Encoding of antennal position and velocity by the Johnstons organ in hawkmoths
Insect antennae function as versatile, multimodal sensory probes in diverse behavioural contexts. In addition to their primary role as olfactory organs, they serve essential mechanosensory functions across insects, including auditory perception, vestibular feedback, airflow detection, gravity sensing, and tactile sensation. These diverse functions are facilitated by the mechanosensory Johnstons organ (JO), located at the joint between the second antennal segment, known as the pedicel, and the flagellum. The pedicel-flagellum joint lacks muscles which means that the Johnstons organs can perceive only passive deflections of the flagellum. Earlier work which characterized the sensitivity and short response time of the sensory units of JO in hawkmoths, showed that their sensitivity to a broad frequency range is range-fractionated. This vastly expands the functional repertoire of the JO. However, it is not clear what components of antennal kinematics are encoded by the JO. Here, we conducted experiments to test the hypothesis that JO neurons encode the position and velocity of angular movements of the flagellum. We recorded intracellularly from the axons of primary sensory neurons of JO while stimulating it with ramp-and-hold stimuli in which antennal position or antennal angular velocity was maintained at various constant values. Our study shows that JO neurons encode angular velocity and position of the antenna in their response. We also characterized the neural adaptation of the responses to angular velocities and positions. A majority of neurons were sensitive to a movement in the ventrad direction, in the direction of gravity. The adaptation and the directional response properties give rise to a nonlinear hysteresis-like response. Together, these findings highlight the neurophysiological basis underlying the functional versatility of the JO.