osmeterium

Osmeeterium is a defensive organ found in most swallowtail butterfly caterpillars (family Papilionidae). It is an eversible, fleshy structure that sits behind the head on the first thoracic segment. When the larva perceives danger, it quickly protrudes the osmeterium, revealing a forked, often orange-colored pair of lobes that can resemble a small snake’s tongue.

Anatomy and deployment: The osmeterium consists of glandular tissue connected to the larva’s thoracic region. In many species it comprises two or more lobes that expand into view within seconds. After the threat passes, the organ retracts back into its housing beneath the mouthparts.

Defense and chemistry: The osmeterium releases a mixture of volatile compounds, including terpenoids, that produce a strong, pungent odor and taste. These chemical cues deter a range of predators, such as birds and arthropods, and may operate in combination with the startling visual display of the bright, orange lobes.

Distribution and etymology: The organ occurs in many swallowtail species worldwide and is considered a characteristic feature of Papilionidae caterpillars. The name osmeterium derives from the Greek osme, meaning “smell,” reflecting its scent-based defensive function. It is not found in most other Lepidoptera families.

In summary, the osmeterium is a rapid, odor-emitting defense used by many swallowtail caterpillars to deter predators, functioning through both chemical emissions and a conspicuous visual display.