Papilio helenus, the red Helen, is a large swallowtail butterfly found in forests of southern India and parts of southeast Asia.
Image: Red Helen (Papilio helenus) at Samsing, Duars, WB W IMG 6219
Image: Papilio helenus fortunius male 20131027
P. h. daksha, Kerala, India
Woodcut from The Fauna of British India, Including Ceylon and Burma
The osmeterium is a defensive organ found in all papilionid larvae, in all stages. The organ is situated in the prothoracic segment and can be everted when the larva feels threatened. The everted organ resembles a fleshy forked tongue, and this along with the large eye-like spots on the body might be used to startle birds and small reptiles. The osmeterial organ remains inside the body in the thoracic region in an inverted position and is everted when the larva is disturbed in any way emitting a foul, disagreeable odor which serves to repel ants, small spiders and mantids. To humans, this odour is rather strong but not unpleasant, usually smelling like a concentrated scent of the caterpillar's food plant and pineapple.
Osmeterium of Papilio xuthus Larva. Upper: Osmeterium partly everted. Lower: undisturbed
Papilio demodocus larva everting its osmeterium fully, and with one horn applying the repugnatorial secretion to the finger that irritated it
