that is a very good question and one i was asking myself the other day, seen that many times and i'm still wondereing what its all about. the only thing i can tell you is that it had nothign to do with the actual alien biology - sometimes in films they film things in reverse for whatever reasons, i think thats all it is, any other suggestions anyone?
the blue field was apparently not to protect the aliens but was used as a warning device to the space jockeys - remember the sound it made when kane broke the field, that (so i read) was meant to alert the jockies. - try going onto 'musings from a jockey's chair' website, its quite good.