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. The number of adult workers per colony may be as high as 40,000. F. obscuripes feeds upon a number of insect species, consumes nectar from homopterous insects they tend, and occasionally eats plant tissue. In the Blue Mountains of Oregon, F. obscuripes has demonstrated the capacity for polydomy. A supercolony in a four-hectare study area near Lehman Hot Springs consisted of 210 active nests with an estimated population in excess of 56 million ants.