 The osmeterium is an organ behind the head that “inflates” into an orange/red Y-shaped growth that resembles a snake’s forked tongue.By the fourth instar, the osmeterium is more than just a startle mechanism and also has bad-smelling and toxic mixture of acidic chemicals.This is only effective on small invertebrate predators, and the caterpillar will try to rub its osmeterium on the predator to deter it.