When I was young, my mother did a lot of home canning. It would always be summer- so it was usually hot outdoors and indoors with the wood stove blazing away it was too. The doors would be wide open and even though the screen doors were tight closed the frequent visits outside to get more fruit or whatever meant the air sometmes was thick with the inevitable flies. It always fascinated me to see that flies could be herded: My mother would open the screen door and with a dish towel flapping she sent the flies out the door.