I've read discussion about this and real professionals say if you glue fly's legs the plane "ain't gonna fly". You need to glue the belly to the small stick called pylon and let legs hang free for optimal steering. Then you get THE performance!
Another guy asked what if he use bees instead of flies? The reply was bee is like a jet engine.
Then there were suggestions on how to set center of gravity properly so the plane really takes off and fly. You need to be very precise about it or it'll crash! You wouldn't want such good performing engines crash, wouldn't you?
Here is another guy's comment:
"...you need to enlighten me on the horse fly bottle rockets. All I can say is that my boss was dumb founded when he entered his small office to find nine of them flying the pattern. I wife was equally unimpressed to find the kids and I assembling the third airplane as the first two buzzed the house."