I imagine they can deflat the bags to some extent (I'm thnking that the structure is lung-like, with numerous small chambers to reduce the loss of lifting agents due to injury)

The way I'm imagining the bodies, the important bits are pretty flat. They don't have skeletons, but insteand their vitals sit in a cartilidge cage. They'll probably be a little more sensitive to changesin gravity than we are, since we evolved to swing through trees, and they evolved to drift over marshes.

(Just as an aside, I wish I had LiveJournal when I was writing Ground Forces.. this is a great way to test ideas.)