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.

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 osmetrium 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.

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

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, b-phellandrene, (Z)-ocimene, (E)-ocimene, b-caryophyllene, (E)b-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, includingα-himachalene,α-amorphene, and germacrene-A, and a few oxygenated sesquiterpenoids.

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