Auditory sensory cells in hawkmoths: identification, physiology and structure

The labral pilifers are thought to contain auditory sensory cells in hawkmoths of two distantly related subtribes, the Choerocampina and the Acherontiina. We identified and analysed these cells using neurophysiological and neuroanatomical techniques. In the death's head hawkmoth Acherontia atropos, we found that the labral nerve carries the auditory afferent responses of a single auditory unit. This unit responds to ultrasonic stimulation with minimum thresholds of 49-57 dB SPL around 25 kHz. Ablation experiments and analyses of the neuronal activity in different regions of the pilifer revealed that the auditory afferent response originates in the basal pilifer region. The sensory organ was identified as a chordotonal organ that attaches to the base of the pilifer. This organ is the only sensory structure in the basal pilifer region and consists of a single mononematic scolopidium and a single sensory cell. In Choerocampina, a homologous scolopidium was also found and is probably the only sensory structure of the pilifer that might serve an auditory function. Since a pilifer chordotonal organ with only a single scolopidium has also been detected in a non-hearing hawkmoth species, hearing in the distantly related choerocampine and acherontiine hawkmoths presumably evolved from a single proprioceptive mechanoreceptor cell that is present in all hawkmoths.