. Inner face of fore femur without erect hairs; dorsal face of hind femur with decumbent to suberect hairs, a few erect hairs near apex; hind tibia with abundant suberect hairs on outer face.

Wings not present on one specimen studied.

Integument: Clypeus shiny, subpolished; with sparse, coarse, setigerous punctures and scattered fine piligerous punctures; frontal lobes shiny, finely, densely and uniformly punctate; frons moderately shiny, finely, densely and uniformly punctate; occiput slightly shiny, finely and irregularly punctate; malar area shiny, coarsely and closely punctate, duller below. Mesoscutum shiny and subpolished, disc with scattered coarse punctures and a narrow band of fine, sparse punctures along midline; parapsis uniformly densely and finely punctate. Scutellum shiny, with scattered, coarse punctures on either side of midline, otherwise with uniformly sparse, fine punctures. Propodeum moderately shiny, strongly shagreened. First tergum moderately shiny, densely and finely punctate on disc; second and third terga similar, but piligerous punctures much finer; fourth tergum a little shinier and more sparsely punctate.

Color: Medium brown, gaster blackish brown, appendages lighter; mandibles and sides of clypeus yellowish.

Type Material
Snelling (1976) - Cotype worker series from Yuma, Yuma Co., ARIZONA, collected 26 Nov. 1910, by W. M. Wheeler. Lectotype worker, by present designation, agreeing with above description, parenthetical data and label data, in. Lectoparatypes in AMNH, and.