Ear Now …
An incredible design in a tiny fly is inspiring engineers
The main reason we can tell from which direction a sound is coming is that there is a slight time difference between the sound waves’ arrival at each ear (inter-aural time difference, ITD), as well as a slightly greater intensity at the nearer ear. But a tiny female fly, Ormia ochracea, can track a male cricket’s chirping so she can lay her eggs on him—even though the fly’s ears are only 0.5 mm apart, meaning that the ITD is only 1½ µs (microseconds = millionths of a second), and the intensity difference is almost nil.
How does she do it? The fly’s eardrums are coupled together by a bridge like a flexible lever. The resulting resonance means that the time difference is, in effect, increased about 40 times, and the eardrum nearest the sound vibrates about 10 decibels more strongly.
Also, the nerves from each of the fly’s ears respond in a type of code so that their time difference is about five times greater still. Finally, the fly’s flight programming is linked to its ears’ signals, so it can tell directions to within 2°, as well as humans can.
This fly’s mechanical and signal-processing technology is being used to improve hearing aids, which normally can’t tell direction, and could be used to design miniature directional microphones.
Once more, the Master Designer has taught the best of human designers some lessons. Sadly, the main research paper called the fly’s ear an ‘evolutionary innovation’, without the slightest explanation of how the mechanical structure and nervous coding system could arise by small mutations and natural selection.
Where does this design, which is going to hasten the demise of the cricket being tracked, fit in with the Fall and no death before sin? As discussed in The Answers Book, ch. 6, insects are probably not nephesh life, so their ‘death’ is Biblically different from that of humans or vertebrate animals. Alternatively, God, who foreknew the Fall, could have designed the machinery and switched it on at the Fall. See also Q&A: Genesis: The Curse.
Mason, A.C., Oshinsky, M.L. and Hoy, R.R., Hyperacute directional hearing in a microscale auditory system, Nature 410(6829):686–690, 5 April 2001; Narins, P.M., In a fly’s ear, same issue, pp. 644–645; Cricket pitch, p. ix.
Wising up to design
The fly in the main article is not the only creature with an amazingly high-tech mechanism in its nerves of hearing to help it to determine the direction of sound.
An owl’s fine directional hearing is also largely due to the way the nerves process the aural time and intensity differences.
Most neurons (nerve cells) ‘fire’ when the incoming signals add up to a threshold, and act ‘like a transistor in an electronic circuit’.
But ‘neurons in the owl’s auditory map multiply’, so each ‘is more like a little processor; computationally it’s much more powerful’.
Peña, J.L. and Konishi, M., Auditory spatial receptive fields created by multiplication, Science 292(5515):249–252, 13 April 2001; Helmuth, L., Location neurons do advanced math, same issue, p. 185.