CAN YOU SAY OSMETERIUM?

By Arthur V. Evans

This summer a cadre of dedicated parents and volunteers joined forces at a nearby elementary school to create an outdoor classroom. The Holton Learning Project Garden includes a vegetable and butterfly garden that will introduce Holton Elementary School students, their families, and the residents of Belleview and beyond to the pleasures and benefits of urban gardening.

Compared to the dreary, sterile plantings of exotic trees, shrubs, and groundcovers found throughout much of the neighborhood, the vegetable and nascent butterfly garden has rapidly become a local hot spot for insects and spiders. As such, it provides an excellent site for macro photgraphy. Since August, I have endeavored to photograph as many of its multi-legged denizens as possible as part of an ongoing effort to document the arthropod diversity of my neighborhood in Richmond, Virginia.

While walking through the garden yesterday afternoon, I noticed several clumps of green spikes rising sadly from the straw-covered beds. I soon confirmed my initial suspicions as to the identity of the culprits that laid these once fat bunches of parsley to waste. At the very base of one of the clumps were two brightly banded larvae of the black swallowtail, Papilio polyxenes, polishing off the last few leaves.

When I knelt down to photograph the ravenous caterpillars, I accidentally brushed up against their food plant. Both caterpillars reacted immediately by assuming defensive postures. Bent over backwards, they spit up green fluid and produced a pair of long tentacles (osmeterium), that resembled bright orange horns. Soon my nostrils were filled with a strong, disagreeable odor that is best described as “spicy vomit.”

The osmeterium consists of two soft, finger-like tubes that are everted from inside the body through a slit in the prothorax just behind the head as a result of  increased blood pressure. This defensive gland is found in the caterpillars of swallowtail butterflies and is coated with highly noxious chemical compounds (2-methylbutyric acid and isobutyric acid) that deter predators, especially ants.

© 2010, A.V. Evans

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8 Responses to “CAN YOU SAY OSMETERIUM?”

  1. Thanks for this post. I briefly saw a black swallowtail caterpillar’s osmeterium recently, but had no idea about what they were or their purpose. Now I know. I didn’t notice any odor, but maybe I just wasn’t close enough.

  2. A quick whiff is worth the experience!

  3. I had a similar experience with monarch caterpillars stripping some flowering Asclepias curassavica plants two weeks ago. No osmetria on these babies, but they sure devastated the plants. As you know, Art, the black-and-yellow larvae sequester cardiac glycosides from milkweeds and the poison is passed on to the adullts, through the pupal stage. We are also seeing migrating monarchs coming through now.

  4. Thanks for clearing this up for me, Doctor! I found several dozen of these little guys in my garden this morning. Glad to know what I’m dealing with, though I have not experienced the spicy vomit secretion. I gave you a shout out in my urban farming tumblr: http://gardanticsrva.tumblr.com/
    Be advised that my blog is notably less scientific than yours.

  5. I was searching the web to identify the caterpillars i found this morning, and when i read they were found in the parsley, I knew that’s what they were. The past few days they have been in my parsley, but leaving my other spices alone. Any suggestions on how to keep them at bay, like a natural deterrent preferably. thank you.

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