If I managed to use this “Key to Cephalotes Species Known from Costa Rica, Based on Minor Workers” correctly, then these images are of Cephalotes basalis.
There were quite a few crawling around on low vegetation.
One interesting thing about this arboreal genus is that some members (perhaps all) have the ability to glide during free fall. If they fall, they use this ability to attempt to maneuver themselves toward their host tree trunk where they have a much better chance of returning to their nest.
Another interesting thing is that some species have a special soldier cast with heads adapted for sealing off their nest entrances. If I’d know that when I saw these, I’d have attempted to locate their nest entrance so I could photograph that behavior.