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western thatching ant

Formica obscuripes

What do they look like?

Western thatching ants have one bump, called a petiole, on the skinny part between their abdomen and thorax. There are a variety of colors and sizes throughout the colony. Workers can be 4.0 to 7.5 mm long and their heads can be 0.94 to 2.1 mm wide. Workers can be grouped into major, media, and minor workers. These ants are likely polymorphic, meaning that despite some big differences in appearance, they are still one species. Workers and breeding ants typically have reddish-orange heads, their thorax can be either reddish-orange or black, their abdomen is black, and their legs and antennae are reddish-orange or black. Smaller workers can also be all black or dark brown. Eggs are creamy white and elliptical shaped. They are about 0.6 mm long and 0.31 mm wide. Larvae are the same size as the egg when they hatch and grow to about 6.0 mm in length. Reproductive pupae are 9 mm long, while worker pupae are 3.5 to 7.0 mm long. (Billick and Carter, 2007; Fraser, et al., 2001; Herbers, 1979; Weber, 1935)

  • Range length
    4.0 to 7.5 mm
    0.16 to 0.30 in

Where do they live?

Western thatching ants (Formica obscuripes) are native to the Nearctic region. They are found across the western half of Canada and the United States. Their range goes as far south as Arizona and New Mexico and as far east as Michigan and Missouri. In the southern half of Canada, they can be found from British Columbia to Manitoba. They are especially common in the Pacific Northwest of the United States. (Crutsinger and Sanders, 2005; Higgins and Lindgren, 2012; Risch, et al., 2008; Tilman, 1978; Weber, 1935)

What kind of habitat do they need?

Western thatching ants build their nests in dry regions, such as dry grasslands, including shrub-steppe habitats and sagebrush, prairies, coniferous forests, dunes, and alpine meadows. Nests are also often found in areas of secondary succession, which is land that has been disturbed by an event such as a fire. Western thatching ants can live in a large range of altitudes. Nests have been found as low as 800 m and as high as 3,194 m, though the most common altitudes are between 1,524 to 2,743 m. Nests are built into the ground, they are often built around a structure, such as the main stem of a sagebrush plant or a fence post. Nests can go as deep as 4 feet into the ground and are typically constructed out in the open. Western thatching ants get their name from a mound of "thatch" that workers build on top of the nest. This thatch consists of twigs, grasses, plant parts, and soil, and can be anywhere from a few centimeters to a meter high. The thatch helps to control the temperature of the nest, allowing the ants to survive in a variety of temperatures, humidity, and weather conditions. Secondary nests are often constructed at the base of plants where western thatching ants farm honeydew from honeydew-producing insects. (Beattie and Culver, 1977; Conway, 1996a; Crutsinger and Sanders, 2005; Higgins and Lindgren, 2012; McIver and Steen, 1994; Mico, et al., 2000; Risch, et al., 2008; Tilman, 1978; Weber, 1935)

  • Range elevation
    800 to 3194 m
    2624.67 to 10479.00 ft

How do they grow?

Western thatching ants go through complete metamorphosis, with egg, larvae, pupae, and adult stages. The first batch of eggs is laid in April. Eggs are laid throughout the summer, until as late as mid-August. Eggs are laid in a brood chamber, as well as other soil chambers, where they develop and are cared for by adult workers. Eggs hatch after 23 to 53 days. Larvae are found in the nest's brood chambers from the beginning of June to the end of August, and become pupae after 7 to 33 days. Pupae that develop into sexual forms are not present in the nests after June, but those that develop into workers can be found into early September. They remain pupae for 31 to 93 days, before developing into adults. By fall, the brood chamber has emptied. Total time of development from egg to adult takes 61 to 122 days. (Weber, 1935)

How do they reproduce?

Mating takes place during nuptial (mating) flights during June and July. Before taking flight, reproductive females and males live in the nest. Reproductive females are present from the end of June to mid-August, while males are present from the end of June to July. When leaving the nest for flight, winged females may be roughed up by workers. Workers pull on female's wings and antennae with their mouths. Flights occur in the morning and typically take place on clear, warm, windless days. Participants often climb nearby structures before taking flight. Nuptial flights may involve a few, to several hundred individuals, who fly to a nearby swarming ground. The same swarming ground may be used from year to year. Females land on plants or grass and perch near the top, with their abdomen sticking up in the air. They likely release a pheromone to attract males and to help the males find them. Males fly around and swarm, attempting to find females. While waiting, females may move their bodies and antennae, as the motion may help males find them. When they have found a female, males attach and remain motionless for 1 to 5 minutes until one of them moves and the male drops off. Males fly off and females remain for a little while. If several males find a single female, they fight until one wins and mates, while the others drop to the ground. Nuptial flights continue each day until the nest is empty of all reproductive females and males. Colonies can have 5 to 16 flights. (Conway, 1996b; Conway, 1996a; Talbot, 1972)

While there has been a large amount of research conducted about the nuptial flights of western thatching ants, very little is known about what happens afterwards. Since colonies move nests or grow by budding, it is not known where females that have recently mated in the nuptial flights go next. They likely return to already established nests to lay their eggs. Colonies typically have two or more wingless queens that lay eggs. The number of queens present may vary significantly, as one colony was recorded as having 198 queens. The first batch of eggs is laid in the nest in April by queens already present in the nest. Eggs are laid in special brood chambers where they undergo metamorphosis and are cared for by workers. (Conway, 1996b; Conway, 1996a; Herbers, 1979; Weber, 1935)

  • How often does reproduction occur?
    Females mate once during the nuptial flight, while males may mate several times.
  • Breeding season
    Nuptial flights occur in June and July.

Colonies of western thatching ants have significant brood care. During the summer, a large brood chamber is built near the base of the thatch. This chamber is divided by twigs that are poked through at all angles and is well insulated. Pupae can be found in the upper part of the chamber, while eggs and larvae are in the lower part of the chamber, as well as in lower chambers in the soil. The smallest workers remain in the nest to care for and feed the brood. Once they reach adulthood, ants become independent and join the colony as workers or sexual forms. The entire brood has left the chamber by the beginning of fall and the chamber is then filled with thatch. There are also nutrients in the eggs provided by the queen for the growth and development of the offspring. (Herbers, 1979; Weber, 1935)

  • Parental Investment
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female

How long do they live?

Most workers live 19 to 44 days after reaching adulthood, an average of 31.6 days, though some go into hiding for the winter and live longer. (Conway, 1996a)

  • Range lifespan
    Status: wild
    1+ (high) years
  • Typical lifespan
    Status: wild
    19 to 44 days

How do they behave?

Like most ant species, western thatching ants live in large colonies, with 10,000 to 40,000 individuals. They forage and do their other activities during the day. In the warmer parts of their range, western thatching ants are active year round, while in the northern regions they spend the winter in their nest and are active from April until October or November. Sexual males and females are able to fly, while workers are wingless. There is disagreement about whether groups of workers have distinct roles. Some researchers say this species does not form these groups, while others have observed 2 or 3 distinct groups. There are a variety of sizes of worker western thatching ants, with no obvious way to group them by size. Workers are often grouped into major, media, and minor workers. Major workers repair the nest and thatch, as well as search for insect prey to bring back to the nest. Majors often work together to take down large insects. These attacks may last as long as 30 minutes. When attacking prey singly, major workers snatch the insect in their mouth and immediately return to the nest. Media workers transport the young between nests, forage for vegetation, and farm aphids. Minors are rarely seen outside the nest, and likely take part in brood care and tend the queens. All groups clean the nest. None of these tasks are exclusively performed by any one group and all sizes contribute when needed, which likely leads to the idea that there are no distinct worker groups. (Billick and Carter, 2007; Conway, 1996b; Conway, 1997; Crutsinger and Sanders, 2005; Heikkinen, 1999; Herbers, 1979; Tilman, 1978; Weber, 1935)

Western thatching ants get their name from the piles of thatch they build to cover their nest mounds. The thatch is created from seeds, twigs, plant stems, grass, and soil. Piles of thatch can be several centimeters to a meter deep and several centimeters to 1.5 m across. This thatch keeps nests at a constant temperature throughout the day, even though nests are typically built out in the open in dry, warm regions. Workers are constantly repairing and adding to the thatch. Many mounds are also built around the main stem of a plant such as sagebrush. Ants chew the bark and spray formic acid at it until it dies, at which point the dead stem can be removed, creating a central passage in the center of the nest. Western thatching ants are polydomous, meaning that one colony is spread out between several mounds. Colonies often switch primary nests. New colonies are formed by budding, when members from an existing colony (usually including a queen), leave and begin a new colony. Secondary mounds may become primary nests, or new mounds may be created nearby, with workers moving between mounds. (Conway, 1996a; Conway, 1997; Herbers, 1979; McIver and Steen, 1994; Mico, et al., 2000; Weber, 1935)

Mound usage by the colony can change throughout the day and year. A nest can have anywhere from 1 to 52 entrances and these entrances constantly change. During warmer parts of the day, ants use entrances that are under cover and use entrances in the sun during the morning and evening. The same principle applies to trail usage. Most trails are constructed under vegetation cover, rarely moving out into the open, keeping themselves cooler in the shade. Ants also remain in the nest or secondary nest during the hottest part of the day, with most of the foraging taking place during the morning and evening. Many colonies have secondary nests. These nests are built at the base of the plants where workers farm honeydew secretions from aphids. Workers use the secondary nest throughout the day, particularly during the warmest temperatures in mid-afternoon. Two types of workers are involved in honeydew farming, tenders and transporters. Transporters spend much of the day in the secondary nest, while the tenders farm honeydew and bring it to the transporters. Transporters collect honeydew in their crop from the tenders and return it to the primary nest. (Conway, 1996a; Conway, 1997; Herbers, 1979; McIver and Steen, 1994; Mico, et al., 2000; Weber, 1935)

Home Range

Western thatching ants stay close to the nest, plants farther than 20 meters from the nest are not usually visited by the ants. In some areas, there can be as many as 115 mounds per hectare. In one study, the closest mounds were 2.36 m, while another study showed nests were usually separated by more than 100 m. (Conway, 1996a; Higgins and Lindgren, 2012; Tilman, 1978)

How do they communicate with each other?

Antennae are one of their most important sensory organs used for smelling, touching, and detecting chemicals. Western thatching ants communicate with other workers by touching each other and feeling their environment with their antennae. Ant crickets (Myrmecophilus manni) live in the ants nests and have learned to use their antennae the way the ants do and trick them into thinking that they are also ants. When foraging for nectar, ants communicate with other foragers. If an ant finds an empty nectary, it leaves a drop of liquid at the junction of the main stem and the stem to the empty nectary. When another ant moves along the stem, they touch the drop of liquid with their antennae and move on without having to visit the nectary themselves. Females release pheromones into the air during mating to attract males. As males fly over the plants where females wait, females also move their body and antennae to signal their location, showing that vision is important in viewing others. (Conway, 1996b; Henderson and Akre, 1986; Talbot, 1972; Tilman, 1978)

What do they eat?

Western thatching ants are omnivores, meaning they eat both plants and animals. They prey on many species of insects, particularly grasshoppers, beetles, pill bugs, true bugs, butterfly and moth larvae, flies, and other ants. They also scavenge dead insects and other invertebrates. Foraging ants bring both living and dead insects back to the nest. They also eat organic matter, nectar, and plant tissues including leaves, galls, and flowers. They scavenge seeds, eating the edible part and storing the rest in their nest. Occasionally, these ants also feed on dead rattlesnakes, birds, and small mammals. Ants typically collect liquid from the carcasses and store it in their crops, returning to the nest and regurgitating the liquid. Western thatching ants also farm honeydew from honeydew-producing insects, including aphids and treehoppers. Honeydew is an important component of their diet, as a significant source of amino acids, carbohydrates, and water. It provides energy for workers, and nutrients for the brood and queen. These ants occasionally also prey on the insects that they tend. (Beattie and Culver, 1977; Billick and Carter, 2007; Clark and Blom, 1991; Conway, 1997; Erickson, et al., 2012; Heikkinen, 1999; McIver and Yandell, 1998; McIver, et al., 1997; Tilman, 1978)

  • Animal Foods
  • carrion
  • insects
  • terrestrial non-insect arthropods
  • Plant Foods
  • leaves
  • seeds, grains, and nuts
  • nectar
  • flowers

What eats them and how do they avoid being eaten?

There are a variety of predators of western thatching ants. As predators themselves, they are aggressive and defend their brood and the aphids that they tend. They can spray formic acid when threatened or attacked. Other insects that gain entry to the nest can pose a threat to the brood. Ant crickets usually live peacefully in the nest, but have been observed attacking larvae. Another ant species, Leptothorax hirticornis, may eat larvae if it gets in the nests, and will also attack and eat isolated workers. Many species of spiders are also predators. Many bird species, including eastern kingbirds, western kingbirds, flickers, and American crows feeding on western thatching ants. Toads are also predators, including Canadian toads and Woodhouse's toads. Bears have also been known to dig open nests to feed. (Conway, 1996a; Conway, 1997; Heikkinen, 1999; Henderson and Akre, 1986; McIver, et al., 1997; Weber, 1935)

What roles do they have in the ecosystem?

Western thatching ants develop mutualistic relationships, relationships that benefit both sides, with many other species. They form these relationships with honeydew-producing species, as honeydew is an important part of their diet. In exchange for collecting and eating honeydew from the insects that they farm, they protect the insects from other predators and parasitoids. They also destroy insects that have been infected by parasitoids before the parasitoid completes its development. The aphids that they tend include Pleotrichophorus utensis, Uroleucon escalantii, and Lachnus allegheniensis, and members of the following genera, Aphis, Macrosiphum, Pleotrichophorus, and many more. It tends the honeydew producing membracid, as well as some jumping plant lice, scale insects, mealybugs, and gall wasps. Thatch nests of western thatch ants provide shelter to many species of insects and other Arthropods. One such insect is the beetle Euphoriaspis hirtipes. All life stages of this beetle can be found within the thatch. The exact relationship is unknown, as the ants do not seem to benefit from having the beetles around and do not even seem to notice them. The beetles can also survive in the thatch without the ants. Other Arthropods found in their thatch and nests include pseudoscorpians, springtails, true bugs, flies, and other beetle species. Larvae of these Arthropods often use the thatch or chambers in the nest for hibernation or development, and feed on decaying matter. The ants mostly ignore them. (Conway, 1997; Erickson, et al., 2012; Grinath, et al., 2012; Henderson and Akre, 1986; Mico, et al., 2000; Risch, et al., 2008; Seibert, 1992; Seibert, 1993)

Ant crickets are one notable species that lives in their nests. Ants are violent towards crickets and will attack if they realize crickets are there. However, crickets have learned to imitate the way ants use their antennae to identify other individuals and trick the ants into thinking crickets are actually ants. Ants then allow the crickets to live in the nest. Crickets even participate in trophallaxis with the ants, which is the regurgitation of food from mouth to mouth. Ants do not seem to gain anything from the crickets' presence, while the crickets get shelter, food, and even attempt to eat larvae if given access. Other ant species have also been found living in the nests of western thatching ants. Formica dakotensis has been found living peacefully in their mounds in Yellowstone National Park. Leptothorax hirticornis may eat larvae and isolated workers given the opportunity and Tapinoma sessile often steals honeydew from within the nest, but western thatching ants do not attack these other ant species. (Conway, 1996a; Henderson and Akre, 1986; Mico, et al., 2000; Risch, et al., 2008)

Western thatching ants play a variety of other roles in the ecosystem. They are prey to many insect and bird species. They also feed on a large number of other insect species. Mites from genus Uropoda can often be found on workers and reproductives, often in the joints of their legs. The wasp Elasmosoma michaeli is a parasitoid of workers. Wasps lay egg in the abdomens of worker ants, killing the ants upon hatching. As a significant aphid-tending species, western thatching ants can play a role in determining the density of other arthropods and herbivores in their habitat. In some habitats, such as a coastal dune habitat, western thatching ants are a keystone species. A keystone species can determine what other species are present in its habitat and the absence of a keystone species can also change what species are present. They reduce other herbivores on aphid-infested plants that could compete with aphids for resources, while also increasing the number of arthropods by creating new shelters by rolling leaves on which the aphids live. Western thatching ants are also known to collect seeds and bring them back to the nest. They eat the edible part and store the rest of the seed in chambers of the nest. These chambers can often be a good habitat for the plants to grow and develop, allowing ants to spread seeds to other areas. (Berg-Binder and Suarez, 2012; Conway, 1996a; Crutsinger and Sanders, 2005; Shaw, 2007)

Species (or larger taxonomic groups) that are mutualists with this species
Commensal or parasitic species (or larger taxonomic groups) that use this species as a host

Do they cause problems?

There are no known negative effects of western thatching ants on humans.

How do they interact with us?

Colonies of western thatching ants likely eat insects and other arthropods that can be pests to their habitat (particularly insects that remove the leaves from plants), as well as insects that could be pests to humans. (McIver, et al., 1997)

  • Ways that people benefit from these animals:
  • controls pest population

Are they endangered?

Western thatching ants are not an endangered species.


Angela Miner (author), Animal Diversity Web Staff, Leila Siciliano Martina (editor), Animal Diversity Web Staff.


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University of Michigan Museum of ZoologyNational Science Foundation

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Miner, A. 2014. "Formica obscuripes" (On-line), Animal Diversity Web. Accessed February 23, 2024 at

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