In April of 2005, I was in Arizona’s Santa Rita Mountains, revisiting my favorite Pseudomyrmex apache nest. I watched the workers crawling on their Oak tree for a while – but then noticed a worker that was not on the tree.
Rather, she was on a thistle near the tree. She was crawling on the flower bud of the thistle. She did that for a few minutes, then started crawling on the stem of the thistle.
The stem was hairy – looking at photographs of it later, the ant was wading her way through a thicket of fibers as she worked her way along the thistle stem.
She would stop and examine the spots on the stem where there were clusters of thorns, or leaves emerging from the stem.
I wondered if she was looking for extrafloral nectaries, perhaps?
Eventually, she found a wet-looking cavity at the base of a leaf/thorn cluster that fascinated her. The diameter of the cavity was a little greater than her head width, and was about about as deep as her head was long.
I wondered what the cavity was, if she had made it herself?
She would put her head into the cavity and remain motionless for 30 seconds or so.
Was she drinking the plant fluid? Was she gnawing the cavity?
She spent a total of perhaps 5 minutes working at the cavity, then began exploring the thistle some more.
My first thought was that the ant was feeding from another insect – a Homopteran, for example. There did not seem to be an insect in the cavity that I could see.
Below is a photo of the cavity after she had gone. All I could see in the cavity was moisture. No insect, no obvious extrafloral nectary, etc.
What on earth was this ant doing? Did it make the cavity? If so, had it made the cavity on a previous trip, then returned to it? Was it using the thistle as a source of moisture, was the thistle juice food?
That’s the sort of location that I’d expect an extrafloral nectary on plants. Was that what the ant was looking for, an extrafloral nectary? Was this a primitive type of nectary?
Pseudomyrmex is a twig-dwelling ant. Some species in the genus (Pseudomyrmex ferruginea) live in the base of hollow thorns which the tree specially grows for them. This location, and the cluster of thorns on the thistle, brought that to mind for me. Is that was this was, a type of excavating/nesting behavior, similar to what eventually became the hollow-thorn dwelling of P. ferruginea?
I’ve got no idea. But it sure was fun to watch, and it’s still fun to wonder about.
