Orchard Swallowtail Butterfly caterpillar

The larvae is in its final instar and will pupate in a day or two When the process of pupation starts the caterpillar is very vulnerable to being parasitised by parasitic wasps. As a defence the caterpillar will when distributed extrude its two Osmeteriums and will spray out a pungent chemical which helps to keep the parasitic wasp at bay. But obviously not all the time.