I wrote this item in 2012. A new piece of related research spurred me to revise and expand. First the 2012 version:
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Wonderful example of what can happen when you decide to look closely at something, instead of fucking around with numbers and potentialities and apocalyptic religious delusions. In other words, when you do ACTUAL FUCKING SCIENCE.
Katydids are thoroughly familiar insects. We’ve all heard them, seen them, probably handled them or batted them away when they jumped on us. We knew that they made sounds with their legs, and that they somehow heard with their legs.
Now someone finally decided to look closely, with amazing results.
A team at Lincoln Univ in Britain used electron microscopes to examine the ears, which are inside the front pair of legs.
I’ve done a 3d pic and animation based on their descriptions.
Each of the front legs has a pair of tiny openings. Behind each opening is a tympanic membrane, much like ours.
I’ve shown one side of the leg here. Each Tympanic Membrane (eardrum) has a ‘plate’ or lever on one edge, serving as a leverage multiplier like our Ossicles. The plate transfers the wave motions to a fluid-filled Acoustic Vesicle, which plays the same role as our Cochlea. The Vesicle picks up the waves from both drums in this leg. It contains hair cells that wave back and forth, sending neural signals to the katydid’s brain. Fluid inside the Vesicle is a dense version of the insect’s blood, again just like the fluid in our Cochlea; and the hair cells are tuned for different frequencies, just like ours.
Eardrums need to avoid being moved by changes in atmospheric pressure, which can be much larger than sound pressure. Mammalian ears balance the pressure on both sides of the drum via the Eustachian tube that opens into the nasal passages, insuring that the middle ear chamber stays at the same pressure as the air outside the skull. The katydid solves the same problem with an Acoustic Trachea, a tube running up the leg, into the body, and thence into one of the insect’s ‘lungs’.
When sound impinges, each eardrum transfers its energy to a ‘pinch’ on one side of the Vesicle.
Aside from the pairing, there’s only one major design difference between mammal and katydid. The katydid’s paired eardrums push on the side of the Vesicle, while our eardrum pushes on the end of our cochlea:
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