[BUZZING] RONALD HOY: I have insects and insect communication in my history-- these days, one might even say in my DNA-- for the past 35 years.
BEN ARTHUR: What happens when sound hits this--
RONALD HOY: I suggested that acoustics is an important part of the mating behavior of many insects.
BEN ARTHUR: Or it could be that it's down here, another ear that we don't know about.
RONALD HOY: The question of mosquitoes I thought had been well-studied by this time. Everybody knows about mosquitoes. Their acoustics must be thoroughly understood.
It turns out that in talking to Laura Harrington, who works on mosquitoes, the acoustic story was by no means totally firm. And so we decided to collaborate on her species, Aedes aegypti, the dengue vector.
What the male and female do is to sing a duet, which is to say they adjust their flight tones to come together at a common tone. But the puzzling thing is they aren't doing it at the fundamental frequency, that is, the low flight tones. They're doing it at a much higher tone, up in the harmonics of the fundamental, at around 1,200.
That was a big surprise. That's the surprise because mosquitoes aren't supposed to be able to hear that high. How in the world are they dueting where they can't be hearing?
In order to investigate the physiology of the mosquito's ear, we enlisted my postdoc, Ben Arthur. Ben is a very talented neurophysiologist, and he turned out to be just the ideal person to make a physiological recording directly from the hearing organ or the Johnston's organ itself to see whether or not it responded to these frequencies.
BEN ARTHUR: So this then is the proboscis. That's what they use to bite you with on a female. Again, this is a male. We know it's a male because it's got really bushy antennae, these things right here.
There's the antennae. That's what they use to hear. And on males, they're really, really bushy so that they can hear the females really far away.
And this is the head right here. You can see its compound eye. And those two things which look like eyes to us, which are right at the base of the antennae, those are actually the base of its ears. And that's where we'll be recording neurons from today.
RONALD HOY: So this really opens up a whole new window on sensory hearing. And it may be solving a puzzle for mosquitologists for decades, and that's that these little critters, these tiny little critters, have something like 7,000 sensory cells in each auditory organ.
What are they doing with almost as many sensory auditory cells as we have? Maybe they have to be tuning themselves very finely in frequency, just as we do. And wouldn't that be an interesting convergence, mosquitoes and people?
BEN ARTHUR: This little box here amplifies the signal coming from those two electrodes. You can nicely see my voice here. But the way we actually study this methodically and objectively is to use a computer to play pure tones to it so we can map out exactly which frequencies it's sensitive to and at different intensities to see what the threshold, how loud the sound has to be, to get it to respond.
That was actually how we first discovered that we were doing things right. I was in here, and the noise wasn't nearly as good as this. I had a much poorer signal to noise ratio. But I had a little bit of a cold that day, and I coughed.
And it was uh-huh. And I saw that. I said, oh my gosh, this is working.
This bottom trace here is the tone that the computer is playing.
And the top trace is the sound that the mosquito is hearing. It's the voltage that's being recorded from the neurons.
And you can see how the response of the neuron on the top is linked to the playing of the stimulus at the bottom.
RONALD HOY: The next time you hear a mosquito buzzing around in your ears, you might be thinking about the fact that there's a lot more going on in this buzzing than just flying over to give you a nibble. In fact, that buzzing sound is a mating call for many mosquitoes.
And that mating call isn't a simple buzz. There's a lot going on, as far as a love duet between male and female. They are interacting on the fly and in song.
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Male mosquitoes have a wing beat frequency of about 600 Hz, females 400 Hz. Yet, when they mate, they are matching tones not at this fundamental frequency but at a harmonic near 1200 Hz. The problem is that the literature reports that male mosquitoes can hear only up to a few hundred Hz and females are completely deaf! Ron Hoy and his postdoctoral associate, Ben Arthur demonstrate how they determined that both male and female mosquitoes can detect tones up to 2000 Hz.
Ronald Hoy is the David and Dorothy Merksamer Professor in Biology and the Department of Neurobiology and Behavior.