When feeding or tending to caterpillars, it is safest not to move a caterpillar unless it is moving its rear legs itself. If it isn’t moving, you can gently tickle the rear of the caterpillar. If it isn’t molting, it will normally move.
Although some species do fine if they are moved when their legs are locked in the silk, moving some species at this time is a death sentence. Monarch caterpillars normally do well when moved while they are molting.
Although caterpillars grow, their skin/cuticle does not grow. This would present a dilemma if it was not for the fact that caterpillars molt.
At a certain point in growth, they lay a mat of silk on an object and lightly lock their legs into it. They release enzymes that dissolve the inner layer of their cuticle. After about a day, the caterpillar’s cuticle splits above the thorax and the caterpillar literally crawls out. Underneath is a new cuticle, a bit baggier and will fill out to a larger size.
It is fairly obvious when a caterpillar is preparing to molt. They withdraw their heads from their head capsules, causing a larger bulge than normal behind the head capsule.
The old head capsule sits on the face of the caterpillar, much like a doctor’s mask.
The caterpillar literally crawls out of its cuticle, raising its hind prolegs high to completely leave the old cuticle behind.
Some species will eat their old cuticle. Others will leave it behind.
A few species leave their old cuticles behind in this manner (below). The old cuticles look as if the caterpillars died and are just dried up, hanging from the plant.
Many species leave their host plants to molt. This can cause confusion to enthusiasts who find a new-to-them species of caterpillar and aren’t sure what to feed them. Gulf Fritillary caterpillars are notorious for molting off their plants.