The osmeterium is a fleshy organ found in the prothoracic segment of larvae (caterpillars) of Swallowtail butterflies including Birdwings. This organ emits smelly 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.