1. Caterpillars of the cabbage mothBarathra brassicae react to low frequency sound stimuli with defensive reactions (stopping, squirming, dropping). 2. In the sound field of a standing wave in a Kundt's tube the caterpillars do not react at places of maximal sound pressure, but only where sound-produced air motion is maximal. It is shown in controls that these reactions are not released by vibration of the supporting platform. 3. The threshold curve of sensitivity to sound is measured in the Kundt's tube for 300 Hz ≦f≦1 000 Hz and in a different setup for lower frequencies. Caterpillars react to sound between 40≦f≦1 000 Hz; threshold is lowest at 100 Hz≦f≦ 600 Hz, where a constant displacement amplitude of 1.5 to 2·10−4cm is sufficient to release defensive reactions to sound. Displacement is therefore regarded to be the effective stimulus parameter. 4. 8 filiform hairs standing dorso-laterally on the thoracal segments are responsible for sound sensitivity. Their length is 508∓26 μm, their basal width 5∓1 μm; each of them is innervated by a single receptor cell. Sound reception thresholds are normal as long as 4 of these hairs are intact; with 2 intact hairs, the threshold is increased threefold and with only 1, sixfold. 5. The conditions of sound—or rather medium vibration—reception in the far-field and in the near-field of sound sources are discussed with reference to possible natural sources of vibrational stimulation for the caterpillars.
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