Dyanmically created levels

I am having problems with duplicating a movie clip and the dups are created on top of the rest of my movie. The clip is a shooting star and it needs to be below all the levels except the background level.
Here is the code:
[AS]function dup() {
for (i=0; i<2; i++) {
mc = _root.fstar_mc.duplicateMovieClip(" mc "+i, (-100+i), {_x:Math.random()*1000/2, _y:Math.random()*200, _alpha:60});
}
}
timer = setInterval(dup, 16000);[/AS]

You can see I set the level to a negative number but that didn’t work. Any one have any suggestions on how to do this???