I've now decided to refer to the caterpillars by numbers because the one I was calling small caterpillar, to start with, is now medium sized. It will now be referred to as Caterpillar One.
Yesterday, Caterpillar One shed its skin again. My photo (above) was taken just after the event. Notice how its face and feet are still fairly clear in colour.
In particular, notice the face plate that fell off after the skin molt process. The face plate is the little black dot you can see below the caterpillar - its lodged in the joint of the lower leaf.
When a Monarch Caterpillar sheds its skin, it eases the unwanted skin downwards from the back of the head. As the skin comes to each set of feet it pulls them through, two by two, and carries on until the skin has been literally 'walked out of'. Sometimes they stay on the plant to accomplish this process. At other times they wander off the plant to find a rough surface that will catch on the skin and help draw it down, as they slide themselves out of it.
The process is called Ecdysone, being the molting hormone of insects.
All insects can only grow by periodically shedding their exoskeleton until the adult (in this case, the butterfly) emerges after the final molt. This is why Monarchs must shed their skin in order to grow bigger.
After each molt, and while the new skin is soft, the caterpillar swallows air to expand its body. Then, when the new skin firms, they let the air out and this provides them with room to grow within the new skin.
During this process of the old skin being 'stepped out of', the old face plate falls off.
I have watched this happen, with fascination. Yes, it's the complete black shiny face plate that drops off to reveal a clear version of the same, underneath. Within a day the new face colours up to black again and the caterpillar gets back to its eating.
Today Caterpillar One is looking magnificent and, this morning, has already an entire leaf.