don't know about a glass of wine that is going to be downed for dinner, but if a fruit fly lands into wine, the wine turns into vinegar eventually if the wine is left long enough to do so...
Represents..nothing. It flops around....flutters its wings and dies. You can either flick it out with your fingernail and finish the glass or pour another and go on with the party.
If this is a saying, then I'm not sure what it represents. But if you are asking if it would actually happen to you, then I would just take it out and finish the wine. It would just drown like any other insect that finds the misfortune of landing in liquids! And by the way, I do have to mention, as I'm sure you are well aware, fruit fly's aren't very good jumpers. They usually fly. I just had so say that! Sorry! lol
Well! If he "jumps" in, feet first, instead of flying in as they normally do, when they're dying for a 'P', don't worry, he's probably just washing his feet off after trotting over a fresh dollop of dog poo! So drink your fill, as advised above!??