The osmeterium is a fleshy organ found in the prothoracic

Prothorax

The prothorax is the foremost of the three segments in the thorax of an insect, and bears the first pair of legs. Its principal sclerites are the pronotum , the prosternum , and the propleuron on each side. The prothorax never bears wings in extant insects, though some fossil groups possessed...

 segment of larva

Larva

A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

e (caterpillars) of Swallowtail butterflies

Swallowtail butterfly

Swallowtail butterflies are large, colorful butterflies that form the family Papilionidae. There are over 550 species, and though the majority are tropical, members of the family are found on all continents except Antarctica...

 including Birdwing

Birdwing

Birdwings are papilionid butterflies native to the Indian Subcontinent, mainland and archipelagic Southeast Asia and Australasia, and are usually regarded as belonging to three genera: Ornithoptera, Trogonoptera and Troides. Some authorities include additional genera...

s. 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.