Osmeterium

Papilio xuthus Larva. Upper: Osmeterium everted. Lower: undisturbed

The osmeterium is a defensive organ found in all Papilionid larvae, in all stages.[1] 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,[2] small spiders[3] and mantids.[4]

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,[5] and storage.[6] of the osmeterial secretion.

Structure

Osmeterium of Pachliopta aristolochiae.

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.[4]

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.,[7] and that of its final instar contains 2-methylpropanoic acid, 2-methylbutanoic acid, and their methyl and ethyl esters as major constituents.[8] 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.[9]

Studies on osmeterial secretion makes it possible to classify the Papilionid species in to two large categories on the basis of the chemical property of osmeterial secretion.[10]

Function

Eversion of the osmeterial gland is demonstrably effective in reducing predation by ants and small spiders,[3] and its chemical constituents have been shown to repel or kill ants,[2] Honda, 1983) and mantids[1][4]

References

  1. 1 2 Chattopadhyay, J. 2011. The structure and defensive efficacy of glandular secretion of the larval osmeterium in Graphium agamemnon agamemnon Linnaeus, 1758 (Lepidoptera: Papilionidae).
  2. 1 2 Eisner, T., and Meinwald, Y.C. 1965. The defensive secretions of a caterpillar (Papilio). Science, N.Y. 150: 1733-1735.
  3. 1 2 Damman, H. 1986. The osmeterial glands of the swallowtail butterfly Eurytides Marcellus as a defense against natural enemies. Ecol.Entomol. 11: 261-265.
  4. 1 2 3 Chow, Y.S. and Tsai, R.S. 1989. Protective chemicals in caterpillar survival. Experientia (Basel) 45 (4): 390-392.
  5. 1 2 Crossley, A.C. and Waterhouse, D.F. 1969. The ultrastructure of the osmeterium and the nature of its secretion in Papilio larvae (lepidoptera). Tissue & Cell 1: 525-554.
  6. 1 2 Lu, Chow-Chin; Yien Shing Chow (1991) Fine structure of the larval osmeterium of Papilio demoleus libanius (Lepidoptera: papilionidae). Ann. Entomol. Soc. Am. 84(3):294-302.
  7. Burger, B.V., Munro, Z., Roth, M., Spies, H.S.C., Truter, V. and Greetsema, H. 1985. Constituents of osmeterial secretion of prefinal instar larvae of citrus swallowtail, Papilio demodocus (Esper) (Lepidoptera: Papilionidae). J. Chem. Ecol. 11 (8): 1093-1114.
  8. 1 2 Burger, B.V., Roth, M., Le Roux, M., Spies, H.S.C., Truter, V. and Geertsema, H. 1978. The chemical nature of the defensive larval secretion of the citrus Swallowtail, Papilio demodocus. J. Insect Physiol. 24: 803-805.
  9. Keiichi Honda, Nanao Hayashi. Chemical nature of larval osmeterial secretions of papilionid butterflies in the genera Parnassius, Sericinus and Pachliopta. Journal of Chemical Ecology June 1995, Volume 21, Issue 6, pp 859-867.
  10. 1 2 HONDA K. 1980: Osmeterial secretions of papilionid larvae in the genera Luehdorfia, Graphium and Atrophaneura (Lepidoptera). Insect Biochem. 10: 583-588.
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