Okay...I don't quite see how this was extended for so long, but xenos have acid in their head.
However, I would like to turn my proverbial eyes onto what Switcher is saying.
I never said that xenomorphs don't eat. They do.
Xenomorphs do not breathe. That does not mean that they don't have the same living requirements of all other organisms.
All organisms need three things to survive: Food, Water, and Shelter.
A xenomorph eats, therefore it has food.
A xenomorph acquires the moisture it needs from its food, therefore it has water.
A xenomorph must rest from time to time, therefore it has shelter.
However, xenomorphs do have blood, acidic or otherwise, in their heads. All organisms with a central nervous system, xenomorphs included, must have blood flow to their brain in order to survive. Without blood circulation, a xenomorph would die.
And thus I clothe my naked villainy
With old odd ends stolen forth from holy writ
And seem a saint when most I play the devil.