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Moths of the genus Celerio, hovering and feeding from blossoms, have been observed to react vigorously to high-pitched sounds. The acoustic receptor appears to involve the labial palps. Its characteristics have been determined from responses made, when the moth is exposed to artificial sound pulses, by an interneuron originating in its head. These responses have been found only in 4 out of 19 sphingid species tested.

Evidence is presented for a novel mechanism mediating the auditory sense in the Choerocampinae (Lepidoptera: Sphingidae). Vibrations in the scale-free medial wall of the enlarged labial palp segment are transmitted by contact to the distal lobe of the pilifer, a derivative of the labrum, and are there transduced into afferent impulses in the labral nerve.