The base was a 12x18" chocolate cake. The caterpillar was made on top using mini ball cakes, trimmed into appropriately shaped segments.
For an event with fewer people, I could have done just the caterpillar on a cake board.
After icing the head of the caterpillar smooth, I partially mixed blue icing into some of the green icing, and loaded that in one bag, and partially mixed yellow into green for another bag, then piped tip 12 dots with points with each bag onto alternate segments for contrast. The feet were Hershey's Kisses.
Overall, I was extremely pleased with how this cake turned out.