The Hickory Horned Devil (Citheronia regalis) is the largest caterpillar in North America. It will eventually burrow into the ground to metamorphose into a regal moth, which has a 6-inch wingspan, but is much less bulky than its caterpillar form. The caterpillar molts four times to become this big (up to six inches long), and is bright green only in its final phase.
Just before pupation, the larva expels its gut and changes color from green to turquoise, the skin of the fully fed creature stretched shiny and tight. They then crawl down the host plant, where they burrow into the dirt and pupate in a well formed chamber at a depth of five to six inches. The pupae are dark brown/black in color, and have a relatively short cremaster. Some pupae overwinter for 2 seasons, perhaps as an adaption to variable and adverse conditions such as fires and flooding, or to maintain genetic diversity across generations.
The caterpillar is harmless, and only looks scary to protect itself while it's this big and easy for predators to see. This one was recorded in Dothan, Alabama. -via Daily of the Day