lime in a Corona bottle

Does anyone know why people put a slice of lime in the mouth of a Corona beer bottle? I was blown away when I found out.

You win the prize. I had lunch in a bar years ago and the bar tender was from Mexico. When he served my lunch, he place a slice of lime by my plate. I asked him why he did that and he said to keep the flies away from my food. He said that's why we put a slice of lime in the mouth of your beer bottle to keep the flies off of it.