The Black Swallowtail Caterpillar with Ostemerium

This little sweetie is growing up on my fennel plant.  His mom, the Black Swallowtail Butterfly laid her eggs on a leaf nearby the fennel.  See those yellow horns on top?  That is his Ostemerium, which is a defense mechanism.  A little yellow internal organ pops out when the caterpillar feels threatened.  (I had to tickle him a little to get the picture.)