The auditory system of blood-sucking mosquito females (Diptera, Culicidae): Acoustic perception during flight simulation

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Abstract
Mosquitoes hear with their plumose antennae which respond to the air movement caused by sound propagation and conduct vibrations to the Johnston’s organ located at the base of each antenna. Each of the two Johnston’s organs contains several tens of thousands mechanosensory cells which detect the displacements of the flagellum and transform them into electric potentials. Hearing plays a very important role in the reproductive behavior of the male mosquitoes. At the same time, our knowledge of hearing in female mosquitoes is very limited and its functional significance is obscure. In this study we measured the auditory sensitivity of 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 with a glass microelectrode from the antennal nerve and the deutocerebral interneurons. Stimulation was applied in two modes: (1) the main stimulus against the background of flight simulation (strong vibration with the typical wingbeat frequency of a given mosquito species) and (2) only the main stimulus without the background stimulation. During the flight simulation, females demonstrated an increased sensitivity to frequencies below 200 Hz. The mean auditory receptor threshold at 80–120 Hz was 45 dB, which was 8 dB lower than that without flight simulation. An additional zone of increased sensitivity was also found at frequencies higher than the simulated wingbeat frequency (the 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 parameters, 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) detection of moving prey, and (3) intraspecific communication. Each of the hypotheses involves the ability to analyze the sound frequency spectrum and subsequent signal recognition.

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