The flea circus was an old vaudeville routine where the performer would present to the audience a team of acrobatic fleas. The fleas would walk on tight-ropes and swing from tiny a trapeze and other various feats of skill and strength.
Of course there were no fleas. In reality, the entire flea circus act is an exercise in pantomime and magic. It's the guy on stage that's making everything move.
The flea circus and its fleas is also the subject of a couple of mediocre Warner Brothers cartoons.
There's a shockwave flea circus at
http://ourworld.compuserve.com/homepages/fuzzybear/fleas.htm