> which leads me to ask: flies? Jo?

The flies is the area to the side of the stage where the flying equipment is based in a traditional theatre design, sometimes in a fly tower. In other words the counterweighted balance system, usually up at a high level where the fly operator can bring the big backdrops and occasionally, Peter Pan, in and out of the stage area.

It's a good place to watch out for ghosts -
www.britishtheatreguide.info/articles/020298.htm