Hips (or bottom, bum, rear-end, butt, whatever you call it!) must come to the surface of the water during butterfly.  
Last night our swimmers practiced with worm drill.  
See below:

As you work on perfecting fly, keep reminding yourself to get your hips up to the surface. When your 
head goes down, your hips should go up.  Tuck your chin to make it easier.

Key words:
"Hips up"                  
"Head go down, hips go up"