The whine of a mosquito in one's ear is one of the most annoying and unwelcome of sounds, yet to a male mosquito, this high-pitched buzzing is music to his ears, or at least his antennae. Joseph Jackson and Daniel Robert at the University of Bristol report in a recent paper some of the acoustic mechanisms that male mosquitoes use to detect nearby females and keep them in range during a pursuit.
Most insects use their antennae to detect sound, and mosquitoes are particularly well-endowed in this area. Sounds cause insect antennae to vibrate, and these vibrations are detected by a large cluster of sensory neurons at the base of the antennae, called Johnston's organ. In a male mosquito, each organ is made up of 16 000 sensory neurons, which is as many as are in the human cochlea. This is quite remarkable considering that humans are about 100 million times bigger than mosquitoes.
This fact is perhaps less surprising when one considers that a male mosquito's raison d'être is to listen for a female, track the sound of her buzzing and mate with her. Also significant is the fact that female mosquitoes are inefficient sound emitters due to their small size relative to the wavelength of the sound they produce. The physics underlying this effect is the same as that which makes a large subwoofer loudspeaker better at pumping out bass notes than a small tweeter speaker ever could.
Previous work by the authors demonstrated that mosquitoes make use of some of the same tricks that vertebrates use to filter and amplify sounds. Scientists previously believed that mosquito antennae vibrated passively in response to sound, however Jackson and Robert showed that mosquitoes actively adjust how much their antennae vibrate. In their latest paper, they attempted to put some of these mechanisms in their ecological context by measuring the response of male antennae to the sound of a passing female. To do this, they presented tethered males with sounds that replicated a female making a linear flyby. As a male listened to this louder and then softer sound signal, they measured his antennae's vibrations using a microscanning laser Doppler vibrometer.
The investigators found that the vibrations of the male antennae didn't vary linearly with the strength of the female signal, which is what you would predict if the antennae vibrated only passively with sound. Specifically, they found that there were two moments during a female flyby when the antennae deflected more than you would predict from the sound input alone, which suggests a process of active mechanical amplification. This phenomenon occurred during approaches when the female signal appeared to be about 2 cm away and getting louder, and again when the female appeared to be about 2.4 cm away and getting softer. The authors point out that the distance at which these effects dominate are far closer than the detection threshold, which is about 10 cm. They suggest that amplifying the signal at this close range may function as an `autofocus auditory telescope' that allows males to lock onto a nearby female's acoustic signal and track her before she gets away.
While it is clear that the mechanical amplification of sound signals by mosquito antennae is an active process (dead mosquitoes don't amplify), it is not known exactly how this is achieved. Because the vertebrate ear is also capable of active amplification, studying mosquito hearing might ultimately help us understand how you detect that faint whine somewhere in your bedroom just after you get into bed, pull up the covers and turn out the light.