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Osmeterium

By Debbie Hadley, About.com

Photo: Entomolo, Wikimedia Commons (CC SA license)
Definition:

A fleshy, Y-shaped gland found in some caterpillars, especially in those in the swallowtail butterfly family. The osmeterium emits a chemical with a foul scent when the caterpillar is threatened.

Common Misspellings: osmoterium, osmetarium

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