The familiar buzzing sound made by a mosquito may be irritating to us humans, but it is an important mating signal. The sound, produced by the beats of the insect's wings, has a characteristic frequency called the "flight tone"; when produced by a female, it signals her presence to nearby males, thereby attracting potential mates.
Attraction is not simply a matter of the male hearing and homing in on the female's flight tone. Females were long believed to be deaf, but two years ago, it was found that courting pairs interact acoustically to create a duet: both male and female modulate the frequency of their wing beats until their flight tones are closely matched, and this process is fundamental to sexual recognition.
In today's issue of Science, researchers from Cornell University report that the hearing organ of one mosquito species is far more sensitive than was previously thought, and that the courtship duet of this species has a frequency to which the insects were believed to be deaf. Another surprising finding they made about the insects' mating behaviour may also provide a means of controlling mosquito populations.
Lauren Cator of Cornell's Department of Entomology and her colleagues performed their experiments on Aedes aegypti, the species of mosquito which carries and transmits the viruses causing dengue and yellow fever. In a set of behavioural experiments, they tethered individual mosquitoes to the ends of insect pins, so that, when suspended in mid-air, they flapped their wings as if in flight. One tethered mosquito was then moved past another stationary tethered one, and the flight tones produced by both was recorded with a microphone.
The researchers then moved tethered females in and out of the 2cm-hearing range of stationary males, in "fly-bys" that lasted 10 seconds. The recordings revealed that in 14 out of 21 pairs (67%) both the male and female modulated their flight tones so that they matched. Previously, the mosquito's hearing organ was believed to sensitive to frequencies in the range of 100-500 Hertz (Hz, cycles per second), but, to their surprise, the researchers found that the frequency of the harmonized flight tone was 1,200 Hz.
Electronically-generated pure tones of frequencies in the range of 100-2,000 Hz were then played, through earbud speakers placed 1.5 cm from the tethered mosquitoes. In these experiments, both males and females quickly altered their flight tones to match those of the played tones. These findings were confirmed by inserting microelectrodes into the mosquitoes' hearing organs, so that the responses of the cells to the electronic pure tones could be measured. Cells in the hearing organs of both males and females were found to respond to electronic tones of frequencies of up to 2,000 Hz.
During the behavioural experiments, unmated females behaved similarly in response to the electronic tones. However, females which had just mated were largely unresponsive to the tones, or to recorded male songs that were played back to them, suggesting that mating somehow decreases their sensitivity to the flight tones of potential mates. This provides a rationale for controlling the number of this mosquito species, and thus reducing the incidence of dengue and yellow fevers, which together affect some 100 million people annually. Sterile males released into the wild may compete for females, making them less sensitive to other potential mates and so reducing their potential to reproduce.
Cator, L. J. et al (2009). Harmonic Convergence in the Love Songs of the Dengue Vector Mosquito. Science. DOI: 10.1126/science.1166541.
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