Caterpillars feed by extending their heads and then curling up as they chew. So the trick was to focus right at the edge of the leaf and wait for the critter to bring it's head up to start the next "row". I got seven frames before it figured out I was close and stopped feeding.
Tech Specs: Canon 80D (F11, 1/250, ISO 100) + a Canon MP-E 65mm macro lens (over 3x) + a diffused MT-26EX-RT with a Kaiser adjustable flash shoe on the "A" head (the key), E-TTL metering, -1/3 FEC, second curtain sync). This is a single, uncropped, frame taken hand held. I'm holding on to the stem of the plant with my left hand, and resting the lens on that same hand to keep the scene steady.