Aphids and other flying bugs were referenced for Dropship designs in an effort to mirror the insectile nature of the Aliens themselves. By contrast, The Mothership was an exercise in iconography, requiring an easily recognisable, imposing piece of
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Like the interface between the Aliens and their organically-keyed weaponry, the Exo Suit went through a design phase that was more bio-mechanical than mechanical. You essentially climb into a big meat carcass and become one with it.