WF showed me some Lime Butterfly (Papilio demoleus malayanus) caterpillars that he was rearing. The caterpillar looked like this below.
However, when it became pissed, two bright red tongue-like organ everted out from the head.
It turned out that the forked “tongue” is an organ called Osmetrium. It is said that it emits some smelly chenicals known as terpene that serves to repel predators. No kidding, this caterpillar even reeled its head up and over trying to strike me when I touched its buttocks!