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Giant Swallowtail: An ugly duckling tale

One of the world's most popular fairy tales is The Ugly Duckling by Hans Christian Andersen. That tale involved a homely bird chick that grew up to be a beautiful swan. The animal highlighted in this post is the lepidopteran counterpart to Andersen's swan.

A spindly little Wafer-ash, Ptelea trifoliata, springs from the ground, its interesting leaves marred by an unsightly bird dropping. But wait! We better look again...

That's no bird dropping - it's a caterpillar! We're meeting one of our strangest butterfly larvae, that of the Giant Swallowtail, Papilio cresphontes. Scores of insects - and some spiders - mimic the appearance of fresh bird poop. Looking like scat can be a good thing, because few predators like to eat bird droppings. So laying on top of a leaf, looking like the fresh aftermath of a Blue Jay's meal, can help an organism to hide in plain sight. Bird dropping-stained leaves are everywhere, and mirroring the look of fecally tarnished foliage is a great way to outwit your enemies.

No one does the bird dropping masquerade better than the caterpillar of the Giant Swallowtail. If there was an Oscar for Best Actor in Fecal Disguise, this would be our winner. These cats are big ones, too, at least when the caterpillar is in its last instar, or growth stage, as is the one pictured in these photos. I was fortunate enough to encounter this Giant Swallowtail caterpillar recently in Adams County, Ohio.

Looking like a fresh, wet bird dropping isn't the only trick that this cat has up its tubular sleeve. If startled by a would-be predator such as a songbird, or possibly parasitoid flies or wasps, the caterpillar flicks out a chemical switchblade. You can often get the caterpillar to pull its weapon by gently tapping it on the back. In this photo, just after a light tapping, the caterpillar has reared its head in the direction of the offending tapper, and is exerting strange red horns called osmeteria. All swallowtail species caterpillars are armed with this piece of equipment.

In the blink of an eye, the besieged caterpillar protrudes its osmeteria to a remarkable length - quite an unexpected and awe-inspiring sight indeed! I would imagine, were you the size of a chickadee or perhaps a menacing insect, these horns being abruptly thrust into your face might just be enough motivation to go elsewhere.

If the physical appearance of these long horns isn't enough, the chemical secretions that they are coated in might do the job. To me, the osmeterial secretions smell rather foul. I was about three or four feet from the caterpillar when I made the photo above, and in no time my olfactory senses were assailed by a distinctly unpleasant odor. I can't imagine that the osmeteria and its associated chemicals hold any charm for birds, spiders, mantids or any other creatures that might threaten the larva.

If all goes well for the rather repulsive caterpillar, this will be the end result, a stunning Giant Swallowtail butterfly. This is the largest species of butterfly in Ohio, and the appearance of one will almost always elicit a positive reaction. The same type of reaction will generally not be offered for the caterpillar, unless the viewer is someone like me who likes and appreciates bizarre larvae.

Giant Swallowtail caterpillars feed on plants in the Citrus family (Rutaceae). In Ohio, that's only two species, the aforementioned Wafer-ash, and Prickly-ash, Zanthoxylum americanum. Neither are true ash, but are related to the orange. Some savvy nurseries that specialize in native plants offer at least the Wafer-ash (the Prickly-ash, true to its name, is quite thorny and less likely to be sold). Try planting some Wafer-ash on your property, and perhaps you can also grow these fantastic beasts.

Following are some Ohio nurseries that sell Wafer-ash:

Keystone Flora

Naturally Native Nursery (extra points for selling Prickly-ash!)

Scioto Gardens


zippiknits said…
We sometimes see them on our citrus trees (3) in season. They are spectacular butterflies!

Thank you for so many great pictures of the caterpillar.

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