That's a major worker. Most likely C. sansabeanus (I have a colony of these) or C. semitaceous. It could also be C. ocreatus but according to Drew they are quite large (like 18mm).
The park closes the gate at 8pm, but the best time to looks for these guys is at night when they come out with a headlamp. That is what I did and found a lot of them.
Edited by nurbs, March 10 2017 - 12:25 PM.