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Broad-headed and short-limbed Polyergus species; easily distinguished by its abundant pilosity. In addition to its abundant pilosity and short scapes, it shows marked preference for Formica cinerea group hosts. Typical habitat: open, moist grassland or sedge meadow (Trager, 2013).
This ant is common in wet to mesic prairies and mesic or wetter old fields of northwestern Indiana and the Chicago Region, and is distributed west to the Rocky Mountains and south to the White Mts. of northeastern Arizona (Trager, 2013).
Trager (2013): "The raiding of Polyergus breviceps follows familiar patterns described for other species.
[Instigated by a successful scout, a dense swarm of raiders heads off, slowly at first, then organizing into a column and hastening the pace. The columns are periodically interrupted as the raid progresses. When this occurs, there is the appearance of the group’s not knowing where to go next. Then, a worker that apparently had run ahead of the main group (a scout/leader?) may be seen to return to the group and recruit the column in the right direction once again. Upon arriving at the host nest, Polyergus workers hesitate and amass outside the entrance, often clearing pebbles, twigs, and other impediments from the entrance of the host nest before they enter. Shortly after the first Polyergus enter, they begin emerging and head home bearing pupae, prepupae, or less commonly, last-instar larvae. Sometimes, especially early in the raiding season, raiders return one or more times the same afternoon to pillage additional pupae from a particularly productive host colony. Less often, a Polyergus colony raids more than one host colony in an afternoon, either simultaneously or sequentially].
Near Taos New Mexico, 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.
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 Formica 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.
P. 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, Colorado, of Polyergus (which I surmise to be breviceps) living with “F. neocinerea” (Formica 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 Formica argentea in less conspicuous nests on the wooded slopes above this meadow. In the Chicago region, tallgrass prairie restoration plantings are colonized by Formica 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 Polyergus topoffi.
Polyergus breviceps normally parasitizes members of the F. cinerea complex; Formica montana in the humid prairies of the Great Lakes and northern Plains states, and Formica canadensis in western mountain meadows. Some samples studied also included Formica altipetens or less often, the less closely related Formica neoclara or Formica 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".
Trager (2013): "Polyergus breviceps 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 Polyergus mexicanus.
Worker description: 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".
Trager (2013): "More pilose examples of Polyergus 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 Formica montana, contrasting with mexicanus’s larger size, and association with Formica 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".
Lectotype designation: Trager, 2013 (w.) Type locality: USA: Colorado, Breckenridge, Summit Co., 39°29′0″N 106°02′0″W. CASENT0179559. Musee d'Histoire Naturelle Genève.
Trager (2013: 512): "As indicated in the synonymy above, the “type series” of breviceps included the original Breckenridge, Colorado 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)".
Trager, J.C. 2013. Global revision of the dulotic ant genus Polyergus (Hymenoptera: Formicidae, Formicinae, Formicini). Zootaxa 3722, 501–548.
Found most commonly in these habitats: 8 times found in garden, 9 times found in Pasture, 2 times found in sedge meadow, 2 times found in low prairie, 2 times found in meadow, 3 times found in Redshank chaparral, 4 times found in Greasewood Desert, 1 times found in sagebrush - rabbitbrush, 1 times found in lodgepole pine forest, 1 times found in Low meadow, ...
Found most commonly in these microhabitats: 5 times under rock, 1 times log stage 4, 1 times log stage 3, 1 times large domed nest, 3 ft. diam., 1 times grassy dome, 1 times domed nest, 1 times dead branch, log stage 3, 1 times sod mound near brook, 1 times raiding party, 1 times mound in wet meadow, 1 times masonry dome nest, ...
Collected most commonly using these methods: 3 times Bay Area Ant Survey (BAAS), 2 times in pitfall trap, 2 times (=incip. col. P1 collected Aug 1982), 1 times hand collected.
Elevations: collected from 40 - 3060 meters, 1836 meters average
Type specimens: Lectotype of Polyergus breviceps: casent0179559; syntype of Polyergus breviceps: casent0912265; syntype of Polyergus rufescens bicolor: casent0912267; syntype of Polyergus rufescens bicolor: casent0911120; syntype of Polyergus rufescens breviceps silvestrii: casent0912266