The contrast is even easier to see below, as one caterpillar hasn't changed as much yet.
After we change their parsley, the caterpillars usually have an active time, moving around and eating. On this day there appeared to be some jostling for position, including pushing and shoving ("Hey! You! Get off of my stem!"), although there were plenty of leaves for all.
Caterpillars are insects, so they have an exoskeleton that doesn't grow. As it gets tighter, a new exoskeleton forms and the caterpillar splits open the old one and crawls out of it. This is called molting.
Anyway, today Marianna saw a crumbled black blob just a few times bigger that a dropping. We suspected it was a discarded "skin" and looked at it under 40X magnification. I wish we could have taken a picture for you--it was so cool! You could see that it was opened (there was an inside and outside, although it was crumpled up), and could see lots of the little black spikes that the smallest caterpillars have.