Written by Joe Ballenger

After our initial launch, we had a number of people submit questions. One of the first, from Antnommer on Twitter, I found particularly interesting because it aligns with my interest in biochemistry.

@SciBugs @BioInFocus @BugQuestions What does an osmeterium smell like? Does the smell differ between species?

— Antnommer (@Antnommer) December 14, 2014

To answer this question, I think I need to split it into 3 parts:

1.) What is an osmeterium?

2.) What does it smell like, and does the smell differ between species?

3.) What chemicals give the osmeterium it’s characteristic odor?

What is an osmeterium?

Image credit: Left, Papilio polyxenes courtesy of Tom Murray under CC attribution via bugguide Right, Papilio multicaudatus courtesy of Mike Quinn under CC attribution via bugguide.

An osmeterium is a defensive organ found on Swallowtail butterfly larvae, and is unique to the family Papilionidae. In the pictures to the left, the osmeterium is highlighted in red. It is exerted when the caterpillar is harassed by a predator, and has a pungent odor which drives the predator away. In some species with eyespots, the osmeterium completes the illusion of snake mimicry by resembling the snake’s forked tongue.

It’s interesting that the osmeterium only repels insects, and doesn’t seem to make a difference when the caterpillar is attacked by a bird.

What does the osmeterium smell like, and does the smell differ between species?

This question is admittedly hard for me to answer, partly because it’s a little bit subjective. Also, I can only answer part of it because the only species I’ve held as a larva is Papilio polyxenes, or the black swallowtail. Most of my experiences with this species also happened when I was younger, and I haven’t encountered one in a number of years. Fortunately, many entomologists weighed in on Twitter.

In my experience with P. polyxenes, I always remembered the larva smelling like parsley which I don’t consider unpleasant. According to Daniel Llavaneras, Battus species have an unpleasant odor, while biologist Thomas Hossie attributes a citrus-y smell to some swallowtail species. I think from this, that it’s apparent that different swallowtail caterpillars have different odors. However the smells I consider unpleasant may be different from what Daniel and Thomas consider unpleasant, so there’s a bit of difference in interpretation here.

There’s another way we can tackle this, though…we can tackle this question through chemistry!

What chemicals give the osmeterium it’s characteristic odor?

Life, at it’s most basic level is a chemical process. Our senses of taste and smell are largely the result of receptors which sense different combinations of chemicals. Different chemical compounds have different tastes and scents. Lots of animals, including swallowtails, defend themselves with chemicals that taste and/or smell bad.

Omura et. al took on this question from an ecological chemistry perspective. They analyzed the osmeterial secretions from 6 swallowtail species from the genus Papilio…or from a closely related genus. They also went a step further, and analyzed the osmeterial secretions from species in different life stages. They found results I thought were interesting.

1.) Osmeterial secretions vary within the lifespan of the caterpillar. Most swallowtails mimic bird droppings when small larvae, and grow up to mimic snakes. The changeover typically happens at the last moult; fourth instar larvae mimic poop, while fifth instars mimic snakes.

The fourth instar secretions are more complex, and mainly consist of terpenoids…chemicals found in a wide variety of fruits and spices. Limonene, for example, would explain the citrusy smell Thomas Hossie described. Fifth instar secretions, on the other hand are far simpler and consist of aliphatic acids…which are similar to the compounds which give rancid butter their smell.

2.) In addition to being different at different life stages, secretions can vary widely between species. Of the 6 species, the only component the fourth instar larvae had in common was a chemical called myrcene, a major component in thyme leaves. In one species, P. demoleus, myrcene comprised .2% by weight of the osmeterial secretion. In another species, P. macilentus, the same component made up about 25% of the secretion. P. demoleus secreted only 7 components, which was by far the simplest mixture. P. paris, on the other hand, secreted a mixture of 24 different components. Between all 6 species, a total of 41 compounds were detected.

The 5th instar larvae, on the other hand, had only 7 components detected between them. These 7 components varied widely as well. Methyl isobutyrate, which smells like rancid fat, varied from 4.6% of the mixture in P. polytes to 76% of the mixture in P. macilentus.

What’s the bottom line?

Unfortunately, the authors of these papers didn’t describe the odors of the insects…which admittedly makes it difficult to connect the chemistry to a specific sensory experience. However, based on anecdotal and chemical evidence, it appears that the osmeterial odor changes both between species and within the life stages of swallowtail butterfly caterpillars.

References

1.) CHATTOPADHYAY J. (2011). The structure and defensive efficacy of glandular secretion of
the larval osmeterium in Graphium agamemnon agamemnon
Linnaeus, 1758 (Lepidoptera: Papilionidae), Turk J Zool, 35 (2) 245-254. DOI:

2.) Ômura H. & Paul Feeny (2006). From Terpenoids to Aliphatic Acids: Further Evidence for Late-Instar Switch in Osmeterial Defense as a Characteristic Trait of Swallowtail Butterflies in the Tribe Papilionini, Journal of Chemical Ecology, 32 (9) 1999-2012. DOI: http://dx.doi.org/10.1007/s10886-006-9124-x

3.) Vegliante F. (2012). Morphology and Diversity of Exocrine Glands in Lepidopteran Larvae, Annual Review of Entomology, 57 (1) 187-204. DOI: http://dx.doi.org/10.1146/annurev-ento-120710-100646