Getting to know thatching ants: a general guide for the Pacific Northwest

Hi there! I’m a student currently researching western thatching ants (Formica obscuripes) in western Washington. Working with iNaturalist to locate colonies has been really useful to me and encouraged me to continue contributing by sifting through ant observations in my region. I would like to bring to attention some information that can improve the site further with this basic guide to managing and identifying relevant observations.

If you’ve ever seen a particularly large anthill covered in dozens, hundreds, or even thousands of black and orange ants, there’s a good chance you’ve come across a thatch mound. In North America, especially the western United States and Canada, these belong to members of the Formica integra species group (in Europe they are built by very similar ants belonging to the Formica rufa species group, and in both Europe and the eastern United States you can also expect to see mounds from ants in the Formica exsecta species group). Often, especially in the Pacific Northwest, observations of these mounds or ants are immediately filed under Formica obscuripes, but this is not always correct. This is because the computer vision software and most users are unaware of and unable to discern minute but important details. At a glance, nearly all ants in the Formica integra species group and the similar Formica sanguinea and Formica exsecta species groups are practically identical. They are all fairly large ants with red-orange heads and black or brownish bodies.

An example of such an ant, in this case Formica obscuripes (Formica integra group):


Original image, all rights reserved

Usually, without decent magnification (usually macro photography or micoscopic analysis) it is impossible to identify these ants to species due to their similarities and range overlaps. The following is a basic, but incomplete guide to breaking these ants down for iNaturalist so that we can mitigate incorrect records. Note that in reality there is much more nuance and that this guide is tailored for the Pacific Northwest, with which I am most familiar. This information is based on personal knowledge and experience as well as Stockan et al. (2016).

integra-group or sanguinea-group?
Members of the Formica integra species group, usually known as thatching ants, are similar in appearance to members of the Formica sanguinea species group. There are a couple ways to discern them, however:

  • The nest: sanguinea-group ants rarely, if ever, construct mounds (piles of dirt and plant materials). They are much more likely to be found living under rocks and especially inside or under logs. On the other hand, integra-group ants may or may not construct a mound depending on the species and location. As a rule of thumb, if a mound is present in the observation, it is Formica integra group. If a mound is not present, identify it as genus Formica (Wood, Mound, and Field Ants).

  • The clypeus: The plate above the mandibles of ants is called the clypeus. The morphology of the clypeus is especially useful in identifying these ants. Formica sanguinea-group ants have a clypeal notch, which is a concave portion at the front of the clypeus. This feature is not present on Formica integra- and Formica exsecta-group ants. This can be difficult to discern sometimes, so if you’re unsure then identify it as genus Formica (Wood, Mound, and Field Ants).

An example of the clypeal notch on the Formica sanguinea-group species Formica aserva:


Original image, public domain

The clypeus of Formica obscuripes which lacks the notch:


Original image, public domain

obscuripes or otherwise?
Formica obscuripes is a bit of a celebrity in the ant world. It is one of the best studied species of Formica and one most widely recognized ant species in the United States and Canada. As a result, most integra-group ants are identified on iNaturalist as F. obscuripes without sufficient evidence. In order to accurately identify F. obscuripes or any other Formica integra-group species, a clear view of several parts of the ant is necessary. To make it quicker and easier to understand, I’ll just go over differentiating F. obscuripes from a few of its lookalikes. For a more precise and thorough list of integra-group species and their identification, I recommend AntWiki’s Key to Nearctic Species in the Formica rufa Group (now recognized as the taxonomically distinct Formica integra group). Please note that if you are inexperienced or unsure it is always best to simply identify an observation as genus Formica (Wood, Mound, and Field Ants) or Formica integra group. That way, those with more experience can take a closer look, and if it cannot be IDed to species we don’t have to worry about a misidentification floating around.

Three of the most likely species to be confused with F. obscuripes are F. oreas, F. obscuriventris, and F, ravida. F. obscuripes can be distinguished from F. oreas by the setae (hairs) on the antennal scapes, which are the first and longest segments of the antennae. In F. oreas, there are numerous erect and suberect (standing up) setae distributed evenly on the antennal scapes. These setae are usually only slightly shorter than the setae on the rest of the ant’s body. In F. obscuripes, there are few if any erect/suberect setae on the scapes and when present they are significantly shorter than the setae elsewhere. The next one, F. obscuriventris, has enlarged clypeal fossae. These are divots where the clypeus meets the rest of the head between the antennae and the mandibles. It can be quite difficult to discern this without experience. The clypeal fossae of F. obscuripes are unremarkable like most other integra-group ants.

An example showing antennal scape setae (red) and clypeal fossae (blue):


From left to right: Alpert, 2011; Prado, 2010; Nobile, n.d. See reference list for licensing information. Images were cropped and combined and names and colored boxes added by author.

Finally, F. obscuripes has extensive, evenly distributed, and usually fairly long erect setae on the head vertex (top of the head from a side profile) and mesosoma (middle section of the ant; thorax). F. ravida and many other integra-group species lack this abundance and even distribution of setae.

An example showing the distribution of setae on the head and mesosoma:


From left to right: Alpert, 2011; Alpert, 2012. See reference list for licensing information. Images were cropped and combined and names added by author

Again, there is a lot more nuance than I would be able to reasonably include in this forum post. My goal here is to bring to light some distinctions between common similar species in the area to improve the quality of community observations and identifications, particularly in the Pacific Northwest. Hopefully this will be helpful to some of my fellow naturalists! Please note that I am also not an expert and this information may not be complete or accurate to literature standards. I am open to feedback and questions.

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References:
Alpert, G. D. (2011). Formica obscuripes MCZ001H [Photograph]. AntWiki. (https://www.antwiki.org/wiki/File:Formica-obscuripes-MCZ001H.jpg). Licensed under CC BY-SA 3.0

Alpert, G. D. (2011). Formica obscuripes MCZ001L [Photograph]. AntWiki. (/wiki/File:Formica-obscuripes-MCZ001L.jpg). Licensed under CC BY-SA 3.0

Alpert, G. D. (2012) MCZ ENT Formica ravida 01hal [Photograph]. AntWiki. (/wiki/File:MCZ_ENT_Formica_ravida_01hal.jpg). Licensed under CC BY-SA 3.0

Nobile, A., n.d. CASENT0005390 Formica oreas Head View [Photograph]. AntWeb v.8.75.4. (https://www.antweb.org/bigPicture.do?name=casent0005390&shot=h&number=1). Licensed under CC BY 3.0

Prado, E. (2010). CASENT0179609 Formica obscuriventris Head View [Photograph]. AntWeb v.8.75.4. (/bigPicture.do?name=casent0179609&shot=h&number=1). Licensed under CC BY 3.0

Stockan, J. A., Robinson, E. J. H., Trager, J. C., Yao, I., & Seifert, B. (2016). Introducing wood ants: Evolution, phylogeny, identification and distribution. Wood Ant Ecology and Conservation , 1–36. https://doi.org/10.1017/cbo9781107261402.002

Apologies for the incomplete links, as a new forum user I am unable to include more than 4 per post.

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Thank you, it was interesting to read about thatching ants.
I wonder if this wouldn’t be a suitable addition to the wiki?

This is pretty cool, thanks so much. I’ll be collecting an ant or two (or three) the next time I come across a nest for later, closer examination under a scope.

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Wonderful post! Really helpful info and photos – thanks for putting in those comparisons.

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Thank you! You’ve changed how I will ID my thatch ant observations now.

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There’s one particular Thatch Ant nest that my colleagues and I have known for over 20 years. Not sure it’s still active, but it may be. Are the queens replaced as they die so a single mound can remain active indefinitely? How long-lived can a colony be?

Multiple species in the Formica integra species group, especially Formica obscuripes, are known to “adopt” queens of the same species during nuptial flights. Virgin queens may wait outside their maternal nest to mate before hurrying back inside, or they may fly in search of mates and then disperse among other colonies of the same species or Formica fusca group host colonies. As far as I know there are no data on how often queens return to their maternal nest or integrate into other nests of the same species – we only know that they do one, the other, or both. It is unclear what the conditions for this are – I have tried unsuccessfully to introduce new queens to a young captive colony whereas other antkeepers have had no problems following the same method. My hypothesis is that one requirement for this behavior, called secondary polygyny, is that the colony must mature enough to be producing its own alates.

In any case, it is reportedly common in the wild to see large colonies with multiple queens and there does not appear to be an upper limit for how many queens can be in a colony at any given time or how long the colonies can perpetuate themselves by adopting new ones. However, as most colonies this large live in sizeable mounds, it is reasonable to assume that the integrity of the mound is a selective factor. Colonies must grow continually to keep up with their growing mounds which, made of plant material, eventually decay and fall apart. Over time an increasing number of workers tasked with mound maintenence must be present to prevent the mound from shrinking and reducing the number of ants that may be able to live in it. It is possible that in the largest colonies, which are usually recorded to have multiple mounds, workers begin constructing new mounds to accomodate the increasing population and mitigate losses from decay. This is, however, just my conjecture.

Additionally, if I recall correctly, Sather (1972) recorded that in F. obscuripes and F. oreas colonies a large number of uniormly sized media workers is a trait of older colonies. This may be useful in making a reasonable guess as to whether a colony is multi-generation, but the data are far from consistent. Perhaps another method is to observe the coloration of different individuals. integra-group species, especially F. obscuripes, can vary immensely in their infuscation (areas of dark coloration), especially between workers of different sizes. However, from my observations in the field it appears that it is not uncommon to see uniform infuscation across workers of the same size in smaller colonies. This would suggest that larger colonies with distinctly varied infuscation among same-size workers have multiple queens, as the coloration of workers will come from the genetics of the queen and whatever males with which she copulated. This is also untested, however, and may not be an indicator of multiple queens from the same maternal colony.

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