(1. The butterfly is an Eastern Tiger Swallowtail, which I chose for its blue and yellow color scheme to match one of Player's "default" outfits from KHUX (not that the art is even in color anyway, but I needed some sort of criteria to work with.)
(2. The caterpillar pictured here technically isn't the right species, but Eastern Tiger Swallowtail caterpillars look a bit too goofy/creepy in my opinion, along with not even looking like a traditional caterpillar. Being accurate would distract from the art too much, so I took a creative liberty and just chose a more standard-looking caterpillar from a similar butterfly species.
(3. Player (KHUX) and their transformation into Player2 (KHML) is represented by the lifecycle of butterflies. The concept I wanted to explore for Player was this: "How does it feel to transform into someone else, remembering bits and pieces of your previous life, but never being able to return to who you once were?" Also, in the KHDR Completion Commemorative Q&A, Nomura describes Player as "melt[ing] into the heart of another person", which brings to mind what happens to a caterpillar in its chrysalis.
(4. Brain is represented by a worm, something caterpillar-like
that is nonetheless not a caterpillar and is incapable of the same metamorphosis. It's also a pun on "brainworm", "bookworm", "computer worm" and "write once read many (WORM)". From Wikipedia: "A data storage device in which information, once written, cannot be modified. This write protection affords the assurance that the data cannot be tampered with once it is written to the device, excluding the possibility of data loss from human error, computer bugs, or malware."
(5. The concept I wanted to explore for Brain was this: "How does it feel to see your friends, your home, your entire world change and evolve around you while you remain the same physically and mentally, trapped in the past, unable to move on?" The worm curled up in the dark/dirt is meant to parallel the caterpillar in its chrysalis, a futile attempt at mimicry.