From what I understand, flies can't fly straight up, they have to fly at an angle, thus making it almost impossible for them to fly out of the holes. I have seet some walk in and out of the hole, but never fly.
I'm not sure what kills them, I would assume it would be dehydration & heat, maybe exhaustion, or maybe injury from banging around on the inside. Either way they are dead. RD is right about maggots if you leave them too long. That normally happens after I've emptied them 2 or 3 times and finally got the problem under control. I refill the bottle and then forget about it for a couple weeks, only to find it writhing with maggots. That is when I normally just toss them.
Something just hit me.....I appoligize for posting this in the dead of winter when most of you don't even have any flies to worry about. Wintertime here is lots and lots of rain, resulting in rehydrating all the dead frogs, rats and birds that died and dried up over the summer. Flies, mosquitoes, mice, centipedes and all kinds of creepy crawlies are at their worse during the winter here....just the opposite of what happens on the mainland.
Cory