The trooper stopped writing the ticket and said"Well yeah, if that's what they are, I never heard of circle flies." So the farmer says "Well, circle flies are common on farms. See, they're called circle flies because they're almost always found circling around the back end of a horse."
The trooper says, "Oh," and goes back to writing the ticket. Then after a minute he stops and says, "Hey...wait a minute, are you trying to call me a horse's ass?" The farmer says, "Oh no, officer. I have too much respect for law enforcement and police officers to even think about calling you a horse's ass."
The trooper says, "Well, that's a good thing," and goes back to writing the ticket. After a long pause, the farmer says, "Hard to fool them flies though."