Underground Cities

[Pandaemonium]

The CB (for colonization) appears to be applicable to being an arthropod, rather than a sub-type, and the DB (for ground combat) to the natural armor casings of arthropods, modified by considerations of weight for gliders and fliers.

 

In my limited understanding, the bonuses tend to be additive, rather than multiplicative, so that if both subterranean arthropod species type and subterranean city were combined, the result would be greater than either alone, although they would not directly effect the amount of the bonus that the other gives.