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Second or third instar caterpillar of Papilio glaucus showing osmeterium
Close up of the osmeterium
Close up of the osmeterium

The osmeterium is a fleshy organ found in the prothoracic segment of larvae (caterpillars) of Swallowtail butterflies including Birdwings and the '-prominent (Notodontidae) moths. This organ emits odorous compounds believed to be pheromones. Normally hidden, this forked structure can be everted when the caterpillar is threatened, and used to emit a foul-smelling secretion containing terpenes. These chemicals vary from species to species.[1]

The fine structure of the osmeterial glands of Papilio demoleus libanius Fruhstorfer (Lepidoptera: Papilionidae) have been studied and found to have three kinds of cells including ellipsoid gland cells.[2]


  1. ^ Eisner, T., and Y.C. Meinwald. 1965. The defensive secretions of a caterpillar (Papilio). Science 150: 1733-1735.
  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.

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