Today one of the caterpillars turned brown. This happens when they are preparing to pupate. I suspect that it also corresponds with a change in microhabitat use. The caterpillars do not grow much after they reach this body size meaning that thier habitat needs likely change from locations with abundant food sources to those that provide a safe place to pupate. Being green makes the caterpillar incredibly cryptic when it is amongst the leave it feeds on, and this brown colour probably serves a similar purpose as the caterpillar looks for a suitable pupation site on a tree branch.
Later today it purged the rest of its waste material. This is a pretty good sign that this guy will pupate tonight.