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Formica obscuripes
	F. obscuripes worker
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Formicidae
Subfamily:	Formicinae
Genus:	Formica
Species:	F. obscuripes
Binomial name
	Formica obscuripes; Forel, 1886

Formica obscuripes, the western thatching ant, is a species of ant in the family Formicidae. It is native to North America. It produces large mounds covered by small pieces of plant material.^[1] The number of adult workers per colony may be as high as 40,000.^[2] F. obscuripes feeds upon a number of insect species, consumes nectar from homopterous insects they tend, and occasionally eats plant tissue.^[1]

Biology

Colonies

F. obscuripes creates distinct, dome-shaped mounds composed of varying materials found in the nest's surrounding environment. This composition is primarily vegetation and is commonly called "thatch". The mounds are typically constructed in areas devoid of cover to expose the nest to sunlight. The size of these mounds is highly variable and is mostly determined by the age and health of the colony. The height ranges from 1 to 18 inches (15–45 cm), although nests of larger heights are not uncommon, and can extend down to 4 ft (1.2 m) in the ground, with chambers present in both the thatched mound and the soil. The condition of the thatched mound is in a constant state of change due to activities by the ants and changes in the environment.^[3] To prevent plants from shading the nest, the ants may chew off the bark at the base of plants growing on or nearby the mound. Formic acid is then sprayed into the open layer eventually killing and felling the plants.^[4]

In the Blue Mountains of Oregon, F. obscuripes has demonstrated the capacity for polydomy. A supercolony in a four-hectare (9.9 acre) study area near Lehman Hot Springs consisted of 210 active nests with an estimated population in excess of 56 million ants.^[5]

Chemistry

The poison sprayed by Formica obscuripes consists of formic acid, eight straight-chain alkanes, a branched-chain alkane and five alkenes. The major component, formic acid, has been found to vary from 8 to 96% and average 73% of the volatiles in the secretion. A variation was also seen with the second most abundant compound, undecane, which ranged from 3 to 63% of the sprayed secretion and averaged 19%.^[6]

Distribution

F. obscuripes is found in a variety of habitats in North America, particularly the United States of America. Most specimens are collected from the Midwest and between the Rocky Mountains and the American west coast.^[7] Populations in the Pacific Northwest may represent a distinct species, Formica planipilis,^[8] and have been described by hobbyists as typically producing larger colonies and mounds than populations elsewhere, though this has not been confirmed. Pacific Northwest populations are additionally distinguished by the unique coloration of the gynes, which sport entirely black bodies with red heads.