{{short description|Odoriferous defensive structure}} [[File:Papilio xuthus Larva 2011-07-01.jpg|thumb|{{center|Osmeterium of ''Papilio xuthus'' Larva. <br>Upper: Osmeterium partly everted. Lower: undisturbed}}]] The '''osmeterium''' is a defensive organ found in all papilionid larvae, in all stages.<ref name=Chattopadhyay/> 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,<ref name=eisner_meinwald_1965/> small spiders<ref name=Damman_1986/> and mantids.<ref name=Chow_Tsai_1989/> To humans, this odour is rather strong but not unpleasant, usually smelling like a concentrated scent of the caterpillar's food plant and pineapple.
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 osmeterium of ''Papilio demoleus libanius'' Fruhstorfer has been studied and found to contain 3 types of specialised cells for synthesis, acid secretion,<ref name=ultrastructure/> and storage<ref name=finestructure/> of the osmeterial secretion.
==Structure== [[File:Papilio demodocus larva IMG 8543s.jpg|thumb|''Papilio demodocus'' larva everting its osmeterium fully, and with one horn applying the repugnatorial secretion to the finger that irritated it]] [[File:Anise swallowtail and European paper wasp.webm|thumb|A European paper wasp interacts with an anise swallowtail caterpillar which uses its osmeterium in defense. Most scenes are repeated in closeup at one-fourth speed.]] The fine structure of the osmeterium gland of ''Papilio'' larvae consists of the following cell types. *Ellipsoid gland cells have an extensively infolded basal plasma membrane, abundant ribosomes and whorls of smooth endoplasmic reticulum. The apical plasma membrane bears long microvilli extending into a mass of granular material containing electron-lucid cavities. Tangential slits occur in the epicuticle. These cells are presumed to be organic acid-secreting cells. *Tubular arm cells contain heterogeneous, electron-dense inclusions, extensively-branched nuclei and large mitochondria, sometimes distended with electron-dense material. The apical plasma membrane bears short microvilli. The inner, dense epicuticle forms a complex ramifying system. These glandular cells presumably synthesize and secrete the mature secretion.<ref name=ultrastructure/> Specific study of ''Papilio demoleus libanius'' Fruhstorfer shows the possible existence of a third type of cell *Slack cells, loosely surround the ellipsoid gland cells. These cells possess papillae, well-developed, rough endoplasmic reticulum, and numerous secretory vacuoles of various sizes and electron density. They may function as storage cells.<ref name=finestructure/>
==Osmeterial secretion== The osmetrial secretion consists of a mixture of volatile organic acids, such as isobutyric acid and 2-methylbutyric acid.<ref name=Chow_Tsai_1989/>
The secretion of ''Papilio demodocus'' pre-final larval instar contains methyl 3-hydroxybutanoate, 3-hydroxybutanoic acid, α-pinene, myrcene, limonene, β-phellandrene, (''Z'')-ocimene, (''E'')-ocimene, β-caryophyllene, (''E'')-β-farnesene, and germacrene A, as well as a number of unidentified sesquiterpenoids.<ref name="Burger_P_demodocus_1985"/> and that of its final instar contains 2-methylpropanoic acid, 2-methylbutanoic acid, and their methyl and ethyl esters as major constituents.<ref name="Burger_P_demodocus_1978"/>
Volatile components of the secretion of ''Parnassius glacialis'' (Parnassiinae, Parnassiini) consisted of isobutyric acid, 2-methylbutyric acid, and their methyl esters. That of ''Sericinus montela'' (Parnassiinae, Zerynthiini) was characterized as monoterpene hydrocarbons comprising β-myrcene (major), α-pinene, sabinene, limonene, and β-phellandrene, and of ''Pachliopta aristolochiae'' (Papilioninae, Troidini) was composed of numerous sesquiterpene hydrocarbons, including α-himachalene, α-amorphene, and germacrene A, and a few oxygenated sesquiterpenoids.<ref name=honda_1995/>
Studies on osmeterial secretion makes it possible to classify the Papilionid species into two large categories on the basis of the chemical property of osmeterial secretion.<ref name=honda_1980/> *One is a group in which the chemical constitution of osmeterial secretion of the last larval instar markedly differs in quality from those of the younger larvae. The results derived from ''Papilio protenor'',<ref name=honda_1980/> ''P. demodocus'' <ref name="Burger_P_demodocus_1978"/> and other ''Papilio'' species (''P. helenus'', ''P. machaon'', ''P. memnon'', ''P. bianor'', ''P. maccki'', ''P. xuthus'', etc., unpublished work) may assign the genus ''Papilio'' (tribe Papilionini) to this group, which can be termed 'heterogeneous type'. *The genera ''Luehdorfia'' (tribe Zerynthiini), ''Graphium'' (tribe Graphiini) and ''Atrophaneura'' (tribe Troidini) apparently belong to the other group, in which no qualitative change of osmeterial secretion occurs at the last larval ecdysis. This group can be designated as 'homogeneous type', which is further subdivisible into three types. **Monoterpene - ''Luehdorfia'' (Zerynthiini, Parnassiinae) **Sesquiterpene - ''Atrophaneura'' (Troidini, Papilionnae) **Aliphatic acid and ester - ''Graphium'' (Graphiini, Papilioninae)
==Function== Eversion of the osmeterial gland is demonstrably effective in reducing predation by ants and small spiders,<ref name=Damman_1986/> and its chemical constituents have been shown to repel or kill ants,<ref name=eisner_meinwald_1965/><ref name="Honda 1983"/> and mantids.<ref name=Chattopadhyay/><ref name=Chow_Tsai_1989/>
==References== <references>
<ref name=finestructure>{{cite journal |last1=Lu |first1=Chow-Chin |last2=Chow |first2=Yien Shing |title=Fine Structure of the Larval Osmeterium of Papilio demoleus libanius (Lepidoptera: Papilionidae) |journal=Annals of the Entomological Society of America |date=May 1991 |volume=84 |issue=3 |pages=294–302 |doi=10.1093/aesa/84.3.294 }}</ref> <ref name=ultrastructure>{{cite journal |last1=Crossley |first1=A.C. |last2=Waterhouse |first2=D.F. |title=The ultrastructure of the osmeterium and the nature of its secretion in Papilio larvae (lepidoptera) |journal=Tissue and Cell |date=January 1969 |volume=1 |issue=3 |pages=525–554 |doi=10.1016/s0040-8166(69)80020-0 |pmid=18631483 }}</ref> <ref name=Chattopadhyay>{{cite journal |last1=Chattopadhyay |first1=Jagannath |title=The structure and defensive efficacy of glandular secretion of the larval osmeterium in Graphium agamemnon agamemnon Linnaeus, 1758 (Lepidoptera: Papilionidae) |journal=Turkish Journal of Zoology |date=2011 |doi=10.3906/zoo-0901-24 |doi-access=free}}</ref> <ref name=honda_1980>{{cite journal |last1=Honda |first1=Keiichi |title=Osmeterial secretions of papilionid larvae in the genera Luehdorfia, Graphium and Atrophaneura (lepidoptera) |journal=Insect Biochemistry |date=January 1980 |volume=10 |issue=5 |pages=583–588 |doi=10.1016/0020-1790(80)90095-5 }}</ref> <ref name="Burger_P_demodocus_1978">{{cite journal |last1=Burger |first1=B.V. |last2=Röth |first2=Marina |last3=Roux |first3=Maritha le |last4=Spies |first4=H.S.C. |last5=Truter |first5=Verona |last6=Geertsema |first6=H. |title=The chemical nature of the defensive larval secretion of the citrus swallowtail, Papilio demodocus |journal=Journal of Insect Physiology |date=January 1978 |volume=24 |issue=12 |pages=803–805 |doi=10.1016/0022-1910(78)90099-9 |bibcode=1978JInsP..24..803B }}</ref> <ref name="Burger_P_demodocus_1985">{{cite journal |last1=Burger |first1=B. V. |last2=Munro |first2=Zenda |last3=Röth |first3=Marina |last4=Spies |first4=H. S. C. |last5=Truter |first5=Verona |last6=Geertsema |first6=H. |last7=Habich |first7=A. |title=Constituents of osmeterial secretion of pre-final instar larvae of citrus swallowtail,Papilio demodocus (Esper) (Lepidoptera: Papilionidae) |journal=Journal of Chemical Ecology |date=August 1985 |volume=11 |issue=8 |pages=1093–1113 |doi=10.1007/BF01020678 |pmid=24310333 |bibcode=1985JCEco..11.1093B }}</ref> <ref name=Chow_Tsai_1989>{{cite journal |last1=Chow |first1=Y. S. |last2=Tsai |first2=R. S. |title=Protective chemicals in caterpillar survival |journal=Experientia |date=April 1989 |volume=45 |issue=4 |pages=390–392 |doi=10.1007/BF01957490 }}</ref> <ref name=Damman_1986>{{cite journal |last1=Damman |first1=Hans |title=The osmaterial glands of the swallowtail butterfly Eurytides marcellus as a defence against natural enemies |journal=Ecological Entomology |date=August 1986 |volume=11 |issue=3 |pages=261–265 |doi=10.1111/j.1365-2311.1986.tb00302.x |bibcode=1986EcoEn..11..261D }}</ref> <ref name=eisner_meinwald_1965>{{cite journal |last1=Eisner |first1=Thomas |last2=Meinwald |first2=Yvonne C. |title=Defensive Secretion of a Caterpillar (Papilio) |journal=Science |date=1965 |volume=150 |issue=3704 |pages=1733–1735 |doi=10.1126/science.150.3704.1733 |pmid=17768875 |jstor=1717449 |bibcode=1965Sci...150.1733E }}</ref> <ref name=honda_1995>{{cite journal |last1=Honda |first1=Keiichi |last2=Hayashi |first2=Nanao |title=Chemical nature of larval osmeterial secretions of papilionid butterflies in the genera ''Parnassius'', ''Sericinus'' and ''Pachliopta'' |journal=Journal of Chemical Ecology |date=June 1995 |volume=21 |issue=6 |pages=859–867 |doi=10.1007/BF02033466 |pmid=24234323 |bibcode=1995JCEco..21..859H }}</ref> <ref name="Honda 1983">{{cite journal |last1=Honda |first1=Keiichi |title=Defensive potential of components of the larval osmeterial secretion of papilionid butterflies against ants |journal=Physiological Entomology |date=June 1983 |volume=8 |issue=2 |pages=173–179 |doi=10.1111/j.1365-3032.1983.tb00346.x }}</ref>
</references>
==External links== {{commons category}} * [http://tolweb.org/Papilionidae Tree of Life]
Category:Swallowtail butterflies Category:Insect anatomy