The ears of Manduca sexta caterpillars

An investigation is described of the hearing mechanism of Manduca sexta caterpillars. We examine whether their hearing is due either to sound-induced motion of the supporting substrate through their feet or through the detection of airborne sound. Behavioral response thresholds were determined due to either base vibration or airborne sound at pure tone frequencies of 150 and 2000 Hz. From the previous studies, the 150-Hz frequency sound triggered the strongest behavioral response for sound levels in the range of 80–90 dB SPL. It was found that 2000 Hz corresponds to the resonance frequency of thoracic hairs using laser vibrometry. Hair motion was measured relative to air particle velocity across 100 Hz to 20 kHz. It was found that the caterpillars’ detection threshold to sound-induced substrate vibration was 1–2 orders of magnitude (accelerometer amplitude) above the caterpillars’ behavioral response threshold to airborne sound stimuli at both frequencies. Since the hairs of other caterpillars are known to enable them to hear, we performed targeted ablation of thoracic and abdominal hairs and compared their behavioral responses to sound before and after ablation. Our results suggest these specific mechanosensory hairs are responsible for detecting airborne sounds at 150 and 2000 Hz and provide evidence of non-tympanal sound detection in caterpillars for these specific frequencies.