# Osmeterium
The **osmeterium** (plural osmeteria) is a forked, eversible defensive scent gland found in the caterpillars of the swallowtail butterflies (family [Papilionidae](https://www.edgepedia.org/papilionidae)). When the larva is disturbed, it extrudes the organ from a transverse slit on the dorsal surface of the prothoracic segment, rears up, and attempts to smear a predator with a strong-smelling chemical repellent.[1](https://www.bugguide.net/node/view/112945) • [2](https://ask.ifas.ufl.edu/publication/IN906/pdf) The organ is one of the three unambiguous morphological characteristics that unite all papilionids and distinguish them from other butterfly families.[3](https://tolweb.org/Papilionidae/12177/2002.02.21)
| Key fact | Detail |
|---|---|
| Definition | A large, colored, V-shaped fleshy process extruded from a transverse slit on the dorsal prothorax when the caterpillar is alarmed[1](https://www.bugguide.net/node/view/112945) |
| Distribution | Present in all swallowtail caterpillars, in every larval instar[3](https://tolweb.org/Papilionidae/12177/2002.02.21) • [4](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae) |
| Secretion | Strong-smelling defensive terpene compounds; the exact mix varies among swallowtail species[1](https://www.bugguide.net/node/view/112945) |
| Defensive display | The larva rears up, everts the osmeteria to release a repellent volatile, and waves its body back and forth[5](https://genent.cals.ncsu.edu/bug-bytes/defense/) |
| Effectiveness | Effective against ants (Eisner & Meinwald 1965); soldier bugs can attack and eat larvae without evoking extrusion (Berenbaum et al. 1992)[2](https://ask.ifas.ufl.edu/publication/IN906/pdf) |
| Etymology | New Latin, from Greek *osme* (smell) plus *terein* (to keep) plus the Latin diminutive *-ium*[1](https://www.bugguide.net/node/view/112945) |
## Structure and eversion
A fully protruded osmeterium assumes a Y-shaped structure: a stout basal trunk or peduncle from which arise two elongated hollow tubular arms. In the final instar of *Graphium agamemnon agamemnon*, the tubular arms measured 7.52 to 9.56 mm long (mean 8.96 ± 0.061 mm, n = 11), with the basal peduncle about 2.07 mm in diameter.[4](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae)
Each tubular arm carries its own gland pore. In *Graphium agamemnon agamemnon* an oval opening of the ellipsoid gland sits on the posterior face of each arm, presumably helping to spray and smear the defensive secretion over the attacker; histology shows two main secretory cell types, ellipsoid gland cells around the pore region and tubular arm cells extending toward the tips.[4](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae) Muscle fibers at the base of the arms drive both protrusion and retraction.[4](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae)
## Chemistry of the secretion
The secretion is a mixture of volatile organic compounds. In *Graphium agamemnon agamemnon* the strong odor comes from volatile organic acids such as isobutyric acid and 2-methylbutyric acid, and the final-instar secretion contains 2-methylpropanoic acid, 2-methylbutanoic acid, and their methyl and ethyl esters as major constituents.[4](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae) The pre-final instar of the citrus swallowtail *Papilio demodocus* secretes methyl 3-hydroxybutanoate, 3-hydroxybutanoic acid, α-pinene, myrcene, limonene, β-phellandrene, (Z)-ocimene, (E)-ocimene, β-caryophyllene, (E)-β-farnesene, germacrene-A, and a number of unidentified sesquiterpenoids.[4](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae)
**Species and instar variation.** The chemical makeup varies across species in both kind and amount, and some species additionally release substantial quantities of methyl 3-hydroxy-n-butyrate along with oxygenated sesquiterpenes.[4](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae) Even within a single larva, the two prongs of the osmeterium in *Papilio demodocus* were found in a number of larvae to produce quantitatively different secretions.[6](https://pubmed.ncbi.nlm.nih.gov/24310333/)
## Defensive behavior and efficacy
When threatened, the larva rears up, extrudes the osmeterium, and attempts to smear the potential predator with the repellent; the display also involves body movements to present the gland and the release of noxious substances.[2](https://ask.ifas.ufl.edu/publication/IN906/pdf) • [4](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae) The repellent is effective against ants, as shown in the classic study by Eisner and Meinwald (1965), but it is not a universal defense: Berenbaum, Moreno, and Green (1992) reported that soldier bugs ([Hemiptera](https://www.edgepedia.org/hemiptera): [Pentatomidae](https://www.edgepedia.org/pentatomidae)) could attack and eat swallowtail larvae without evoking extrusion of the osmeteria.[2](https://ask.ifas.ufl.edu/publication/IN906/pdf)
The defense is also finite. In one experiment, repeated attacks by predators exhausted a swallowtail caterpillar's osmeterial secretion, and ants were then able to prey on it successfully.[4](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae) Irritant compounds in general can induce cleaning behavior by a predator, giving the prey time to escape.[5](https://genent.cals.ncsu.edu/bug-bytes/defense/)
## Appearance and development
The organ's color varies by species and stage. The osmeterium of the black swallowtail (*Papilio polyxenes asterius*) is bright yellow-orange.[2](https://ask.ifas.ufl.edu/publication/IN906/pdf) In a newly described *Heraclides* swallowtail from North America, the first-instar osmeterium is yellowish, and early-instar larvae tend to use it right away when disturbed.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC4296521/) In *Graphium agamemnon agamemnon* the protruded structure is shiny yellow-green, and earlier instars possess fewer papillae, more distantly placed, than the final instar.[4](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae)
## References
1. [osmeterium, osmeteria, BugGuide.Net](https://www.bugguide.net/node/view/112945)
2. [Eastern Black Swallowtail: *Papilio polyxenes asterius* (EENY-504), UF/IFAS Extension](https://ask.ifas.ufl.edu/publication/IN906/pdf)
3. [Papilionidae, Tree of Life Web Project](https://tolweb.org/Papilionidae/12177/2002.02.21)
4. [Chattopadhyay, J. (2011). The structure and defensive efficacy of glandular secretion of the larval osmeterium in *Graphium agamemnon agamemnon* Linnaeus, 1758. Turkish Journal of Zoology 35(2): 245-254](https://www.researchgate.net/publication/278078292_The_structure_and_defensive_efficacy_of_glandular_secretion_of_the_larval_osmeterium_in_Graphium_agamemnon_agamemnon_Linnaeus_1758_Lepidoptera_Papilionidae)
5. [Defense, ENT 425 General Entomology, NC State University](https://genent.cals.ncsu.edu/bug-bytes/defense/)
6. [Constituents of osmeterial secretion of pre-final instar larvae of citrus swallowtail, *Papilio demodocus* (Esper), PubMed](https://pubmed.ncbi.nlm.nih.gov/24310333/)
7. [A new *Heraclides* swallowtail (Lepidoptera, Papilionidae) from North America, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4296521/)
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*Topic: Encyclopedia › Life and health › Animals › Invertebrates › Arthropods › Insects › Butterflies and moths › Butterfly biology and behavior*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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