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People who go to u-pick vegetable farms usually come home with lettuce or corn or tomatoes. When I went to a u-pick vegetable farm near Phoenix, Arizona, I came home with photographs of ants, instead.

It wasn't surprising to see numerous circular mounds with a single entrance hole in the center.

The nests are made by a common ant in the low desert, Dorymyrmex bicolor. As I've written previously, D. bicolor seems to prefer to nest along dirt paths or roads. The garden had plenty of those.

Here's another nest, again with an entrance hole in the center, and covered with active ants.

The workers here were pulling out clumps of what looked like dirt. Can you see the single petiole that is characteristic of the species?

After seeing about three dozen or so circular nests like those above, I found this one.

Same rough shape, but notice anything different?

Where are all the ants?

From another view, the entrance hole is actually blocked with dirt.

Finally, I can see some ants, but those aren't Dorymyrmex bicolor workers.

The workers exiting from this nest are uniform in color. They have a petiole and a postpetiole.  Notice anything else about them?

The Dorymyrmex worker ants from the same perspective have large eyes. See any noticeable eyes from this view?

Having no noticeable eyes is a characteristic of Neivamyrmex army ants (Note:  They do have a single eye facet, but it isn't obvious.)

The exact species is much more difficult to figure out.

Wayne P. Armstrong found some similar Neivamyrmex near the Superstition Mountains in Arizona, which are close to the farm where I found these ants. Gordon Snelling identified Armstrong's ants as Neivamyrmex leonardi. The ones I found could be N. leonardi or one of several similar species.

In any case, Neivamyrmex army ants resemble their larger namesakes because they are always on the move. They don't make permanent or long term nests like the Dorymyrmex, but instead raid nests of other ants stealing the brood for food. Seems like in this case a Dorymyrmex bicolor colony was a target of their raid. Armstrong reports Neivamyrmex workers raid Pheidole nests, as well.

So, I didn't bring home lettuce, but maybe something even better from the u-pick farm.

Have you ever seen Neivamyrmex army ants?

1

It is spring and the flowers are blooming in Arizona.

Do you recognize the plants? I believe they are smallseed sandmat, Chamaesyce polycarpa.

In any case, ants were all over them.

On the day I visited, Dorymyrmrex bicolor workers were everywhere. Getting a little closer...

I could see the front of the ant's head was covered with pollen.

Two more workers, with equally yellow mandibles.

The Dorymyrmex workers were definitely visiting the flowers* (see below).

I also saw Pogonomyrmex californicus workers in the sandmat.

They weren't visiting the "flowers," though.

The Pogonomyrmex workers were searching under the plants. I saw a lot of gasters in the air. Perhaps they were searching for seeds? I also wondered if there were extrafloral nectaries under the leaves or on the stems that were attracting the ants.

Frankly, I wasn't that familiar with these little plants, so I wasn't sure where the nectaries were.

Upon investigation, it turns out that what look like *flowers* are actually special flowering structures unique to euphorbs called cyathia (singular cyathium). What look like anthers are actually male flowers and at the center is a female flower. The dark reddish areas near the center are the nectar glands within the cyathia. (For more details about the flowering structures see Wayne's Word (scroll to absolute bottom of post) or the flower structure of euphorbs.)

In any case, it seems like this plant would be a great one to add to an ant garden. I'm looking forward to learning more about it's life cycle and how ants interact with it.

What do you think?

3

Dorymyrmex bicolor ants are common in Arizona, especially in open areas. In a previous post, I wrote about how to identify them.

A few weeks ago I visited an agricultural research station and found almost a monoculture of Dorymyrmex.

The first thing you notice about Dorymyrmex bicolor colonies is their neat circular mounds. (Most of these were conveniently located along the dirt roads.)

Each reflect the color of the soil beneath the surface. This one wasn't a perfect circle.

Nor this one.

Tofilski and Ratnieks (2005) studied the mound formation of two colonies of Brazilian Dorymyrmex. They found the worker ants removing excavated material from the nest deposited their loads at the crest of the mound and beyond, preventing the material from rolling back into the entrance hole. They also indicated that workers maintained the circular shape, when one side was removed, by climbing the area with the least slope to deposit their loads.

Ratnieks noted that ants of other, larger species had difficulty climbing the mounds, thus suggesting the mounds served a protective function.

As you will see in the next few photographs, the Dorymyrmex were carrying out clumps of soil and related materials the morning I visited.

Because ants at all the mounds were so busy removing material, I began to wonder about it. Were the clods of soil and pebbles rolling into the entrance hole over night? Perhaps there had been a wind storm or vehicle disturbance?

By the way, you might recognize these ants under a different name. Prior to a revision of the genera, Dorymyrmex bicolor was known as Conomyrma bicolor. Under that name, they had a brief burst of limelight when Möglich and  Alpert (1979) found they exhibited tool use by dropping small stones on their competitors, particularly Myrmecocystus workers. Finding ants that stoned their enemies caused quite a stir, as you can imagine.

Thinking about that, I also began to wonder if perhaps Dorymyrmex bicolor worker ants also drop stones on conspecifics, that is other Dorymyrmex bicolor nests. With the name change, it is somewhat hard to track the literature. Does anyone know? Anyone studying Dorymyrmex bicolor?

If not, perhaps it would be worthwhile to visit these fascinating ants some evening.

References:

Adam Tofilski and Francis L. W. Ratnieks. (2005). Sand Pile Formation in Dorymyrmex Ants. Journal of Insect Behavior, 18(4): 505-512.
.pdf will load immediately at http://www.cyf-kr.edu.pl/~rotofils/tofilski_ratnieks_2005.pdf

Michael H. J. Möglich and Gary D. Alpert (1979). Stone Dropping by Conomyrma bicolor (Hymenoptera: Formicidae): A New Technique of Interference Competition. Behavioral Ecology and Sociobiology, 6 (2):  105-113.
free .pdf available at Springer
http://www.springerlink.com/content/p16717229406004l/