osmeterium

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os·me·te·ri·um

O0137650 (ŏz′mĭ-tîr′ē-əm)n. pl. os·me·te·ri·a (-tîr′ē-ə) An eversible organ behind the head of certain caterpillars, containing strong-smelling chemicals that deter predators.
[New Latin osmētērium, from Greek osmē, smell.]

osmeterium

(ˌɒzmɪˈtɪərɪəm) n, pl -ria (-rɪə) (Zoology) zoology a gland in some caterpillars that secretes foul-smelling substances to deter predators