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.
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.