I wouldn't feed wild caught flies. They are filthy and probably filled with parasites. There a few places where you can order healthy flies raised as feeders. You can get maggots and let them change as you need them or get them ready to turn into flies.

You can cut a piece of waxworm (maybe a quarter of an inch or so?) off the head end. It will move around for quite some time. It would probably be a more manageable piece.

Andy