Osmeterium
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 (not unlike a snake 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 pleasant, usually smelling like a concentrated scent of the caterpillar’s food plant and pineapple. The constitution of the osmeterial secretion varies from species to species and contains monoterpene hydrocarbons, sesquiterpenic compounds or a mixture of aliphatic acids and esters The fine structure of the osmeterium of "Papilio demoleus libanius" Fruhstorfer has been studied and found to contain 3 types of specialised cells for synthesis, acid secretion, and storage of the osmeterial secretion. Structure. The fine structure of the osmeterium gland of "Papilio" larvae consists of  the following cell types. Specific study of "Papilio demoleus libanius" Fruhstorfer shows the possible existence of a third type of cell Osmeterial secretion. The osmetrial secretion consists of a mixture of volatile organic acids, such as isobutyric acid and 2-methylbutyric acid. The secretion of "Papilio demodocus" pre-final larval instar contains methyl 3-hydroxybutanoate, 3-hydroxybutanoic acid, α-pinene, myrcene, limonene, β-phellandrene, ("Z")-ocimene, ("E")-ocimene, β-caryophyllene, ("E")-β-farnesene, and germacrene A, as well as a number of unidentified sesquiterpenoids. and that of its final instar contains 2-methylpropanoic acid, 2-methylbutanoic acid, and their methyl and ethyl esters  as major constituents. Volatile components of the secretion of "Parnassius glacialis" (Parnassiinae, Parnassiini) consisted of isobutyric acid, 2-methylbutyric acid, and their methyl esters. That of "Sericinus montela" (Parnassiinae, Zerynthiini) was characterized as monoterpene hydrocarbons comprising β-myrcene (major), α-pinene, sabinene, limonene, and β-phellandrene, and of "Pachliopta aristolochiae" (Papilioninae, Troidini) was composed of numerous sesquiterpene hydrocarbons, including α-himachalene, α-amorphene, and germacrene A, and a few oxygenated sesquiterpenoids. Studies on osmeterial secretion makes it possible to classify the Papilionid species into two large categories on the basis of the chemical property of osmeterial secretion. Function. Eversion of  the osmeterial gland is demonstrably effective in reducing predation by ants and small spiders, and its chemical constituents have been shown to repel or kill ants, and mantids.