Hearing is an active process in mosquitoes

Abstract

Auditory sensitivity and frequency selectivity are enhanced by active mechanics. Known from vertebrates, active audition is established for an invertebrate. The auditory mechanics of antennal sound receivers in mosquitoes exhibits key diagnostic features of active mechanics: (a) physiologically vulnerable sensitivity and tuning. The mechanical sensitivity and tuning sharpness of the receiver decrease after death. (b) Vulnerable nonlinear mechanical response. Nonlinearity is expressed as amplitude-dependent damping: lower stimulus intensities result in larger response gain and sharper resonance peaks. This band-limited nonlinearity disappears post-mortem. (c) Hypoxia sensitivity. In females, transient exposure to CO2 reversibly reduces tuning sharpness. In males, the effect is opposite: CO2 sharpens tuning and elicits large vibrations in the absence of acoustic stimulation. Hypoxia-induced vibrations are unrelated to external forces; they are autonomous and self-sustained, and forcibly generated by an internal motor. (d) Autonomous antennal vibration (AV). AV is also induced by the injection of dimethylsufoxide (DMSO). DMSO-induced AV persists (90 min) at large amplitudes (>400 nm). Antennal deflection shapes rule out the involvement of muscles, narrowing down the source of mechanical activity to the auditory sense organ at the antennas base. [Work supported by the Leopoldina Academic Society, the Swiss National Science Foundation, BBSRC and the University of Bristol.]