Just read through this post and would like to add my two cents on the alien biology. There are such things as drones. The official name of the original alien created by HR GIGER was called an 'Alien Drone' These are the young aliens who have not yet matured, the dome over the head acts as a protective cover to sheild the alien whilst it is still in development. Once fully matured, the dome then adapts and 'shrinks' if you will and reveals the indented alien skull underneath, this is now a fully grown alien which was called a 'warrior' by James Cameron.
We all know the designers each wanted to add their own special touches to the aliens but the thing you are forgeting (as AVPFREAK mentioned) is - you cant just have a random change in the movie's continuity, it messes the whole equilibrium up of the alien mythos, there has to be an explanation for these changes within the movie for the fans to actually appreciate, i mean, Ripley is not going to turn around mid way through the film and say "I've never seen one like this before, it's a james cameron alien.'
Point is, there are fan based biological reasonings for the changes behind the aliens, and no we're not talking about the pathetic comics and games, they just simply put the word ridiculous on the map, As a hardcore fan of the alien series for over 15 years i for one welcome the addition of the biological explanations, the more detailed ones make sense and add a sense of depth to the movies.
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