Exploration of Neuronal Responses to Auditory Stimuli in Dragonflies

To date there is no scientific evidence that dragonflies (Odonata), have a nervous system equipped to process auditory stimuli. Even with considerable research on these creatures due to their specialized vision and flight mechanics, there is no evidence that dragonflies have ears or even auditory neurons. Last year student Andrew Hamlin and Professor Robert Olberg recorded neuronal responses in the dragonfly to auditory stimuli of 100-2000Hz sounds (Olberg and Hamlin, unpublished). This year our research was aimed at understanding a sensory modality that was previously unknown in dragonflies, the sense of hearing.  In order to investigate this question we used behavioral and electrophysiological studies on the live dragonflies Anax junius and Aeshna constricta. Behaviorally, dragonflies were loosely tethered to a standing mount allowing free movement while computer-generated sound stimuli were played to the animal and video-recorded. Electrophysiological studies were done by extracellular recording of the ventral nerve cord to observe neuronal activity in response to these computer-generated frequencies (50Hz – 22KHz). Due to the electrical and mechanical properties of a speaker, low range frequencies (100-200Hz) were used to conduct sound waves that directly contacted the dragonfly (near-field sound) while staying out of the electrical field of the speaker itself. This inhibited the electrical field of the speaker from being picked up in the extracellular recordings. We observed body movements to near-field sound waves in behavioral studies that backed up the initial observations.  Our electrophysiological studies showed that sound waves do not stimulate an auditory sense through a tympanum but apparently stimulate mechanoreceptors on the body. This mechanoreception is subtle and highly dependent on the quality of the recording but does exist. Our findings suggest that the behavioral responses of dragonflies to loud sounds may be mediated by mechanoreceptors, such as sensory hairs, distributed across the body of the dragonfly.