We did some baby sitting with a teen aged Imperial Moth caterpillar this fall, feeding it sweetgum leaves daily until it pupated. I had intended to get some pictures of the pupation but like a sneaky teenager, it changed overnight. Ours looked like this one above from UFL Entomology.
They start out life as incredibly cute little "cats" with stripes and a few tufts of non-stinging hairs. Like many lepidoptera, they may snack on their egg case before finding leaves of oaks, maples, sassafras, or sweetgum. Later instars, like below, develop a spiky look of the punk generation of teens.
The final instar crawls into the soil and leaf litter to pupate and spend the winter. It comes equipped with a couple of pointed tail pipes that are said to help it dig its way out in the spring before emerging as the full grow silk moth. If you have trouble visualizing it wiggling to the surface, just watch this video of a pupa in action.