AUDITORY SYSTEM OF BLOOD-SUCKING MOSQUITO FEMALES (DIPTERA, CULICIDAE): ACOUSTIC PERCEPTION DURING THE FLIGHT SIMULATION
Mosquitoes hear with their plumose antennae, which respond to the movement of the air particles during sound propagation and conduct vibrations to the Johnston's organs located at the base of each antenna. Each of the two Johnston's organs contains several thousand mechanosensory cells that detect the displacements of the flagellum and transduce them into the electric potentials. Hearing plays very important role in reproductive behavior of the male mosquitoes. At the same time, our knowledge of the hearing in female mosquitoes is very limited and its functional significance is unclear. In the current study we measured the auditory sensitivity of the female mosquitoes and investigated how the flight conditions affect their hearing. We studied mosquitoes of three species: Anopheles messeae, Aedes excrucians and Culex pipiens pipiens. The neuronal responses were recorded from antennal nerve and deutocerebral interneurons using a glass microelectrode. Stimulation was applied in two modes: 1) the main stimulus against the background of a strong vibration with the typical wingbeat frequency of a given mosquito species which simulated the flight and 2) only the main stimulus without the background stimulation. It was found that during the flight simulation females demonstrated the increased sensitivity to the frequencies below 140 Hz. Auditory receptor thresholds at 80-120 Hz were equal to 45 dB (on average) which is by 8 dB (2.5 times) lower compared to the stimulation without the flight simulation. Also we found the area of increased sensitivity at the frequencies higher than the simulated wingbeat frequency (so-called image channel). Our analysis of frequency tuning curves measured from the receptors and auditory interneurons shows that mosquito auditory neuronal complex consists of several subsystems which have different frequency tuning, and suggests the possibility of spectral analysis of sounds. Three hypotheses could be proposed on the function of hearing in female mosquitoes: 1) predator avoidance, 2) moving prey detection and 3) intraspecific communication. Each of the hypotheses involves the ability to analyze the frequency spectrum of a sound and subsequent signal recognition.