What kind of caterpillar is this?
Your caterpillar is obviously that of a Sphinx moth, but the exact species is difficult to determine. Since you found it in the compost pile and given the season, it was likely seeking a place to pupate. As caterpillars mature and begin the process of pupation, their colors will intensify or change, sometimes dramatically. As we searched for a likely identification of your caterpillar we considered the possibility that it is a 'White- lined' Sphinx, There are several other realistic possibilities.