What's a circle fly?

"Yep," the farmer said as he watched the trooper shoo away several flies.
"These flies sure are terrible," the trooper complained.
"Yep," the farmer said. "Them are circle flies."
"What's a circle fly?"
"Them flies that circle a horse's tail," answered the farmer. "Them are circle flies."
"You wouldn't be calling me a horse's tail, would you?" the trooper angrily asked.
"Nope, I didn't," the farmer replied. "But you just can't fool them flies."
Originally published December 07, 2002.