The Promethea caterpillars are changing to second instar. They’re seven days old.
They have not all shifted yet – they may have hatched across two days, or some may just be a little delayed. You can see that the remaining first-instar caterpillars are smaller, and also yellower than their second-instar siblings.
You can see – especially on the stripe behind the head – that they’re beginning to develop a white, flaky cortex – as they get older, it will develop almost into a crust. I don’t know whether this helps to make them unpalatable to birds, or what.