We know so little about the fallen anatomy. How do the fallen eat? How do they....well, you know.....reproduce?
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Well they're basically giant spider-people so they probably inject something inside their prey to liquify it and suck it out through a straw. Reproduction is egg-based. All those engrams dropping out and rolling around when they die? Those are their eggs. Science!
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With their mouths. It's a mask they carry. And reproduce with their penis, which is covered by armor. :D