Osmeterium

Close up of the osmeterium

The osmeterium is a defensive organ found in all Papilionid larva, in all stages. The organ is situated in the prothoracic segment and can be everted when the larva feels threatened. When not everted the organ is stored inverted within the body. 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 also exudes a foul smelling liquid which serves to repel ants, small spiders and mantids.

The constitution of the osmeterial secretion varies from species to species but on the whole is made up of one of the following

• monoterpene hydrocarbons
• sesquiterpenic compounds
• a mixture of aliphatic acids and esters

It has been found that in certain species the constituents of the secretion varies between the 4th instar and the 5th instar. Based on this the papilionid species can be grouped into 2 sets. One where the quality of the secretion varies and the other where it does not.

The fine structure of the osmetrium of Papilio demoleus libanius Fruhstorfer has been studied and found to contain 2 types of cells

1. Ellipsoid gland cells which might be organic acid-secreting cells
2. Tubular arm cells which might be tasked with synthesis and secretion of the mature secretion
3. A third, Slack cells which might serve as storage cells, has also been reported.

Structure

The fine structure of the osmeterium gland of Papilio larvae consists of 2 main 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.

Specific study of Papilio demoleus libanius Fruhstorfer shows the possible existence of a third type of cell

• Slack cells, reported for the first time, 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.

Osmetrial secretion

Studies on osmeterial secretion makes it possible to classify the Papilionid species into at least two large categories in terms of the chemical property of osmeterial secretion.

• 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 (HONDA, 1980), P. demodocus (BURGER et al., 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.

Function

Eversion of the osmeterial gland is demonstrably effective in reducing predation by ants and small spiders (Damman, 1986), and its chemical constituents have been shown to repel or kill ants (Eisner and Meinwald, 1965; Honda, 1983) and mantids (Chow and Tsai, 1989)

External links

• Tree of Life