Formica obscuripes

Known as the western thatching ant, Formica obscuripes makes large nest mounds of thatch. This is a temporary social parasite of Formica fusca group ants during their colony founding period. Nest are often located along the edge of meadows and other open areas. Many myrmecophiles and several inquiline ants can be found within these host nests.

Identification
Formica obscuripes ants build nests deep into the soil and create a mound of thatch (twigs, grasses, and other vegetation) that can be more than 1 foot above ground. Nests are normally in an open area exposed to direct sunlight.

Distribution
Southern Canada and northern states from Indiana and Michigan westward to British Columbia and southward to California, Nevada, Utah, Colorado, and New Mexico.

Distribution based on Regional Taxon Lists
Nearctic Region: Alberta, Canada, Canada, United States.

Habitat
In New Mexico (Mackay and Mackay 2002) - Prairies, sagebrush, mixed deciduous forest up to pinyon-juniper, ponderosa pine-riparian.

Biology
Formica obscuripes twig and earthen mound nests contain a diversity of myrmecophiles including species of beetles (Cremastocheilus, Haeterius), Fly larvae (Microdon), and many others. Workers feed on a variety of insect prey including caterpillars and tend homopterans for their honeydew.

Nests are usually in open areas devoid of cover but sometimes in open woods. They are begun at the base of small plants (frequently sagebrush). Extensive use is made of thatching. The mature nest consists of a large mound of collected detritus. An important feature is a large brood chamber in the center of the thatch in which all the brood is kept. Sather (1972) provides a comprehensive account of the growth and maturity of mound structure.

For New Mexico (Mackay and Mackay 2002) - This species is not common in New Mexico. It nests in thatched mounds or under logs or stones (usually partially covered with thatch). Workers tend aphids. It is polygynous (multiple queens in the nest). This species does not have typical mating flights; instead small numbers leave the nest throughout the season. Dealate females were collected in late June and early July.

Nevada, Wheeler and Wheeler (1986):

Formica obscuripes is scattered throughout the state north of the Hot Desert (i.e., north of latitude 38°N). We have 60 records from 44 localities; 4,300-10,480 ft. (80% between 5,000 and 9,000 ft.). Twenty of the records were from the Cool Desert (l from a Sarcobatus Subclimax and 1 from a disturbed area), 5 were from the Pinyon-Juniper Biome, 6 were from the Coniferous Forest Biome, and I was from the Alpine Biome. The nest is typically a dome-shaped thatch mound which was usually circular in basal outline but often elliptical. The thatch was mounted on an earthen base 5-8 cm high and was greater in diameter (43-150 cm, average 88 cm) than the thatch. The thatch itself was piled in the center of this base and measured 30-138 cm (average 66 cm) in diameter and 13-43 cm (average 30 cm) in height. The composition of the thatch was opportunistically determined, i.e., it depended upon the available plant material. Apparently when a favorite material was sufficiently abundant, the thatch was homogeneous; this was true especially of pine needles and juniper sprays. There are numerous entrances throughout the thatch; we counted at least 50 in one mound. The foregoing might be called typical, but many variants occurred; e.g., 2 nests were under stones and 1 was under a log lying on the ground. A common variant was a long pile of messy thatch along a prostrate sagebrush trunk.

Colonies were populous and the workers were very aggressive. When a colony was disturbed the surface of the mound was soon covered with workers. Many assumed the defensive position: head up and mandibles widely spread; gaster turned forward under the thorax and ready to spray formic acid into any wound made by the mandibles. Many workers started spraying at the beginning of the disturbance and soon there was an invisible cloud of formic acid vapor above the nest that was irritating to human eyes and noses. The bites of the workers were also annoying.

Under normal conditions workers would not expose themselves to direct sunlight during the hot hours of the day, but they worked diligently in any shaded areas no matter how small. Just what they were doing was hard to determine. Seemingly they were removing sticks from the thatch and putting them back at a slightly different angle.

This species was tending Aphis incognita (Hottes and Frison) at Bunker Hill (-16N-43), Lander Co., 8,100 ft. and Brevicorne symphoricarpi (Thomas) (both Homoptera: Aphididae; det. W.B. Stoetzel) on Symphoricarpos vaccinoides, on Murry Summit, White Pine Co., 8,200 ft.

Nomenclature

 *  obscuripes. Formica rufa r. obscuripes Forel, 1886b: xxxix (w.) U.S.A. Emery, 1893i: 650 (q.m.); Wheeler, G.C. & Wheeler, J. 1953c: 165 (l.); Hung, 1969: 456 (k.). Raised to species: Creighton, 1950a: 492. Senior synonym of aggerans: Forel, 1914c: 619; Creighton, 1940a: 1; of melanotica: Creighton, 1950a: 492. Material of the unavailable name rubiginosa referred here by Creighton, 1940a: 1. See also: Weber, 1935: 165.
 * aggerans. Formica rufa subsp. aggerans Wheeler, W.M. 1912c: 90 (w.) U.S.A. Wheeler, W.M. 1913f: 430 (q.m.). Junior synonym of obscuripes: Forel, 1914c: 619; Creighton, 1940a: 1.
 * melanotica. Formica rufa subsp. melanotica Creighton, 1940a: 1, fig. 1 (w.q.m.) U.S.A. [First available use of Formica rufa subsp. obscuriventris var. melanotica Emery, 1893i: 650; unavailable name.] Junior synonym of obscuripes: Creighton, 1950a: 492.