Flies and Chickens

I recently used the expression 'running round like a blue arse fly' to an American who asked me about the origin of this expression. Can anyone help me put him (and me) out of our misery?? Thanks.

Origin I don't know, but the fly in question is a bluebottle. Strange expression as they don't normally run anywhere.

Certainly the fly in question is a bluebottle, but I think I'm right in saying that the expression originally was "buzzing around like a blue-arsed fly", which makes a lot more sense. One more normally tends to "run around like a headless chicken".