Ronald R. Hoy
Parasitoid tachinid flies and crickets
NPR/NGS Radio Expeditions
2 Jun 1999
New YorkTompkins County
- Ithaca; Cornell University; Seeley G. Mudd Hall
- 42.44722 -76.47917
Stereo=1; Split track; Neumann KMR 81
Show: Fly¿s Ear
Log of DAT #: 1
ng = not good
ok = okay
g = good
vg = very good
Ron, just say who you are and who you are in relation to this story. What do you do in the story of the fly¿s ear?
Okay. My name is Ron Hoy. I¿m a professor in the section of neurobiology and behavior here at Cornell. My professional specialty is bioacoustics and I¿ve been studying insect sounds and how they hear for over 25 years and one of the most interesting stories that I¿ve been lucky enough to come upon is the story of the cricket and the fly.
It almost sounds like a parable, the cricket and the fly. Tell me, from the fly¿s perspective, what is the problem that it¿s got to solve in its relation to the cricket.
Okay, well, this is a very, this is an unusual, this is an unusual fly in the sense that it¿s a parasitic fly, and as I think many people know about parasites, there¿s a strong linkage between a parasite and its host. That is, there¿s, any given parasite has to find a specific host in which it either goes after or raises its young. In this case of this particular fly, which is called ormea ocracia?, which I¿ll refer to as the ormea fly, and which inhabits the southern parts of the United States but has in fact gone cross the waters. It¿s now found in Hawaii as well. This fly must find a particular kind of cricket in which to raise its young and so the problem for any parasite is to find its host and many parasites use their eyes, so there¿s visual spotting of the host, others smell out their hosts, they sniff them out, but if you¿re a fly having to find a cricket, perhaps the most obvious and conspicuous signal that a cricket gives off is its song. Now a cricket sings, not for the benefit of a fly, but a cricket sings. And by the way, only male crickets sing. A cricket sings in order to attract females for mating. And what the fly has done is break in on this communication system, so this means that the fly has to, in order to find a cricket by song, it has to hear. Now for flies, this becomes a particular problem, because while some flies do produce sounds and can hear these sounds, it is the case that just about all flies that have been documented to have sounds are in the low range. They¿re sort of low down in the sort of the humming range, in the hundreds of cycles per second, sort of like, little bit higher than radio, than a¿.cut. (7:11- laughing, joking). 7:24- The problem for flies is that very few have been known to hear the high frequencies that you find in a cricket¿s song. In fact, no fly that we¿d ever heard of at all. It¿s just not known. So it turns out that what this fly is done, presumably, is had to invent, in an evolutionary sense, a cricket-like ear.
In order to find these crickets that are singing.
Exactly. In order to find the cricket. And so it had to solve an evolutionary problem, because while we know about, while we know about, about fly sounds, for example, a mosquito, everybody knows about the ¿bzzzzzzzzzz¿ of a mosquito. It turns out that those frequencies are only about 400 or 600 hertz. So they¿re little frequencies, whereas a cricket¿s song is about 5000 hertz. So it¿s 10 times higher. It requires a different kind of ear. And the kind of ear that an insect needs to hear a cricket song is not associated with any known fly.
Well, how is it that this fly did it?
This is an evolutionary innovation. You threw me a curveball, because actually, the evolutionary story is a whole different thing. So, should I just go ahead and do that?
Okay. It turns out that this, that, the fly was able to modify some of the, was able to modify some membranes on its thorax, or if you want to think of,.most people don¿t know what thorax is, so think of it as the fly¿s chest right behind its head, to thin it out to serve as a pair of membranes, eardrums, as it were. And that was part of the innovation, the other thing was that it had to sense up a sensory organ to this thin membrane. Now in your ordinary fly, for example, a horsefly or a housefly, the precursors for these structures are there, but they¿re much too thick in a housefly, for example, to serve as a membrane, as a eardrum and the sensory organs contain for too few sensory nerves and they¿re just not, they¿re not sensitive to the small mechanical vibrations which constitute sound. So what the ormea fly did was to modify them in evolutionary time and to perfect this system as a hearing system so now it has extraordinarily thin membranes for an eardrum and it has a very elaborate sensory organ to transduce those vibrations, sound vibrations into nerve signals.
Does the housefly hear anything?
As far as we know, the housefly is not endowed with a sense of hearing.
So this fly, this ormea fly, is really different from other flies.
Well, it¿s, yes it is. In that it can hear the high frequencies of cricket sound. Mosquitoes are a form of fly and they do hear the sounds of prospective mates. That humming which is so annoying to us is actually, a buzzing sound can be heard by other mosquitoes.
So this fly has managed to develop an ear, but this not something that was widely known. I mean, this is a fairly recent discovery that this fly can hear and can hear a cricket.
That¿s right. In fact, the story only, the story dates to about the middle 70s when a man named Bill Cade, who was then a graduate student at the University of Texas, was studying mating behavior in crickets and studying cricket song. And what he found that was when he was playing cricket songs in the field through his tape recorder and through a loud speaker, that flies would show up. And this seemed a little strange, and then he noticed that in some of the flies that he kept in captivity, they seemed to die prematurely and he found little pupae, or cocoons outside them, and when they emerged, they came out, turned out to be the same flies that were coming to the speaker. So Bill ascertained that in fact these flies were attracted to cricket songs and they were parasitizing the crickets. He tested this in a critical way. What he did was to make sure that there wasn¿t anything special about a cricket, so he used model crickets, that is, dead crickets on top of a loudspeaker, and they came right to this and they were drawn right to the loudspeaker. So sound alone, sound by itself is sufficient to attract these flies
Would they go and put their larvae on the dead cricket that was on top of the loudspeaker?
Yep. Dead or alive. So he used to tether live crickets to the loudspeaker as well and they were attracted to both. But yes, they would, they would either drop, they would either, the female flies would leave their larvae and, by the way, these flies are different in that rather than leaving eggs which hatch into maggots, they¿re carrying live maggots and very small maggots, like half a millimeter long, but they would deposit these maggots either directly on the cricket or very nearby. These maggots are a little different in that they¿re quite active and they will actually search out and can find the host.
One maggot per cricket or can one cricket support more than one?
That¿s, that¿s a, that¿s a good question. It turns out that in nature the maximum number of crickets, or the average number of crickets, sorry, the average number of maggots per host is about three, so the average parasitic load then is about 3 to 4 per host. 3 or 4 maggots per host.
Do all the maggots survive?
By and large, yes. If any makes it through, if one makes it through, by and large, they all do. And it takes about, the life cycle is about a week, so from the time that a maggot comes in, enters the host, and, as I said, it¿s about half a millimeter long at that stage, until the time it burrows out of the cricket a week later, there¿s been a tremendous amount of growth so that it¿s a good third of an inch long by the time it emerges from the host, and so, and the three or four all leave in a time span of about an hour.
And the cricket at that point¿
The cricket at that point dies. In a, as an insult to injury, what these maggots have done, they, of course, they grow tremendously, as I said, from a half, half, from a half a millimeter to about a third, to a third of an inch long. They¿re growing and growing and growing. But as their exit act when they chew their way out, they void their guts of everything that had been accumulating and this acts to, apparently poisons the cricket. But the cricket, crickets don¿t survive any longer than about 6 hours to a day after the flies leave.
You¿ve described the actions of these larvae inside the cricket over the course of this week and it¿s quite remarkable what they¿re doing and what they¿re not doing.
Yes. Dr. Shelly Adamo, who is my postdoc at the time, did a very thorough study in which she wanted to know just what happened, she wanted to follow the life cycle of a maggot, from the time it got in until the time it leaves because of this explosive growth. And what she found was that they, as soon as they get inside the cricket, they lodge in the muscles up close to the head and there they stay for about three days and they grow and grow, but they¿re not, they¿re essentially drinking juices. They¿re not messing around with or eating any of the nearby muscles because those are the muscles that the cricket used to walk and to fly and to perform its normal life functions with. After about three days, they detach and then they reattach back at towards the end of the cricket in the abdomen and there they begin feeding voraciously on the fat bodies, as it were, of the cricket. So they¿re eating fat and then they¿re eating, they feed on abdominal muscles, which are presumably important to the cricket, but not important to the major functions of locomotion, respiration, or in fact, mating or fighting. So that¿s what they eat. It is very interesting, Alex, what you mentioned, what they don¿t do. They don¿t eat the nervous system, they don¿t eat the digestive system or the gut, and they don¿t eat any of the essential muscles of flight and they don¿t seem to eat the reproductive organs of the cricket either. So they¿re quite choosy.
In fact, I think your studies have found that these male crickets, they are all male crickets, that these, that the male crickets are more aggressive after these larvae are inside them.
Yeah, we used to set up, Shelly used to set up these little contests where she would pit, put two male crickets together, only in a, and when you do this with two normal crickets, they immediately start fighting. And fighting is accompanied by butting and lashing of the antennae and also a lot of strigulating, song, so that they¿re signing all the time, singing an aggressive song. What Shelly found was that in the first few days after infestation, the male crickets were a little touchier. They were more aggressive. They seemed to be, they seemed to be feistier than a normal cricket, but after, about halfway through the infestation period, about four or five days along, then they were much quieter and they weren¿t quite as aggressive, but Shelly found that even up to a day, the day of, the day before and the day of the time when the larvae would burrow out a week later, that some of those males were still capable of putting up a pretty good fight. So it¿s pretty remarkable what these crickets are able to endure even though they were essentially mortally infested.
With more than one larvae, I mean with three or four larvae.
That¿s right, three or four larvae. And these things are filling up, it¿s taking up most of, these things have taken up most of the space of the gut. If you, for scaling, these larvae are about the size of the, of the little, of the little creatures that popped out of the movie Alien if you remember that. But the, back to the cricket, the crickets not only can fight. They court and mate and they can continue to do so throughout their period of infestation. So while the crickets are definitely, no doubt they¿re not, if they have feelings, they may not be feeling fully up to snuff, but they can do, they can carry out many of their essential life functions. Certainly they feed very well. Their eating, drinking, their grooming, that is their picking, picking dust particles off of them so, so that¿s a normal activity, you can¿t really tell that they¿re compromised until just a few days before they, before they die.
How long would a cricket live if it did not have this¿
Probably, they¿d live on normally for another month or 6 to 8 weeks, but once, once you¿ve got parasites in, you¿ve got about a week to live.
Do the crickets seem to know that they have the larvae in them in any way?
That¿s a good question. Of course, we¿d like to know what a cricket knows, period. But, they don¿t behave in any particular, in any particular way that would give away the fact that they¿re infested. We had looked for it, for possible giveaway signs that they were sick. For example, do they sort of hunker down? Do they sort of crawl into a space and just sit there and be inactive? Do they not sing? Do they not walk around? But as I say, when we put them in these behavior tests, they performed all the normal acts of behavior and they seemed to feed and drink normally. So they weren¿t acting in any obvious way compromised by the infestation.
When you look at biologically at this fly, what are, what is it that this fly has done that other insects haven¿t done with its, with its ear? I mean, the problem for a fly of hearing something, I mean, the idea of a fly even developing an ear organ is pretty extraordinary for¿
Yes, well, the job of hearing is at least two fold. First, an ear, an animal has to use an ear to detect an acoustic signal, so it needs to be sensitive enough. And then the second thing is, presumably, you¿re gonna use, put that ear to use. After you detected something, what else do you want to know? You want to know where it is. So you want to, in terms of the reporter¿s question, you need to know, an animal wants to know, ¿Who is it?¿ and ¿Where is it?¿, and so the, or ¿What is it?¿ And so the ¿who and what is it?¿ are detection, are detection and identification functions. The ¿where is it?¿ is localization, and all animals that can hear have to solve this. I mean, we, presumably, we or our ancestors did, too. And you need to, there¿s a sound out there and that sounds could be either prey or a predator, so it¿s an old story. But at any rate, those are the two functions, and in the case of the fly, it had to evolve an ear that was sensitive specifically to cricket songs and it had to be able to localize that song. And the remarkable thing that the fly has done is to with its ear, with its ear, to localize a cricket song. So the detection of the song, the ear that it evolved is, turned out to be remarkably rather like a cricket¿s ear. But the way in which it localizes a sound source, a cricket song, is very different from a, from the way a cricket does it, and that¿s, that of course, became the engineering side of the story. And that¿s, it turned out to be a very remarkable thing because this innovation was much more extensive than we had thought at first.
Are there other insects that have this kind of pattern, that have, you know, one species has, what other incidences are there of this, this, what other things identify a host just by listening and have developed an auditory sense that allows them to find their host, that relies chiefly on hearing? You know, allows them to replicate the ear of the host?
Well, it turns out that there is another fly which is not related to the ormea fly. It¿s in different category. You might know this in a familiar sense, these are called flesh flies, sarcophagetes, and these are, but there are members of this family which have developed a parasitic lifestyle, and their hosts are cicadas and these insects, called emblomisomids, also have evolved an ear which can detect the sounds of emblomisomids by a mechanism remarkably like that of ormea but different in quite interesting ways and we¿ve just begun over the past couple of years, we¿re investigating that ear as well. But, you know, a lot of insects do, do have timpinal-like ears. I can give you a few examples. Most people don¿t think of praying mantises as having ears, but indeed, they have ears. We all know that crickets, katydids, grasshoppers, and cicadas must have ears because they¿re making sounds. And indeed that¿s the case, but it turns out that scara beetles, it turns out that have ears. It turns out that, that tiger beetles have ears. And of course, many moths have ears, as Kenneth Roder showed almost, almost 50 years ago. And green lace wings have ears, and it turns out that probably even some butterflies have ears.
But they use them to hear other butterflies, presumably.
Yes. Butterflies use them to hear butterflies, and many of the other ears. For example, the mantis, the moth, and some of these other night-flying insects use their ears to hear predators. Specifically bats, which use ultrasound. They use biosonar to locate them, and so these are ultrasound-sensitive ears, which they, which the insect uses to detect and localize the sound source and fly away from it.
You mentioned earlier that, that a hearing foundation has been helping you, supporting some of your work, anyway, for as long as 25 years.
Oh, indeed. This hearing foundation is the National Institutes of Health, specifically the National Institutes of Deafness and Communicative Disorders, the hearing institute, and they have had a record of supporting comparative studies in hearing, meaning that, I mean many animals use their ears, frogs, birds, other mammals besides ourselves, but I¿m, I¿ve been very pleased that the NIH has been willing to support a comparative study of insectine hearing and it gives me great pleasure at this stage in my career to say that studies in basic research on what might seem to be kind of an esoteric piece of knowledge, like a bug¿s ear, who would care, that in fact it might actually come to something. And so we were all very, very pleased, we being me and my Swiss collaborator, Daniel Robert, who is now with the University of Zurich, and Ron Miles, whom you¿ll speak to you in a few minutes from Binghamton University. That, in fact, the principles that we¿ve been able to learn from this fly¿s ear might have practical application in hearing aid applications.
Did you, I mean I can imagine people saying, ¿Spending deafness research money to study hearing in flies? For God¿s sake, who cares how the flies hear? Let¿s worry about how people hear.¿
That¿s right. And I think that if one wanted to just straight away solve the problems of how do we make a better hearing aid, well, an awful lot of money this century has already gone into to designing, to designing, coming up with better hearing aids, and I¿m not saying that the audio engineers aren¿t going to eventually do it, but here, we¿ve only been studying this fly¿s ear for about 6, 7 years now and there have been some new principles found here that might just be applicable. And you might say, ¿Well, but why? Why a fly¿s ear?¿ Well, flies are small. You know, they¿re fly-sized. And so, so they¿ve already solved the miniaturization problem and certainly one of the things that you want in a hearing aid microphone, especially a highly directional one, is that you need for that on board microphone to be really, really small. And the fly in nature has already solved that. It¿s already solved the problem of having exquisite sensitivity with a tiny, tiny ear. And by tiny, I mean the whole extent of the ear is just a couple of millimeters in breadth and maybe half a millimeter, so it¿s a half millimeter by a couple of millimeters, and this is two, that¿s two eardrums worth of ear. So the miniaturization problem, the sensitivity problem, are something that I would have to say are the envy of auditory engineers and the fact that, the fact that it¿s, you put it into this whole package, sensitivity plus the ability to localize the sound. That¿s pretty remarkable.
Is there something, you¿ve said that the fly, you mentioned downstairs, that the fly has come up with a third way of hearing?
Yes. The, well for, not necessarily of hearing, but to localize sounds, a directional to, as a directional sound receiver. So there are, we humans have what¿s called pressure detectors and in fact, most, many mammalian ears are just simple pressure detectors where each hearing organ is separate and uncoupled from each other and the sounds are, the sounds are recombined, so to speak, the process in the brain. But other animals, primarily really small animals like, like crickets and cicadas have another way of localizing sound and that¿s that the two ears are coupled together internally by a system of air-filled tubes and this set of tubes introduces slight delays, acousticians call them phase shifts, which allow each hearing organ to be activated slightly differently from the other. The fly, as Ron will explain, in fact uses the third way, which, as far as we know, hasn¿t been described in any other animal, and that¿s mechanical coupling. That the two eardrums are actually linked mechanically to each other and through some devilishly clever mechanism, they¿re able to take advantage of this coupling in order to discriminate whether sound¿s coming from the left or from the right.
Let me ask you, in the contest between the fly and the cricket, whose side are you on? Who do you admire?
Ah, gee. You know, for twenty years, for twenty years I¿ve been studying crickets and for the last five or six or so I¿ve been studying flies. I, you know, I¿m a kid from the Disney generation so I¿m always rooting for Jiminy Cricket, but, and, I don¿t think, I don¿t think Michael Eisner has heard yet about, about these flies. I guess some, but some insect¿s got to wear the black hat. But I suppose, but I do admire the fly in its, in that it has, it had a model, it had to, it needs to live the acoustic life of a cricket. That is, it has to, in order to lead this parasitory lifestyle, it had to come up with a cricket-like design. And it did it better. I would maintain that the fly¿s ear is more sensitive than a cricket¿s and its ability to localize sounds is every bit as good as a cricket¿s, so, and it came along a lot later in evolution, of course, because first there was a cricket and only a lot later in terms of tens of millions of years did the fly come along. But I think, you asked me a direct question. I basically root for the crickets.
You root for the crickets. But in this contest between the fly and the cricket, you¿ve got to know that the fly is going to win. I mean, the fly has really developed a very acute and useful sensitivity to this cricket, it can find a cricket. It¿s better, as you¿ve pointed out, it¿s much better at finding a cricket in a room than you or I would be.
Absolutely, and if there were as many flies out there as there are crickets, that would pretty much be the end. We would hear one more season¿s worth of crickets and then it¿s all over, but, but fortunately, for the crickets, that is, and for those of us who love their songs, there are an awful lot more crickets out there than there are flies. Now you might, you might ask the question, and it¿s probably coming next, ¿Well, why is that?¿ And I¿m gonna, I¿m gonna pass to the evolutionary biologist. All we know is that from field experiments, infestation, infested populations of crickets tend to be spotty. It¿s not, it¿s not general. And it, so that not all of the population is hit by parasites, so there don¿t seem to be pandemics, if you will, of flies upon crickets. And presumably, this is, there¿s some equilibrium here at work that I leave to the population biologist to solve. I have my hands full trying to record from their nervous systems and just figure out what they¿re, how they¿re processing sound.
Let me ask you to do the same way we started with Ron. (Who I am) ¿yeah, and your relationship to this project. Kind of your formal title and your relationship to this project.
Okay, I¿m Ron Miles. I¿m a professor in the Department of Mechanical Engineering at the State University of New York at Binghamton. My area of research is vibrations and acoustics and I got involved in this project because Daniel Robert and Ron Hoy knew that I worked in this area, partly because my wife works in the same department as them. And they had found this very unusual looking ear and asked me to help them figure out how it works, so we started up a collaboration there several years ago, mostly to try and understand the mechanics of how the ear responds to sound.
When you began considering the problem of the fly¿s ear, what was it that struck you about it? What seems difficult about what the fly is doing? What¿s interesting to you about it?
Well, what was interesting to me was that here¿s an ear or a pair of ears that are really, really close together. They are right next to each other. And yet the animal can respond very differently when sound comes from different directions, so the animal can clearly tell where sound comes from, but its ears are right next to each other. And that was a puzzle, because practically every other animal that we know about and that has a pair of ears, they try to get their ears as far apart as possible. You know, humans can localize sound because your ears are on opposite sides of your head and when sound gets to one ear before the other and its also shadowed somewhat by the head, you get different signals at the two ears and the brain processes that to tell you where the sound comes from. And lots of other animals do the same, depending on the size of the animal, they can get their ears further apart. In the fly, the fly¿s too small to get its ears very far apart, so the ears are right next to each other, and yet it¿s obviously really good at localizing sound. And that was the puzzle we were trying to figure out.
When you say it¿s really good at localizing sound, have you measured, is there a way to measure its ability to localize sound as opposed to the ability of a cricket or indeed of a human?
Well, I think you can do a behavioral study to try and quantify how good it is at localizing the sound and that¿s really something that Ron Hoy does. But we know that this animal is really good at localizing sound based on observing it.
And as a, but as a mechanical principle, if you were looking at this creature, and guessing how good is this thing going to be able to localize sound, you would say, couldn¿t do it, because physically, it just doesn¿t have the equipment that we think of when we think of ears.
Right. If you had a pair of ears that were stuck right next to each other, it certainly wouldn¿t be clear that it would be able to localize sound. But what we did to try and sort that out was to take the fly and set it on a little tether and play sound to it from different directions and actually measure the vibration of the eardrums using a laser vibrometer which can measure the response of a point that you focus the laser at and by playing sound from different directions, we found that the ear, the eardrum that¿s closest to the sound source responds a whole lot more than the eardrum that¿s on the other side of the fly, so we showed that mechanically, the vibration of the ears is highly directional and that must be how the fly can localize the sound.
If you and I hear something, do our ears vibrate at the same rate, at about the same rate if we hear a sound, we hear it about the same in both ears?
When sound comes from one side, say from your right side, your right ear hears it before the left ear and it also hears it with a little bit different amplitude, a little louder, because of the shadowing from your head that depends on how big your head is, but your two ears will receive somewhat different signals because of the fact that they¿re on opposite sides of your head. If they were right next to each other, they¿d both get about the same sound pressure and you wouldn¿t be able to tell where sound came from.
So how is it that the fly can do it?
Well, the trick that the fly uses, that as far as we know, is different than has been found in any other animal, is to have the two ears mechanically coupled together, and the basic idea is that you could think of these two ears as connected through something like a flexible teeter-totter where if it were a rigid teeter-totter, when you pushed down on one side, the other side would pop up, and they would just vibrate out of phase. Because of the flexibility in the middle, if you were to push on both sides equally, they¿d both move down. But it turns out that when there¿s sound coming from one side, the total motion of the ears is a combination of those two kinds of motion: one where they¿re going out of phase or rocking in opposite directions and another where they¿re going together and it¿s possible with the right set of mechanical properties to get those two kinds of motions to cancel on one ear and add on the other. So it, it achieves this by a mechanical structure that couples the two ears, and that, as far as we know, hasn¿t been found in any other hearing organs before.
How long have you been studying acoustics?
Well, let¿s see. Since about 1970, I guess. I¿ve been interested in it for a long time. I¿ve been working it since, well, for about 25 years, I guess.
And it that time, have you ever seen a hearing device like this before?
No. I don¿t think anyone has as far as I know. The basic principle that¿s being used here is similar to something that engineers have known for a long time. What¿s different is that in the fly, it does this mechanically, through a mechanical coupling between the ears.
You could build a microphone that would do the same thing as the fly¿s ear is doing, but it would be an electronic device and it would be fairly big and cumbersome.
Yeah, in fact, it¿s not too hard to make a directional microphone by just taking a pair of nondirectional microphones, which you could think of as the two ears, and then processing the sum and the difference of those two signals and you could create a output signal that would be directional, and that¿s something that engineers have known about since the early 30s and it¿s been used in stereo microphone technology. What is new here is that this is a mechanical structure that accomplishes the same operating principle but it does just simply by a mechanical structure and there¿s no electronics involved.
You said to me earlier that you don¿t think it¿s really possible to explain this. Or let me modify that- you don¿t think it¿s really possible to explain this to me- to an engineer perhaps, to a mathematician, to someone who really understands mechanics, but it¿s quite complicated.
It¿s sort of hard to explain I think, because, in order to get it all to work, you need to have the right set of mechanical properties in the system in order to have these two types of motions, this out of phase and in phase to combine right and the basic principle is not real complicated, but it takes some detail to explain it.
You¿re attempting actually, explaining it is one thing, but you¿re going beyond explaining it, you¿re gong to try to duplicate it.
Yeah, once we understood, or thought we understood really how it works, what the essential mechanism was behind it then, the next question was, since I¿m an engineer, I like to figure out what I can do with that. And to me the fly¿s ears are essentially a small directional microphone and the question was, what can you do with a very small directional microphone. I mean, we know, have known, for many decades how to make directional microphones. In this case, this is an opportunity to make a very, very small directional microphone because the entire pair of ears is only 1 or 2 millimeters across, and I believe that¿s smaller than any other directional microphone that¿s been made. And what we¿re trying to do now is to make use of what we know about this pair of ears to fabricate a mechanical structure using silicon micromachining with a colleague of mine here at the Cornell Nanofabrication Facility, Norman Thien. We¿re trying to fabricate a microphone diaphragm that operates on the same principle by mechanically coupling the two sides or two microphones with a similar principle that¿s used in the fly¿s ear to achieve a very small directional microphone.
You were saying earlier that what¿s neat about this, what¿s neat about looking at the fly is that it¿s already solved the problem of being very small and very directional which is something you want to achieve.
Yeah. The fly, I see it, is an existing prototype. It¿s known that there is a mechanical structure that could be built that does this. So that gives us some confidence that it¿s possible to make one of these things, `cause nature certainly has. The challenge that we have is to figure out how to fabricate it so that it works the same way, and, of course, the first step was to understand how the fly¿s ear works, and I think we know that, and the next task is to engineer a design out that works in a similar way. Get the properties right, the physical properties, the stiffness, the mass, and the damping of the system right, so that it works under the same principle. And I think that¿s possible.
You think you can do it?
Yeah, I think we can do it. And we also have been supported by the National Institutes of Health, which, in order to fabricate a microphone, which is the same institute that supported Ron Hoy, to do the basic science part of this and they¿re interested in now supporting the application of this. The application that we¿re working on is to develop a very small directional microphone for hearing aids. That is a place where you need a very small directional microphone and everybody who uses, or many people who use hearing aids, would like to have very small, cosmetically acceptable hearing aid that fits in the ear and there¿s a lot of interest in having a microphone for that hearing aid that will help reject sound that comes from behind the hearing aid wearer. So if you could make a microphone that fits in the hearing aid that would tend to respond more to sound from the front, which is the direction that someone would be speaking from, then that¿ll help the hearing aid wearer communicate and understand speech.
Tell me the kinds of things that are in a hearing aid now. In a hearing aid that¿s the size of an eraser on the end of a pencil.
Well, over the past 10 or 15 years there¿s been a huge increase in hearing aid technology. You can now buy digital hearing aids that have basically a microphone and a preamplifier and an analog to digital converter which converts the signal into digital, into bunches of ones and zeroes and then a little computer in there, a digital signal processor, which can process the signals and then a digital to analog converter that converts it back into a regular analog signal and then a little amplifier and a little loudspeaker which sends the signal into your eardrum. And all of that can fit inside a little in-the-ear hearing aid. The digital signal processing that¿s available now can significantly increase your ability to communicate in a noisy environment and that has been a big plus for hearing aids. It¿s been shown than in addition to having this digital signal processing capability within hearing aids, if you rely on a directional microphone to help cancel out and get rid of some of those unwanted sounds, that will also help your speech intelligibility.
Hearing aids now, they don¿t have this directionality, they don¿t, they¿re not good at figuring out where sounds are coming from.
Well, some hearing aids are available that do have directional microphones. Certainly behind-the-ear hearing aids, it is possible to get a directional microphone in those and there are some in in-the-ear hearing aids, but it¿s very difficult to pack a directional microphone that works well into an in-the-ear hearing aid. What we have in the fly¿s ear is really a whole new way of doing it. That, certainly in the case of the fly, has been shown that you can make a directional microphone or a directional ear very small, that would easily fit inside a very small package in an in-the-ear hearing aid.
If you had to guess, when do you think people who are hard of hearing, if everything worked well, when might your hearing aids be out there?
Actually hearing aids. We are, we plan to have a prototype microphone completed within the next three years. Actually getting it within a hearing aid is going to require additional work, to get it into a production situation. What we¿re trying to do now is show that we can make the microphone diaphragm, which is a pretty good challenge on its own. But we¿re certainly hoping that this is something that will get applied in a hearing aid.
I think it was your observation¿.
We were talking earlier about how this technology is going to be really useful in the next 10 or 15 years because this whole generation, new generation of people for whom hearing aids are going to become a subject of increasing interest.
Yeah, I think that as the baby boomers age, and the fact that the baby boomers brutalized their ears, certainly when they were younger, I think there are going to be more and more people needing hearing aids and certainly just with the aging population, hearing aids are gonna become a bigger business, so there¿s going to be an increasing need for good hearing aids. I think that another aspect of it is that a lot of us have grown up expecting good sound. People are interested in good sounding stereos and everything and just aren¿t willing to tolerate rotten-sounding hearing aids. And that will add some incentive to hearing aid manufacturers to make high fidelity hearing aids.
Do you recall the moment when you sort of figured out how it was the fly was hearing and I wonder if the mechanical engineer in you felt some kind of awe or admiration for this, I think you¿ve described it, used the word elegant to describe the flies, what the fly is doing.
I do remember when I figured out how it worked. And that was in here, in Ron Hoy¿s lab, we were looking at some fly ears under a microscope and I found, or we found, that if you look under a microscope and press on one ear, the other ear pops up. And it was clear then that the two ears were mechanically coupled. And immediately in my mind popped this little image of a lever, or a teeter-totter across, with some flexibility in the middle. And I said, ¿I know how it works.¿ Course, I didn¿t understand how it works in great depth, but I knew that somehow this combination of an out of phase motion and an in phase motion or the two sides going together and the two sides going apart, must be the key to how this thing accomplishes its directionality, and that was the moment that I felt that I really understood how it works. And it took me a long time after that to get to a point where I thought that I understood it well enough to actually make one, which is another level of understanding. Really got to know how to do things before you can make one of your own.
But when that light went off for you, what did you think about that fly?
I thought that was extremely cool. I was very excited, you know. It¿s great fun to look at a system, a mechanical system, and suddenly learn how it works. I think it¿s absolutely remarkable. Especially knowing how hard it is to get all the physical properties right in order for the principle to work. That, that¿s very impressive to me. There are a lot of things that have to come together for this thing to work well and of course that¿s true all over nature. You know, it¿s kind of amazing that things work as well as they do.
In addition, you had co-discovered something that truly is new. I mean, ears have been around for a long time and we¿ve been studying them for a long time. You found something that is truly new.
Yeah. We don¿t know of any other ears that have been described that work this way. It is, it¿s quite an unusual system. What is also kind of interesting to me is that the basic operating principle can be related to what was known by engineers in the 30s, but this is a new way of making it. No one, as far as I know, ever tried to make one of these by a mechanical structure. We¿ve tried to do it by electrical processing, and with amplifiers and things like that, but to make it with a mechanical structure is an idea that we got just by looking at the fly.
You¿re not a biologist.
No, but I¿ve learned a few things by, well, I¿m married to one, but also I have been spending a lot of time with Ron Hoy and learning a lot about fly ears and bug ears and I find them very exciting. It¿s a lot of fun as an engineer to get to look at a system like this and just look at the physical principles and try to understand how it works. That¿s really the science and I think by combining the science with some engineering to actually apply this to a physical system and try and make one is a lot of fun.
Is there an evolving synergy of mechanical engineering and biology in terms of trying to understand and replicate how things work?
Well, there¿s a lot of interest these days in biopneumetics and looking at biological systems to try to understand how to make engineering devices and systems. There¿s a lot of things that we can learn by looking at nature. To me, this is just another example of why we want to help preserve nature. A lot of people talk about saving the rain forest because, you know, you might find some neat chemicals and medicines by looking at things out there, but here¿s a mechanical system that we¿ve learned about by looking at nature and I think there¿s a lot of things that we can get by studying natural systems that sometimes people haven¿t really paid much attention to.
There are answers out there.
There are bound to be lots of answers out there. Lots of systems that engineers could learn from out there. We¿ve only looked at a couple of different flies and found some extremely interesting things about them. It takes a combination of understanding the physical principles they¿re using along with some sense of what you would do with those physical principles. So combining engineers and biologists, I think, is a very good thing.