Ant of the Week: Trachymyrmex arizonensis

The ant of the week is one of those amazing ants not many people get to see.

Photograph by Michael Branstetter / © AntWeb.org / CC-BY-SA-3.0 from Wikimedia

You can probably tell right away that this worker is a leafcutter in the tribe Attini because of the spines and spikes. Ants in the genus Trachymyrmex grow a fungus garden like other attines, although they are much more likely to collect plant debris from the ground for their gardens than actually cut fresh leaves. They are the original composters.


Photograph by April Nobile / © AntWeb.org / CC-BY-SA-3.0 from Wikimedia

The bumps, called "tubercules," are characteristic of the group. Trachymyrmex arizonensis is known for the prominent tubercules on the first tergite of the gaster, as well as by the shape of the carinae (ridges on the front of the head). (See Rhabling, et. al. 2007). Unlike the spectacular polymorphism of Atta leafcutters, the workers are mostly monomorphic in Trachymyrmex.

Several species, including Trachymyrmex arizonensis, live in southeastern Arizona in the more mountainous regions. All members of Trachymyrmex are found in the New World.

Colonies are relatively small, with mature colonies up to about 1000 workers. Most Trachymyrmex nests are pretty much unnoticeable, but T. arizonensis does produce a yellowish mound. (You can be sure I will be on the lookout for those from now on.)

Are you bothered by the dust and debris on the ant in the photograph? Entomologists have long noticed that Acromyrmex ants are covered with fuzz, which looks a bit like dander. Because most ants are shiny clean, Cameron Currie decided to examine the debris closer. The fluff turned out to be bacteria. Not a disease-causing bacteria, but unusual bacteria capable of making its own antibiotics. When leafcutter ants grow the fungus they use as food, parasitic and competing fungi are a constant threat to their underground gardens. While the workers tend their gardens they distribute the helpful bacteria from their bodies, which inhibit the growth of harmful bacteria and fungi. There have even been suggestions that leafcutter workers have those unique crevices and bumps on their bodies as places for the helpful bacteria to grow. Given the whitish appearance in some of the photographs online, it is very possible that Trachymyrmex has its own bacteria, (although Dale Ward identifies it as a mineral deposit.)

Does anyone have a colony of these ants? Anyone working on Trachymyrmex? I would love to learn more about them.

References:

C. Rabeling, S. P. Cover, R. A. Johnson, and U. G. Mueller. (2007) A review of the North American species of the fungus-gardening ant genus Trachymyrmex (Hymenoptera: Formicidae). Zootaxa. 1664: 1–53. There is a link to a free .pdf in this list of publications. (Encyclopedia of Life also has the key).

Bert Hölldobler and E.O. Wilson. 2011. The Leafcutter Ants. W. W. Norton & Company, New York. (More info at previous post.)

Dale Ward has a nice series of photographs with captions at Ants of the Southwestern United States.

Alex Wild of Myrmecos also has a number of photographs, including one of a tiny species, Pyramica arizonica, that lives in the nests.

Mingzi M Zhang, Michael Poulsen, and Cameron R Currie. (2007). Symbiont recognition of mutualistic bacteria by Acromyrmex leaf-cutting ants. The ISME Journal. 1: 313–320

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