I've seen this kind of damage before when I lived in the Dallas area. I came out one morning and my entire moon plant was eaten down to the stem - gone, in one night. Each of those stem stubs used to have a leaf on them.
Even though this moon plant is quite large, I'm pretty sure the caterpillars, if left to their own devices, will devour the entire plant in just a couple of days. I don't feel much like sharing my plants with the caterpillars, especially since this plant is blooming anew every night and has dozens of buds ready to pop.
It took some careful searching, but I finally did find about five of these caterpillars. They are beautiful in their own way and very cleverly designed so that they blend in with the plant. It's as if God created the moon plant, then thought, "Hmm, now I need something to keep it humble," and then He created this perfectly matched creature with a voracious appetite.
I probably didn't find all of them and will awake tomorrow morning to more of my moon plant having been devoured, but at least, I've saved a bit of it. Oh, and I fed those chubby little critters to the chickens. It's only fair.
And, just because it's sort of pretty, here's a picture of the seed pod for the moon plant.