Peter Laufer's book, The Dangerous World of Butterflies, takes you to Shady Oak Butterfly Farm. In the book, he writes of Edith "boop-booping" Monarch butterfly caterpillars. In this video, Edith "boop-boops" caterpillars as you watch them dance to sound. They jerk their heads violently in reaction to loud "Boops".
In some instances, a caterpillar will react so violently to sound that it will literally fall off a milkweed plant to the ground.