Monarch caterpillars famously withstand the toxic sap of milkweeds. They themselves become toxic to predators by eating milkweed. This gaudy circus look is the opposite of camouflage: it’s a warning!
But they don’t want to drown in the sap. This caterpillar chewed away at the stem, or petiole, of this leaf to cut the plant’s circulation to this particular leaf. Of course, this undermined the leaf’s stability, and with the weight of the very hungry caterpillar underneath it, the leaf drooped vertically.
After cutting the leaf’s throat, so to speak, the VHC turned around and started munching away at the leafy greens.
Look at those antenna go…These two eating scenes were taken four minutes apart.
Nearby was a younger caterpillar, that is, an earlier instar or stage.
A third, late-instar stage specimen had just finished a leaf; you can see how this leaf, too, is bent downwards from the petiole undermining. 
A lot of milkweed goes into a caterpillar. Some of it comes back out.
VHC was a favorite of my children way back when.