Larvae of C. borealis Boisduval respond to the low audio frequencies of 330 to 360 and 175 to 180 Hz. The response consists of throwing the two caudal tentacles anteriorly while extending eversible flagella from the two tentacles. The flagella bear what appear to be scent storage and releasing structures which are described herein. The defensive mechanism may function to repel Cotesia sp. (Hymenoptera: Braconidae) parasitoids. Integument; Integumentary glands; Sensory reception; Behaviour; Agonistic behaviour; Communication; Acoustic signals; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Cerura borealis (Notodontidae); Repugnatorial glands; Eversible larval flagella, probable hymenopteran parasite repellent; Sound reception; Low frequency similar to hymenopteran parasite, larval response; Instinctive behaviour; Larval posture & flagella eversion in response to low frequency sound; Defensive behaviour; Response to low frequency sound, analysis & function, larvae; Hymenopteran parasites; Cotesia; Parasite sound production, host defensive response Cotesia (Braconidae); Sound characteristics; Low frequency flight sound, response of lepidopteran host larvae; Lepidopteran hosts; Cerura borealis; Host larval response to parasite flight sound