Butterflies in the stomach?

There is this english saying: "butterflies in the stomach" to describe nervous feeling when you are about to "face the music" (exams, interviews, answering summon from your boss, walking down the aisle of your wedding..etc). I would really like to know who and how this saying originated as it is very accurately describe the nervous + nausea feeling. Esepcially when you are about to have a job interview the same day afternoon whereby this job determines your happiness for the rest of your life... I am feeling exactly like this... interview this afternoon...wish me luck!!!!