I have seen "flea circuses" on TV when I was a kid, possibly on the old Ed Sullivan show. You could see a little chariot (or whatever) moving around the tabletop, but you had to take the "ringmaster's" word for it that it was being pulled by a flea. I may have believed it as a kid, but I now think it was a great leg-pull. They probably used hidden magnets under the table to move the chariot.

Another thing I "saw" was a miniature trapeze that began to swing when the invisible "flea" jumped on it. Come to think of it, I seriously doubt that a flea would have enough mass or momentum to start a trapeze swinging.

One thing I am curious about: When flea circuses were in vogue, how many audience members believed in them, and how many were in on the joke? (I wonder the same thing about professional wrestling, tabloid newspapers, and TV evangelists.)

Great thread! It may not be musical, but it certainly concerns our common heritage and folklore, which is close enough for my money.