Polyergus breviceps

Trager, James C., 2013, Global revision of the dulotic ant genus Polyergus (Hymenoptera: Formicidae, Formicinae, Formicini), Zootaxa 3722 (4), pp. 501-548 : 511-513

Polyergus breviceps

Figures 6, 7, 8

Polyergus rufescens breviceps Emery 1893: 666 . Lectotype worker (here designated) USA, COLORADO Summit Co. Breckenridge. [MHNG, CASENT0179559] (examined).

Polyergus breviceps: Kannowski, 1956: 185 ; Wheeler, 1968: 163.

Material of the unavailable names fusciventris, silvestrii, montezuma, referred to breviceps by Creighton (1950), here referred to P. bicolor , P. mexicanus , and P. topoffi .

Former syntypes (N=3 on a single pin, including lectotype) HL 1.36–1.40 (1.37), HW 1.32–1.36 (1.35), SL 0.90–0.96 (0.92), ½ VeM 3–6 (5), ½ PnM 12–20 (16), WL 1.96–2.02 (1.99), GL 1.64–1.88 (1.77), HFL 1.32–1.36 (1.35), CI 97–100 (98), SI 66–71 (68), HFI 97–103 (101), FSI 138–53 (147), LI 3.32–3.40 (3.37), TL 4.96–5.28 (5.14).

Measurements (N=58) HL 1.24–1.60 (1.46), HW 1.28–1.64 (1.48), SL 0.84–1.08 (0.98), ½ VeM (3, one specimen) 5–17 (9.52), ½ PnM 8–22 (14.97), WL 1.88–2.40 (2.14), GL 1.64–2.60 (2.13), HFL 1.30–1.72 (1.52), CI 0.96–1.08 (1.01), SI 61–78 (67), HFI 94–115 (103), FSI 138–170 (155), LI 3.12–3.96 (3.60), TL 4.96–6.52 (5.73).

Worker description. Polyergus breviceps is more narrowly defined here than has been conventional in North American ant taxonomy. It is a broad-headed and short-limbed species, but most easily distinguished by its abundant pilosity. This is among the two smaller Nearctic species, though averaging somewhat larger than partially sympatric bicolor , and larger than some isolates of mexicanus .

Head suborbicular to (less often) subquadrate, its length and breadth about equal, or not uncommonly the breadth a bit greater, sides quite rounded, outer margins of eyes not or at most slightly extending beyond sides of head; vertex flat or broadly and shallowly concave, the flat portion or concavity about as wide as the space between mandibles; vertex pilosity conspicuous and abundant, usually 16–24 (6–30) macrosetae; scape not reaching vertex corners by about twice its maximum diameter, clavate in the apical third; pronotum usually with 22–36 (16–44) erect setae, including a few shorter ones near the lower margins; mesonotal profile flat or very weakly convex for most of its length; propodeum evenly rounded; petiolar dorsum rounded and shallowly emarginate; first tergite densely pubescent; first tergite pilosity flexuous, basally suberect and distally subdecumbent, about as dense in posterior half of tergite as in its anterior half, appearing to be in 5 or 6 transverse arrays.

Head matte; mesonotum matte; gaster matte; slightly shining lateral portions of all tagmata in some specimens. Color usually dull red with infuscation of dorso-posterior portions of tergites. Pilosity matching color of body to slightly darker, pubescence yellow gray.

Discussion. Polyergus breviceps is here restricted to a broad-headed and short-limbed species, one most easily distinguished by its abundant pilosity. In addition to its abundant pilosity and short scapes, it shows marked preference for F. cinerea group hosts and their typically open, moist grassland or sedge meadow habitat. More pilose examples of mexicanus found both at the northern and southwestern portions of its range are those most likely to be confused morphologically with breviceps . Where mexicanus and breviceps occur together (in close parapatry) in the West, mexicanus occurs in well drained soils, often in conifer forests on podzols, while breviceps occurs in wet meadows with organic-rich soils. Even when its pronotal and mesonotal dorsal pilosity is abundant, P. mexicanus lacks the pilosity on the sides of the pronotum, lacks or at least has little vertex pilosity, has a shinier head, and is deeper red in color, with gray rather than yellowish gray pubescence. In the Chicago region, breviceps is readily distinguished, even in the field, from the local version of mexicanus by its clearly smaller size, and its association with F. montana , contrasting with mexicanus ’s larger size, and association with F. subsericea . In the Dakotas and Rockies, a closer examination may be required to discriminate breviceps from other Polyergus , though these other congeners are more often found in upland prairie, open woodland or forest, rather than the usually moist (including saline and alkaline) meadow habitats preferred by breviceps . True breviceps does not occur in Pacific Coast states or provinces.

The majority of the literature regarding P. b re v i c e p s regards either mexicanus or topoffi (Topoff 1982, 1985, Topoff et al. 1984, 1985a, 1985b, 1988a, 1988b, 1989, Topoff and Greenberg 1988, Topoff 1990, Topoff and Mendez 1990, Topoff and Zimmerli 1993, Zimmerli and Topoff 1994, and included references), and true breviceps is in fact little studied. As indicated in the synonymy above, the “ type series” of breviceps included the original Breckenridge, CO material, plus two samples collected later at other localities that I identify as bicolor and mexicanus . I restrict the type to the Breckenridge material (CASENT0179559), and exclude the other samples, one being a sample of bicolor (CASENT0179561) and the other a sample of mexicanus (CASENT0179560). Once the characterization of breviceps is clear, it may be noted that there is remarkably little variation in this species, notwithstanding its broad but spotty distribution. Western samples have slightly, but insignificantly shorter average SL and HFL.

Etymology. Emery coined this name from the Latin “brevis” plus “-ceps” to mean short-headed (in contrast to the more elongate head shape of rufescens ).

Natural history. This ant is common in wet to mesic prairies and mesic or wetter old fields of northwestern IN and the Chicago Region, and is distributed west to the Rocky Mountains and south to the White Mts. of northeastern AZ.

The raiding of P. breviceps follows familiar patterns described for other species. I have not directly observed mating and colony foundation, but have seen alates fly from the nest several hours before the late afternoon raids, and I also have seen a lone, dealate gyne wandering near a mound of F. montana in a prairie near Chicago. It would seem such lone gynes are capable of colony foundation, even with this rather aggressive host and its populous colonies. Polyergus breviceps is naturally a species of wet and mesic prairie and meadow habitats, though it persists in drier, but formerly wet, locations after habitat degradation and hydrological disruption, if the host remains abundant (I have observed this both in CO and IL). Wheeler (1910, p. 477) describes a situation near Florissant CO, of Polyergus (which I surmise to be breviceps ) living with “ F. neocinerea ” ( F. canadensis ) in conspicuous mounds raised above the moist soil of a mountain meadow, and what he took to be the same species (but which I surmise to be mexicanus ) living with F. argentea in less conspicuous nests on the wooded slopes above this meadow. In the Chicago region, tallgrass prairie restoration plantings are colonized by F. montana in just a few years, and breviceps seems to arrive almost or indeed concurrent with them, just a few years after conversion from plowed crop land. This may occur through breviceps gynes teaming up with young F. montana gynes or incipient host colonies, as has been reported for P. topoffi . (See account for this species, below.) Polyergus breviceps normally parasitizes members of the F. cinerea complex; F. montana in the humid prairies of the Great Lakes and northern Plains states, and F. canadensis in western mountain meadows. Some samples studied also included F. altipetens or less often, the less closely related F. neoclara or F. occulta . A few samples have been found from drier western grassland sites with these less pilose hosts, and these breviceps seem to average a bit less pilose than those with cinerea group hosts, but still have telltale pronotal lateral pilosity. These also differ in proportions (narrower head, slightly longer limbs) from typical P. breviceps , and may represent another species or a hybrid. Near Taos NM, I once observed raiding columns from two colonies of this species cross paths, resulting in a battle lasting two days, including over night, with high mortality. One of the colonies disappeared after this.

Distribution of studied specimens. ARIZONA Apache Co. Williams Valley 33°51.8’N 109°13.2’W 8690’ Elev. 12-X-2004 #3490 RA Johnson (RAJC); COLORADO Alamosa Co. Alamosa, 2286m. Pasture 28-VI-1945 E. V. Gregg (FMNH); COLORADO Boulder Co. Niwot. Minims. Nest in garden. (colony P1) 16 Aug. 1982 JC Trager (incipient colony, JCTC); COLORADO Boulder Co. Niwot. (=incip. P1, Aug. 1982) 4 Jul. 1985 JC Trager (JCTC); COLORADO Boulder Co. Lagerman Reservoir, Niwot, 1548m. 26.x.1961. Mound in wet meadow. R. E. Gregg (FMNH); COLORADO Park Co. Hartsel VII-4-32 Creighton (FMNH); COLORADO Park Co. Taryall. R Savolainen #102/98 (JCTC); COLORADO (Gunnison Co?) Snodgrass Mtn. Gothic. 2920m. Masonry dome nest. J. Atticott (FMNH); COLORADO Gunnison Co. Crested Butte Savolainen 1998 43/98, 45/98; COLORADO Gunnison Co. Farnum Peak GG 1998 Savolainen 102/98, 108/98, 110/98 (JCTC); COLORADO Gunnison Co. Gothic 1998 Savolainen 39/98, 47/98, 49/98 (JCTC); COLORADO Routt Co. Steamboat Springs VII-1-1943 6800’ Owen Bryant (FMNH); COLORADO Weld Co. St. Vrain Nuclear Site, Platteville 1478 m 10-VII-1976 Col. W. Brewer (FMNH); IDAHO Twin Falls Co. South rim, Snake River Canyon Twin Falls. 1067m, 11.vi.1967 sagebrush-rabbit brush. Mound near brush. R. E. Gregg w/ F. neoclara (FMNH); ILLINOIS Cook Co. Chicago VII-10-33 M Talbot (JCTC); ILLINOIS Cook Co. Chicago 4-30-33 M Talbot #33-45 (JCTC); ILLINOIS Cook Co. US 45 Chicago Pond on top of mound M Talbot #33-221 (JCTC); ILLINOIS Cook Co. Chicago 8-102 140 St. off Halsted 10-30-38 AS Winds (JCTC); ILLINOIS Cook Co. Chicago 142 St. and Halsted, Harvey low prairie #11 13.VII.1939 R. E. Gregg (FMNH); ILLINOIS Kane Co. FermiLab prairie planting, on interp trail. 1800hr 23 Aug 2008 Abundant F. montana (lone gyne, JCTC); ILLINOIS Lake Co. Site 16 SBM 415 31 May 2011 Sean Menke (JCTC); ILLINOIS Will Co. Mokena 96th Ave. near W. L. S. Transmitter 4-IX-1942 prairie #24 R. E. Gregg (FMNH); INDIANA Lake Co. Hammond mound by ditch 4-27-1933 M Talbot #33-30 (JCTC); IOWA Dickinson Co. Iowa Lakeside Lab. August 1997 R Savolainen #s 63/97 & 67/97 (JCTC); NEBRASKA Morrill Co. North Platte River Wetlands. Salt Marsh. J. Jurzenski Coll’n #Morr 01-07-25-01; NORTH DAKOTA Barnes Co. 2236 19.VII.1963 G.C. & J.N. Wheeler (LACM); NORTH DAKOTA Barnes Co. Conservation Area 19-VII- 1963 #2236 G.C. & J.N. Wheeler (LACM); NORTH DAKOTA Cavalier Co. Waterloo Twp. 13/7/54 #77 Don Sather LACM); NORTH DAKOTA Grand Forks Co. 45 Mekinock 10-VII-1932 C.V. Johnson (LACM); NORTH DAKOTA Grand Forks Co. Powell VIII-1-1932 #701 C.V. Johnson (LACM); NORTH DAKOTA Grand Forks Co. T152NR52W Sec. 9 26-VII-1959 #2166 G.C. & J.N. Wheeler (LACM); NORTH DAKOTA Grand Forks Co. Oakville Township Oakville Prairie Sec. B 14-VII-1949 #408 G.C. & J.N. Wheeler (LACM); NORTH DAKOTA Grand Forks Co. 16-151-51 13-VII-1964 2552 G.C. & J.N. Wheeler (LACM); NORTH DAKOTA Ramsey Co. Twp. Fancher Sec. 22 VIII-17-1951 #157c and IX-11-1951 # 188 P. B. Kannowski (LACM); NORTH DAKOTA Ramsey Co. Twp. DeGroat Sec. 18 IX-18-1951 # 168 P. B. Kannowski (LACM); NORTH DAKOTA Ramsey Co. Twp. T53R64 Sec. 16 IX-30-1951 # 221 P. B. Kannowski (LACM); NORTH DAKOTA Walsh Co. Twp. Ardoch s.4 2-VII-1950 # 180 W. E. LaBerge (LACM); NORTH DAKOTA Ward Co. Kenmare VII-27-1954 G.C. & J.N. Wheeler (LACM).

Figures

FIGURES 6 – 8. Lateral, full face, and dorsal views of Polyergus breviceps worker Polyergus breviceps CASENT 0281065. Photographs by Shannon Hartman from www. AntWeb. org.