Hanging bats is the subject. The question is which of those two words is the real subject.
We aren’t going to be out hanging bats like ornaments from a tree. If we were, then hanging would be the subject and bats the object of that hanging, and hanging therefore a gerund in the old grammar books. That would make hanging the real subject.
But it isn’t. It’s the bats themselves who are hanging from the ceilings of the the caves like so many furry stalactites. They are bats a-hangin’ — they are “bats pendant”, if you would. Therefore, hanging is here acting as an adjective, which makes it a participle in the old books. Bats is the real subject.
But it’s really just an -ing word in both cases, and not always of much use to try to figure out which particular flavor we think we’re looking at.