In early April of 2004, my wife and I were walking our dogs through a patch of the Sonoran Desert in Southern Tucson, Arizona. We would casually stop to look at the various ant nests as we went along. We had been on this route many times, and most of the nests were old friends.
When we stopped to look at one of our Novomessor (Aphaenogaster) cockerelli nests, we were surprised by the level of activity. The ants were clearly upset about something – there were guard ants all around the nest entrance, and we could see ants within the entrance tunnel milling about.
Looking more closely, we noticed a stream of small ants that was running by the entrance of the Novomessor cockerelli nest.
Army Ants! I ran back to the house as quickly as I could to get my camera.
These North American Army Ants that we were watching are not as spectacular as the famous Army Ants of the tropics. Ours are much smaller, for one thing. This particular group of ants is Neivamyrmex nigrescens. It’s one of the more common and widespread of the North American Army Ants.
The Army Ants were running in a mostly continuous stream of ants, staying mostly within the confines of a roughly 3 inch trail. The stream of Army Ants came out of a hole in the soil, ran along the surface for 10-20 yards, then disappeared back into the ground.
Neivamyrmex in general are fond of eating other social insects. That explains the nervousness of our Novomessor cockerelli nest – the trail was passing about a foot away from the entrance to the Novomessor nest.
Every now and then a Neivamyrmex would wander too close to the Novomessor nest, and one of the Novomessor workers would lunge forward and grab the Army Ant.
The much-larger Novomessor workers made short work of the individual Army Ants. The Novomessor guards would be quickly overwhelmed by the Neivamyrmex if the main group of Army Ants determined there was a Novomessor nest so near the raiding party.
After killing an individual Army Ant, the Novomessor workers would stand over the dead Army Ant, periodically mouthing it. It was as though the Novomessor wanted to make sure the Army Ant was really dead.
The Army Ants are mostly blind. Their world is primarily a world of touch and of odors – they don’t have the big compound eyes of most ants.
The Army Ants ran along, so densely packed in their trail that they were nearly touching each other as they streamed past the Novomessor nest. You could see them quickly tapping their antennae along the ground, or dragging them along as though tasting the ground.
They were running along so quickly that it was tough to get a photo of a single ant – I found that I needed to place objects in or near their trail, then wait for one of the ants to climb onto it and pause when she lost the trail of her companions in order to get a clear, in-focus shot.
I noticed that some of the Army Ant workers were much hairier than the others. They were almost shaggy. I’ve read that this is due to the age of the worker – younger workers have not worn their sensory hairs away yet.
One big advantage of the tight raiding trail that the Neivamyrmex maintained was that I was able to lay down next to the trail and get some eye-level photos without getting stung. That was a big plus.
I stayed and watched the raid for 30 or 40 minutes. The Neivamyrmex did not break into the Novomessor nest while I was watching. There were only occasional incursions by individual Army Ants into the Novomessor nest while I was watching, and the Novomessor workers dealt with the Army Ant individuals promptly. And thoroughly..
The Novomessor nest was still in existence the next day. I wonder how many of these raids it had seen.
And I wonder how many of these Army Ant colonies I’ve walked right by without even noticing them.
