Here's a few somewhat bad pictures of the workers, which are fairly large at around 5-8 mm long. They sort of look like something that should be able to sting, but I couldn't induce them to do so. They enthusiastically ate dead mosquitoes.
At first I thought they might be some sort of Myrmica, but then I found a queen of what I believe to be the same species crawling around on the ground (around 15 mm) and that clearly wasn't what it was. What really stood out was the long, thin pedicle and the strange shape of the abdomen.
I also saw some other cool critters on that trip, including Pogonomyrmex, huge colonies of mound- building Formica and giant dampwood termites... maybe for another post.
