THE OSMETERIUM: SPOTLIGHT ON CATERPILLAR ANATOMY
This week's Caterpillar of the Week, spicebush swallowtail (Papilio troilus), has an extra-special body part that some of you may have observed at our shows. It's a two-pronged structure called an osmeterium, which stays hidden under the caterpillar's skin until the caterpillar feels threatened. When the caterpillar is squeezed, poked, or prodded, out pops the osmeterium! It emerges from the caterpillar's thorax, just behind the head, and may mimic a forked tongue, helping the caterpillar look more like a snake. All swallowtail species have an osmeterium. While we don't have a picture of a spicebush swallowtail with its osmeterium out, here are two pictures of other swallowtails' osmeteria.
Besides potentially helping make swallowtails appear more intimidating to their predators, osmeteria also contain chemical defenses, which scientists have been studying for at least 50 years. When a swallowtail caterpillar extrudes its osmeterium, glands in the structure produce a liquid chemical cocktail, which the caterpillar rubs on its provoker--an ant, a spider... or a TCL staff member's finger! In the case of swallowtails' natural predators (such as ants, spiders, and mantids), these chemicals usually send the offender away in a hurry. I personally find the smell intoxicatingly delicious!
Studies have turned up some fascinating details about swallowtails' unique osmeterial secretions. There can be some variation in chemical recipes from species to species, and also between early-instar and final-instar caterpillars of the same species. Within a single species, caterpillars at the same instar will likely have the same ingredients in their secretions, but the amounts of each ingredient can vary widely. Even within an individual caterpillar, each of the two osmeterial prongs can secrete different recipes of the same chemicals!
The predator-deterring chemical ingredients are various types of acids. Surprisingly, these acids are usually not present in swallowtails' food plants! Rather, swallowtails' bodies synthesize them on their own, probably using chemicals that do occur in their food plants.
One sneaky predator appears to successfully evade swallowtails' osmeterial defense--the soldier bug. Soldier bugs pierce their prey with their needle-like proboscis, which they can use like a straw to slurp up caterpillars' insides. They are also deterred by swallowtails' chemical secretions. But interestingly, when black swallowtails were attacked by soldier bugs in a study, the swallowtails never everted their osmeteria! It turns out that soldier bugs may be using their own chemical warfare, in the form of anesthetic chemicals, which may numb the swallowtails' bodies to their bite!
We're excited to tell you more about other anatomical features of caterpillars in future posts! Are you interested? What would you like to learn more about? Let us know in the comments!
TCL Education Director