The Big Oni is one of those bosses who eats the protagonist alive and whole. When he opens his mouth, it is damn wide. Fortunately, the screen cuts to black as soon as he places Momohime in it. But the thing is, his stomach is dark, your character is basically stepping in its acid, and you can see the insides moving for a bit.
One unnerving detail about the phantom spiders is that their green bellies don't look like a spider's, but that of a human skull's. Honen later explains that Tsuchigumo creates phantom spiders by digesting skulls. They are quite similar to the Headcrabs but with a different color theme, and you can easily identify a face/skull in the phantom spiders' bellies.
Also, in Tsuchigumo's lair, you can see its recent victims being bound with webs - Honen, a young monk, Torahime and another body which already looks like a dried out, almost-skeletal corpse wearing a monk's robes. If you would watch the latter's idle animation, you can notice that it is still breathing.