So, I've found oak galls to have a suprising diversity of ants in them.
The live oaks around here have these hard galls about 1" in diameter.
Here are some ants I've seen in them:
Camponotus impressus
Camponotus etiolatus
Camponotus sayi (a queen!) -- I moved her to a test tube.
Crematogaster laeviuscula
Crematogaster punctulata
Pseudomyrmex gracilis
Temnothorax sp.
Next time you go anting, try breaking open some oak galls.