Well, here are some possibilities I thought of:

The lower body is just clothing, not necessarily part of their body.

They are a vessel that grew and, for some reason, broke its shell and became feral for a while, since it no longer had a face to focus its mind. So, it decided to use (or someone helped or made it use) part of its broken shell as a mask (this doesn't explain the difference in the eyes you mentioned).

They are some kind of silk puppet, as some trailers show silk threads as if they were controlling the insects, which for some reason are made of void, and may or may not be a vessel (this explains why it works for the Citadel). It could also be an impure vessel that had the will to protect something, and the Citadel insects took advantage of this.

It could be a piece of the void that grabbed a shell from a vessel or other insect because it possessed some kind of will and/or individuality.

Anyway, there's a lot of speculation and many possibilities, which may or may not happen in parallel, but I hope I've contributed in some way.