Hearing and acoustic interaction in mosquitoes

Johnston, who discovered the mosquito auditory organ at the base of the antennae 150 years ago, speculated that audition was involved in mating behaviour. Indeed, the Johnston’s organ (JO) is now known to detect the whine of flying mosquitoes. Analysis of sound recordings of flight tones from tethered, flying, mosquitoes revealed that opposite-sex pairs, when within their acoustic near-fields, attempt to frequency-match the harmonic components of their flighttones. Same-sex pairs actively avoid frequency-matching. Mosquitoes of the species Toxorhynchites brevipalpis, where the flight-tone frequencies of males and females are similar, attempt to match the fundamental frequency of their flight-tones. Haemophilic, vector-carrying mosquitoes Culex quinquefasciatus and Anopheles gambiae ss, where the fundamental frequency of the male flight tone is about 1.5 times that of the female, frequency-match harmonic components of their flight tones. Usually the male’s 1st harmonic with the 2nd harmonic of the female flight-tone. In Burkina Faso, where two morphologically similar molecular forms aggregate in the same swarms but rarely hybridise, frequency-matching of flight-tones may perform a pre-mating barrier and a form of subspecies recognition. We discovered that frequency-matching occurred significantly more frequently between same-form male-female pairs of flying, tethered mosquitoes, than when each member of the pair was of a different molecular form. The bandwidth and tuning of sound-evoked flagellum vibrations and the JO’s electrical responses to this mechanical input were measured using laser interferometry and extracellular electrodes, respectively. For the first time we showed that distortion products, recorded from the flagellum and JO, could provide the neural basis for frequency-matching at frequencies beyond the range of the JO’s electrical responses. We also discovered that spontaneous oscillations of the antennae are produced by physiologically-sensitive mosquitoes. Through temperature-control and injection of pharmacological agents into the JO, evidence is presented advocating dynein as the molecular motor responsible for powering these oscillations.