I'm not a fan of flies, mainly because like lb says, who knows where they've been once they fly into your house and land on things ! In the summer though, those big ones zoom in and zoom out again fairly quickly, as all the doors at the back are wide open. However, IF they don't fly straight out and start annoying both me and my dog, they will be zapped, especially if I am eating and they are trying to land on my butty !! However, I won't kill wasps, bees and spiders and even though not a fan of wasps, I will try and get them out because they do have a purpose, they are pollinators, like bees.