This is one of the butterfly parasites--the tachnid fly.  It lays its egg inside a caterpillar and the larva slowly eats the poor caterpillar from the inside out.
Note red eyes and prickly back.

My last 7 crysalises are awaiting good weather before the butterflies eclose (emerge).  Meanwhile, this wooly bear caterpillar found a home in my garage after munching a final meal!