Formica obscuripes

Known as the Western Thatching Ant, Formica obscuripes is a dominant component in many habitats across much of western North America. Their large thatched mounds and huge colonies with tens or hundreds of thousands of workers are distinctive and when encountered there is little doubt as to their identity. When at high density they seem to suppress other Formica diversity in the vicinity of their nests. Workers are variable in size (polymorphic), forage during the day in large numbers across the ground and on vegetation, and perform nest maintainance on their mounds and in the immediate vicinity. They are host to a handful of other ants (inquilines and parasites, including Formicoxenus hirticornis) and a large number of guest insects (myrmecophiles and opportunists).

Distribution
This taxon is especially prevalent in the Pacific Northwest and is uncommon in New Mexico. A collection of Formica obscuripes was made at Ft. Davis, Texas, in 1902 (specimen in AMNH) but it has not been collected since and it is unlikely to occur in this area.

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

Habitat
Formica obscuripes inhabits a wide range of habitats, including grasslands, prairies, sagebrush, shrub-steppe, mixed deciduous forest up to pinyon-juniper, coniferous forests, ponderosa pine-riparian areas, coastal and inland dunes, and alpine meadows. Nests are often found along the edges of meadows and other open areas.

Nests
This species builds large mounds consisting of small pieces of plant material with tunnels and galleries extending into the soil below the mound. It is one of the only true thatching ants, with mounds constructed completely of vegetative debris (soil is used in most "thatching" ants). New colonies are typically established in an open areas devoid of cover, under a small piece of rotten wood or small stump, with this wood remaining at the base of the nest as the nest grows over time. The mound is built using various material, commonly short twigs, grass stems, conifer needles, and other material, with specific composition based on available plant material from the area around the nest. The materials used can change over time as the surrounding vegetation changes. The size of mounds is highly variable and depends on the age and health of the colony. In their early stages nests are under logs or stones usually partially covered with thatch, with the thatching growing as the colony grows. The height of typical mounds ranges from 25mm to 500mm, although larger heights are common with mounds reaching nearly 2m in some cases. 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. In large colonies workers are known to chew bark at the base of plants growing on or near the mound and spray formic acid into the open layer to kill the plant.

Colonies are polydomous (single colonies have multiple separate nests) and polygynous (single colonies have multiple queens). The number of workers and individual mounds in a colony can be exceptionally high. Near Lehman Hot Springs, Blue Mountains, Oregon, a supercolony was found that occupied an area of 4 hectares and consisted of 210 active nests with an estimated population in excess of 56 million (McIver et al., 1997).

Mounds are thought to collect solar radiation, providing warming during cooler periods, while the organic material may generally moderate temperatures and humidity levels within the nest.

Nest site selected in open areas devoid of cover. Nest begun at the base of some small plant (frequently sagebrush). Extensive use made of thatching. The finished nest consisting of a large mound of collected detritus. (Creighton, 1940)

Feeding
Formica obscuripes are general omnivore-predators, obtaining food primarily by scavenging or preying upon insects and other arthropods. Foraging takes place both on the ground and on vegetation, including high in trees. They also harvest honeydew from aphids and other homopterans and are known to occasionally eat plant tissue.

Colony Founding
These ants are temporary social parasites of Formica fusca-group species. New colonies are established when an inseminated F. obscuripes queen enters the nest of a Formica fusca-group species and is accepted by the host workers. The host queen is eventually killed or driven off, and the host workers raise the brood of the invading queen. Eventually, only F. obscuripes workers remain as the original host workers die off.

Assocation with Other Species
Nests of Formica obscuripes provide a home for many insects and other athropods. These include species of beetles (Cremastocheilus, Euphoriaspis hirtipes, Haeterius), pseudoscorpians, spring­tails, hemipterans and dipterans (Microdon). The larvae of these species often use the thatch or chambers in the nest for hibernation or development and also feed on decaying matter. Formica obscuripes is also the primary host of two inquiline ant species, Formicoxenus diversipilosus and Formicoxenus hirticornis.

Nevada
Wheeler and Wheeler (1986) provide the following notes on this species in Nevada:

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.