I read many years ago in an old Aliens pen and paper RPG book some things that may or may not be true to the real story behind the aliens. Granted I was very young, so I may be wrong, but in that book, it mentioned that aliens did consume flesh, and it was when that amount of food was available that queen development could occur. When not in the presence of a major food source, the aliens could actually survive in a hibernating state (i.e. attached to the walls like resting) by eating the metals in the environment (creating that hive look wherever they are) and by absorbing a natural electrical current in the environment through their bodies.