Back in the days when milk was stored in a real jug and not a plastic container in the frig, creative house keepers found a way to keep the flies out of the milk jug....a Milk Jug Cover or Doily. I have many of these lovely little pieces of needlework history from my grandmother. They are pretty, round crocheted "doilies" with beads or buttons attached around the edges. The beads or buttons gave weight to the doily so it didn't fall or blow off the top of the jug.
Here is a crochet pattern I found in an Australian newspaper from 1937. Now you can make your own milk jug doily.