What is "osmeterium"

n. (context zoology English) One of a pair of scent organs in the larvae of certain swallowtail butterfly.

The osmeterium is a defensive organ found in all Papilionid larvae, in all stages. The organ is situated in the prothoracic segment and can be everted when the larva feels threatened. The everted organ resembles a fleshy forked tongue not unlike a snake tongue and this along with the large eye like spots on the body might be used to startle birds and small reptiles. The osmeterial organ remains inside the body in the thoracic region in an inverted position and is everted when the larva is disturbed in any way emitting a foul, disagreeable odor which serves to repel ants, small spiders and mantids.

The constitution of the osmeterial secretion varies from species to species and contains monoterpene hydrocarbons, sesquiterpenic compounds or a mixture of aliphatic acids and esters

The fine structure of the osmetrium of Papilio demoleus libanius Fruhstorfer has been studied and found to contain 3 types of specialised cells for synthesis, acid secretion, and storage. of the osmeterial secretion.