How do insects hear?

How do insects sense sound? We hear from a scientist using this information to design the world's smallest microphone...
09 September 2014

Interview with 

Daniel Robert, University of Bristol


African Field cricket Gryllus bimaculatus


Humans and other mammals collect sounds using their eardrums which vibratefield crickets and these vibrations are transferred through 3 tiny bones called ossicles to a structure in the inner ear which is called the cochlea and this converts sound waves quite literally into brainwaves.  But it turns out that insects might be able to teach us a thing or two about sensing sounds and even show us how to build super small microphones and hearing aids.  This is the brainchild of Professor Daniel Robert who's a researcher at the University of Bristol.

Daniel -   Well, insects hear through tiny little organs, tiny little ears that also have eardrums that are structured a bit differently.  What's interesting about insects is their diversity because we can find ears virtually everywhere on the body - from the wings of some butterflies, the chest of some flies, and the legs even of some crickets.

Chris -   When you say you can find little ears there, if you look at those hearing organs, how do they differ from my ears for example?

Daniel -   First of all, they're much smaller. The Cochlea you have is roughly a centimetre across.  These ears are down to half a millimetre or even smaller sometimes.  Now, they're also structured very differently.  Even though they do the same job for those of the crickets for instance, their ears are structured in a way that we couldn't imagine before.  

Chris -   Now, we talked about these three tiny bones, the ossicles - hammer, anvil and the stirrup in a human.  Do insects have tiny bones as well?

Daniel -   For us, in order to convert the acoustic wave that is in the air and push that energy into the cochlea, we require that amplification that the middle ear bones are generating, some sort of a clever 3-part leverage system which is quite complicated.  In insects, we don't find that.  In these particular crickets, what we find is a simple lever system that does the centrifugation but in much, much simpler way anatomically.  And that's kind of an exception and we were pretty excited to find out because that gives us a particular solution to the same problem of amplification, but with a much simpler system.

Chris -   When you say a lever system, this would be a bit like jacking up a car.  Basically, you put one thing under the car wheel and then have a very long handle, and you're basically making the job much easier to do by having a very long handle.  The insects are turning these low-energy soundwaves into vibrations in their hearing organ by having this similar sort of lever system.

Daniel -   Exactly.  That's a good description because with a very small force at one end of the lever, you generate very large force at the other end.  But the large force has a long displacement if you want and the other side has a short displacement.  But no matter what, your car is going to be lifted.  The same principle applies to shifting these vibrations into the inner ear of the insect.  That works for an awful lot of frequencies which is the beauty about it.  You can have lower frequencies and higher frequencies.  So there, we learn a lot about how the systems having evolved for 200 million years can be very efficient at capturing sound in way that we've never imagined before.

Chris -   So, your goal would be to take the structure of what a grasshopper uses to pick up soundwaves and convert them into nerve signals and copy that, so that you could instead have for instance, a tiny microphone.

Daniel -   Exactly.  We want to be inspired by the process that evolution has come up with, and maybe more excitingly even, is to borrow one process from a grasshopper, another process from a fly, another from a butterfly, and makes some of a chimera - a mix between these different processes that are good at different things - detecting the direction of sound, detecting a nice amplitude, being very sensitive, or as we said before, being able to push that to some form of cochlea to draw an analysis of the different pitch of the sound.  So with that, we can generate tiny microphones.  We hope that would emulate what we've learned from the tiny ears of insects.

Chris -   Why do you think insects have evolved to use this system and we evolved another system that what you're saying is, turns out to be less good than what the insects have got?

Daniel -   That's a very difficult question to answer.  What we know about insects is that they have under their skin, just about everywhere a mechanoreceptor.  That means little cells, little neurons that are sensitive to tiny vibrations.  So, what insects have as an advantage, having the skeleton outside as opposed to us is that it can just thinner a little bit of their skin over here, make a little lever over there, they're very modular and they're very flexible in their anatomy - one other diversity.  So as a result, what we observe today is a series of different solutions to the same kind of - if you want - engineering problem which is the detection of sound.  So, our ears are deep-down into our skull and they are embedded in vast amounts of fluid.  For insects, this is not the case.  They're much smaller and much lighter which allows them to be much more sensitive and accurate.

Chris -   But given how tiny they are, this means that they will respond very well to high frequency sounds, I would presume.  But does that mean that were you to make a microphone, modelling or based on the insect structure, it would have a problem detecting lower frequency, bigger wavelength soundwaves?

Daniel -   Insects are very happy to listen to high frequencies, up to 150 kilohertz, reminding you that we stop at 15 or 20 kilohertz as humans.  So, insects are very good - particularly crickets - at measuring the sounds that come from bats.  So what we do there is extract the knowledge if you want and then push this data into a mathematical model in a computer.  And then we start another process of optimisation where we ask basically that system we have by modifying ever so slightly some bits of the system like the eardrum to respond to lower frequencies.  We're trying to adapt that to lower frequencies to make this compatible with human voice, human hearing.

Chris -   How do you build a plastic model of an insect ear?

Daniel -   Well, for this, we use new tools that have been available now for a little while and that come from nanotechnology.  Some very fancy 3D printers that can print very tiny structures.  So, we used that, scaled it up a little bit and build it on little organs that are half a millimetre or a millimetre across that emulate the shape and form of these insects' ears.  And we do the same test again as we did with the true insect.

Chris -   And if you are successful and you can produce these miniature microphones, what application will you have for them and why will they be better than what we currently have?

Daniel -   Our hope here is to be able to borrow the best of different species, make a microphone that will work a little bit like a cricket ear and then incorporate what flies are doing with sound for directionality.  And then at the end, perhaps have a microphone or two, or three, or perhaps 10 in one package, and put it in a hearing aid to be able to do what we call auditory scene analysis.  A microphone that would be clever enough, intelligent enough in the biomimetic sense to detect sounds that perhaps normal microphones with just one membrane are not able to detect.  So, we're getting there.  We're not quite there yet, but we think that we have here a solution inspired by nature that could be useful for auditory processes indeed.


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