Myrmecocystus depilis

Identification
See the description below.

Distribution
United States, Mexico. Western Texas to southern Nevada, south to central Mexico.

This taxon was described from Mexico.

Biology
Snelling (1976) - Known habitats for this species range from Creosote bush Desert to Trans-Pecos shrub savannah in the United States, with the bulk of the records in Grama-Tobosa Shrubsteppe. Since many records attributed to Myrmecocystus mimicus by Wheeler (1908, 1913) and Creighton (1950) are actually based on this species, it follows that many of their behavioral observations refer to depilis. The notes on "mimicus" by Cazier and Statham (1962) are also based on depilis. These investigators studied depilis in southeastern Arizona. Their study area was on an alluvial fan comprised primarily of silt, sand and soil up to a depth of 12 inches underlaid by caliche. The principal cover was tarbush, Flourensia cernua D.C., with an open stand of creosote bush, Larrea tridentata (D.C.) in the northern half of the plot.

In the study plot (about 50,184 sq. yds.) more than 60 nests were present. Distances between individual colonies ranged from 14 to 132 feet, the average being 49 feet. Of 42 colonies studied, 19 had asymmetrical entrance holes; the average east-west diameter was 1.30 in. and the average north-south diameter 0.92 in. The 23 with more symmetrical entrances averaged 0.93 and 0.84 in. respectively. Of those with measurable tumuli 10 of the asymmetrical nests had tumuli averaging 7.17 in. in diameter. The tumuli were found to be rounded craters consisting of small or medium sized pebbles and soil, surrounded by discarded bits of plants, insects and miscellaneous debris.

Maximum foraging activity during the observation period (2 Aug. -12 Sept. 1959) took place when there had been a rain on the preceding night and daytime temperatures were between 80° and 98° F. or on warm, overcast days threatening rain. Both individual wandering and directional column foraging were noted. Ants were not found visiting flowers for nectar during August and September, but were found to bring back large amounts of dead and living arthropods. By contrast, during October and November, they were seen to be "... feeding on or gathering the pollen and nectar from at least two plants, Parthenium incanum H.B.K. and Euphorbia albomarginata Englm. and may have been getting honey-dew from a species of Aphid ... on the latter plant."

These authors show that the scarabaeid beetle Cremastocheilus stathamae Cazier is regularly associated with this Myrmecocystus; and C. constricticollis Cazier was found once. Bombyliid flies were seen to apparently oviposit at the nest entrances and an unidentified dipterous larva was present in material excavated from a nest. Case-bearing chrysomelid beetle larvae, possibly of the genus Saxinus, were found just inside the nest entrance on 11 Sept.

Cazier and Statham found repletes present in the colonies which they studied. The author has collected them from a nest excavated 6.1 mi NE of Apache, Arizona.

Nomenclature

 *  depilis. Myrmecocystus melliger var. depilis Forel, 1901c: 135 (w.) MEXICO. Snelling, R.R. 1976: 52 (q.m.). Raised to species: Snelling, in Hunt & Snelling, 1975: 22.

Description
Snelling (1976) - This ant has been repeatedly confused with Myrmecocystus mimicus. Curiously, Wheeler (1908) listed this as a variety of mimicus, although depilis had a priority of seven years! The two ants are superficially similar, but media and major workers of Myrmecocystus depilis always possess long, slender flexuous hairs on the pronotum, some of which equal or exceed EL. Also, the third gastric tergum usually has conspicuous appressed pubescence, except in the smallest workers. The minor workers are more difficult to separate, but those of depilis possess hairs of very variable length on the pronotal dorsum, a few of which are exceptionally long, about three times as long as the shortest hairs. In mimicus minors the pronotal hairs are uniform in length.

Much more vexatious is the problem of distinguishing between this and Myrmecocystus nequazcatl for the two appear to be closely related. The head and thorax are consistently and uniformly light ferruginous in nequazcatl workers, light to dark brownish in depilis. Erect hairs are less abundant on the scape of depilis workers, those of any row for the most part separated by more than their own lengths; the hairs are more numerous and closer in nequazcatl, The longest pronotal hairs are as long as, or exceeding, the eye length in depilis, about as long as the minimum eye diameter in nequazcatl.

The sexual forms of the two species are very similar, but those of depilis have shorter occipital hairs in the female and the mesoscutal hairs of both sexes are shorter. In females of depilis the eye is smaller, EL 1.00-1.27 X IF (EL 1.40 X IF in nequazcatl). Males of depilis also have somewhat smaller eyes, OMD 1.13-1.54 X EL (OMD 0.78-1.00 x EL in nequazcatl). These differences, although slight, appear to be consistent. The ranges of the two species become quite close in coastal areas of Sonora, Mexico, and there is no evidence to suggest that the characters tend to break down in these areas.

Worker
Snelling (1976) - Malar area with few or no erect hairs in frontal view; gaster blackish, head and thorax extensively infuscated; some pronotal hairs longer than EL in large workers; large workers with abundant, medias and minors with scattered, pubescence on disc of third tergum. Female: OMD 1.6 or more x EL; longest occipital hairs about equal MOD, longest scutal hairs about 0.5 x MOD; parapsis closely, uniformly punctate; malar area with fewer than eight erect hairs. Male: Posterior half of mesoscutum very superficially tessellate; gastric pubescence very sparse; longest occipital hairs about 0.75 x MOD; longest scutal hairs less than 0.75 x MOD; OMD 1.13-1.54 x EL.

Measurements. HL 0.90-1.53; HW 0.77-1.50; SL 1.10-1.67; WL 1.3-2.3; PW 0.6-1.0.

Head. Distinctly to slightly longer than broad, CI 80-98 (89), shorter than scape, SI 105-128 (120); sides of head, in. frontal view, straight in small workers, slightly convex in large workers, slightly convergent toward mandibular base. Occiput, in frontal view, gently and evenly convex in small, medially flattened in large workers. Eye small, 0.89-1.36 x first flagellomere; OMD 1.45-2.00 x EL. Mandible with seven teeth.

Thorax: Slender, PW 0.40-0.46 (0.43) x WL. Propodeum, in profile, with basal face flat or slightly convex, juncture with posterior face narrowly rounded.

Petiole: In profile, thickly cuneate, summit bluntly rounded; in frontal view, crest evenly rounded, rarely with shallow median notch.

Vestiture: Pubescence sparse on head, abundant on thorax and first three terga (sparse or absent on third tergum in media and minor worker).

Cephalic hairs numerous on frons and occiput, some on occiput exceptionally long, slender and flexuous, subequal to EL; malar area, in frontal view, with at most three widely spaced, erect, short hairs, often none. Promesonotum with numerous erect hairs, some long, slender, flexuous (in large workers equal to or exceeding EL, in small workers usually about equal to MOD); base and side of propodeum with abundant erect hairs, some of which may equal or exceed 0.5 x MOD. Side and crest of petiole with numerous erect hairs, some of which may equal longest of propodeum. Terga with numerous long, erect hairs, longest on disc of second segment more than 0.5 x MOD; longer on apical segments and on sterna. Scape with erect hairs short, sparse, hairs mostly separated by more than twice their length; more abundant on femora and tibiae, but still sparse (those of any row on tibiae mostly separated by about twice their length).

Integument: Head moderately shiny, lightly shagreened, with scattered fine punctures in malar area and on face below vertex; occiput behind ocelli densely, finely punctate; frontal lobes closely, more coarsely punctate. Thorax slightly shiny, lightly shagreened and closely micropunctate, more coarsely so on propodeum. First two terga slightly shiny, lightly shagreened and closely micropunctate; third tergum similar (majors and most mediae) or subpolished and very sparsely punctate (minors and some mediae).

Color: Head, thorax, petiole and appendages medium ferruginous, often almost entirely brownish, but with brownish infuscation at least on vertex, propodeum, petiole and legs; gaster medium brown to blackish brown.

Queen
Snelling (1976) - Measurements. HL 1.77-1.97; HW 1.83-2.04; SL 1.63-1.73; WL 4.0-4.5; PW 2.2-2.6.

Head: Usually slightly broader than long, CI 100-107, longer than scape, SI 88-93. In frontal view, margins nearly straight, slightly convergent toward mandibular base. Occiput, in frontal view, flattened in middle, sides evenly rounded onto lateral head margins. Eye small, 1.0-1.27 times length of first flagellomere, OMD 1.62-1.93 x EL. IOD 2.9-3.7 x OD. OOD 4.3-5.7 x OD). Penultimate segment of maxillary palp narrow, spindle-shape, narrower at apex than at base. Mandible usually with eight, sometimes with seven, teeth.

Thorax: Robust, PW 0.52-0.59 x WL. Posterior half of mesoscutum and anterior two-thirds of scutellum flattened, scutellum convex behind. Propodeum, in profile, with base narrow, poorly defined, broadly rounded onto posterior face.

Petiole: In profile, compressed-cuneate, crest narrowly rounded; crest, from front, distinctly angularly notched; from above about twice wider than long.

Vestiture: Erect hairs general on head, least abundant on malar area, where fewer than six are present on each side; occipital hairs variable in length, longest about equal to MOD; longest hairs on clypeal disc equal to about 0.5 x MOD. Mesoscutal hairs sparse, short, longest about 0.5 x MOD; scutellar hairs more variable, some about equal to EL; longest pleural hairs about equal to MOD; propodeum with numerous erect hairs on side and across base, longest equal to about 0.5 MOD. Petiole with numerous short, erect hairs on side and crest. Terga with numerous erect hairs, progressively longer on succeeding segments, longest on disc of second segment about equal to MOD. Fore femur without long erect hairs on inner face, a few short, erect hairs near lower margin. Tibiae with numerous suberect hairs which are about as long as minimum thickness of hind tibia. Scape with numerous fine erect hairs on outer and lower faces. Fore and hind wings without fringe hairs.

Pubescence sparse on head, most conspicuous on malar area and occiput; moderately long and dense on most of thorax, inconspicuous on scutum (except on parapsis and along anterior margin), sparse on scutellum; dense on first four terga.

Integument: Head moderately shiny, surface microreticulate; frontal lobes densely punctate, punctures of several sizes; clypeus duller, with scattered coarse punctures; face more sparsely and coarsely punctate than frontal lobes; malar area similar, but punctures denser; occiput finely and densely punctate. Mesoscutum shiny, center of disc impunctate, otherwise disc sparsely punctate, punctures of two sizes, parapsis with uniformly dense fine and irregularly scattered coarse punctures. Scutellum finely punctate, punctures evenly spaced, separated by about a puncture diameter. Mesopleura slightly shiny, with fine punctures separated by one-fourth to one-half a puncture diameter, those of anepisternum a little finer and more uniformly spaced than of katepisternum; metapleura and propodeum dull, roughened, with close, fine punctures and scattered coarse punctures. First four terga densely, finely piligerously punctate, without areas of sparse punctation of discs.

Color: Head light ferruginous, thorax and gaster medium to dark brownish, mesoscutal disc sometimes lighter; antennae and legs light brownish. Wings whitish, subcostal vein dark brownish, remaining veins and stigma yellowish brown.

Male
Snelling (1976) - Measurements. HL 0.83-0.93; HW 0.83-0.97; SL 0.93-1.03; WL 1.83-2.34; PW 1.13-1.30.

Head: Slightly longer than broad to slightly broader than long, CI 96-104, shorter than scape, SI 107-119; in frontal view, sides straight, evenly convergent below; occiput, in frontal view, evenly convex from side to side, without perceptible lateral angles. OMD 1.13-1.54 X EL; OOD 2.4-3.0 x OD; IOD 2.3-3.0 x OD. Mandible without pre apical notch or basal denticles.

Thorax: Robust, PW 0.53-0.64 x WL. Propodeum with narrow basal face broadly rounded onto posterior face.

Petiole: Cuneate in profile, summit narrowly rounded; crest, in frontal view, weakly notched to broadly, shallowly concave.

Vestiture: Pubescence everywhere sparse, more conspicuous on pleurae and sides and base of propodeum. Pilosity short on frons, longer on malar area and behind eye, longest on occiput, longest occipital hairs about equal to MOD. Mesonotum with numerous erect hairs, longest less than 0.75 x MOD; some scutellar hairs longer, subequal to EL; longest pleural hairs about equal to MOD; side of propodeum with long, flexuous hairs, basal face with shorter hairs. Petiole with short, erect hairs on sides and crest. Gaster with long, flexuous hairs, longer and more abundant caudad. Scape, femora and tibiae with abundant erect to suberect short, stiff, acuminate hairs. Wings without fringe hairs on apical or posterior margins.

Integument: Head moderately shiny, distinctly shagreened, more distinctly so on malar area and gena; with scattered, obscure, piligerous punctures. Discs of scutum and scutellum shiny, very superficially shagreened, parapsis duller, more sharply shagreened. Pronotum, pleurae and propodeal base and sides duller, distinctly shagreened; disc of propodeum shiny and smooth. Most of dorsum of first tergum and narrow discal areas of second and third terga, nearly smooth, shiny and very superficially shagreened; remainder of these and other segments duller, more distinctly shagreened; all segments with scattered piligerous punctures.

Color: Blackish brown; mandibles, antennae and legs medium brown. Wings whitish, veins and stigma pale brownish.

Type Material
Snelling (1976) - Two cotypes from Pacheco, Zacatecas, Mexico, no date, collected by W. M. Wheeler. One, without gaster, selected as Lectotype, is in the ; the other is in the.

Additional References

 * Holldobler, B., Grillenberger, B. & Gadau, J. 2011. Queen number and raiding behavior in the ant genus Myrmecocystus (Hymenoptera: Formicidae). Myrmecological News, 15, 53-61.