Sunday, August 27, 2006

Insect ears

I work on crickets and when i tell people about one of the things that always gets them is that crickets have ears on their legs. Look just under the leg joint in the bushcricket picture, can you see that slightly inflated region? Thats the ear (well sort of, it leads to the ear, thats a tympanal slit).

Insects have ears, literally, all over. They've evolved independantly among insects almost a dozen times over. They're on legs, abdomens, and they're on wings! In the neuropterans, ie lacewings, like the one in the picture, the ears are on the wings!

Well, mosquitoes and bees hear with their antennae. Actually, if I have it right, they hear in the near-field, so they detect particle motion not pressure, unlike the other ears I'm talking about. The resolution those systems manage is so great that they apparently can pick up even motion at even angstrom scales! Thats pretty impressive, and possibly almost on the absolute borderline of physical possibility.

The other weird ear in the insect world belongs to mantises, they have a single ear. Now if you remember your basic physics, sound is scalar and does not have directional cues. So to tell where sound comes from animals have evolved two ears and several physical and computational tricks. But this hinges on having more than a single ear, which the mantises dont have. Why is that?

The thing about ears in insects is that they seem to be easy to build. Ingredient one : you need an air space which insects have everywhere because of their tracheal breathing tubes.

Ingredient two and three: receptors and neurons. First you need something to sense the vibrations of the tympanic membrane you strung across this empty space. Then some neurons to carry these signals away to the 'brain'.

And those are everywhere in insects as well. See they (actually all of us) have these receptors called proprioceptors that detect mechanical stimuli. They're meant largely to figure out things like ok, legs bending now, and now the wings out, theres x amount of resistance on leg 2 and so on. To provide feedback to the brain on how well its commands to the motor system are being carried out. Now these have neurons connecting them up to some ganglion and then to a higher 'brain'. So if you can rearrange your proprioceptors to detect tympanal vibrations and your neurons to carry the signals, voila, you have a ear!

Actually why spend all that energy on making a ear though? Predator avoidance comes the pat reply, particularly, BATS!

You know this cool thing I read about this proprioception stuff, in one of the first insect walking robots they made, they didnt think about the interaction between the robot and the surface it was to walk on. And there was no feedback of the kind proprioceptors provide. If memory serves, that robot tore itself apart as it walked!


Kalyan said...

Great post. Do keep posting more of these insects gyan.. people really need to go beyond big mammals and even birds :)

Natasha said...

Thanks Kalyan, I'll keep doing just that...