When someone quotes 'I'm running around like a blue-assed fly', what is the symbolism of the blue-assed fly?

Question

I know it means that you are flat out, but why a blue-assed one?

Answer 1

This is a reference to the 'blue bottle fly' whose back end is blue. They are particularly buzzy and annoying, like "green head" flies. Then there are the red-eyed flies. I don't know much about them, but I guess you could run around like one if you wanted to.