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Eastern Black Swallowtail Butterfly Caterpillar

Papilio polyxenes


White caterpillar with yellow-and-black markings feeding on fennel. My daughter discovered that when you squeeze it, what look like yellowish-orange horns emerge from its head. She has a good sense of smell and says they emit a scent when they emerge. We found out these are a defense mechanism specific to Swallowtail larvae, and they are called "osmetrium." According to Wikipedia, "This organ emits smelly compounds believed to be pheromones."


A fennel plant in our yard.

1 Species ID Suggestions

Eastern Black Swallowtail
Papilio polyxenes Black Swallowtail

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InannaBintAli 9 years ago

Thought about it some more and went and looked at the caterpillar food source for these two types of Swallowtails. florida33girl is right. Black Swallowtail butterfly caterpillars would feed on fennel. The Tiger Swallowtail caterpillars wouldn't. Thanks!

InannaBintAli 9 years ago

It could be an Eastern Black or an Eastern Tiger Swallowtail. We have both butterflies in our yard, and I'm not sure which one this caterpillar turned out to be.

alicelongmartin 9 years ago

The fennel plant looks pretty bare, time for him togo on to his next stage.

Spotted by

Kansas City, Missouri, USA

Spotted on Jul 1, 2010
Submitted on Jul 10, 2011

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