Why cannot people hear a house fly walking?
Answers
Answer:
The speed at which those images are processed by the brain is called the "flicker fusion rate". In general, the smaller the species, the faster its critical flicker fusion rate - and flies, in particular, put us to shame.
Professor Roger Hardie, from the University of Cambridge, investigates how flies' eyes work, and he has an experiment to determine their flicker fusion rate.
IMAGE SOURCE,SPL
Image caption,"How d'ya like them apples?" For flies, time drags more slowly than for people
"The flicker fusion rate is simply how fast a light has to be turning on and off before it's perceived or seen as just a continuous light" says Prof Hardie.
Roger inserts tiny glass electrodes into the living light sensitive cells of their eyes - photoreceptors - before flashing LED lights at faster and faster speeds. Each flash of the LED produces a tiny electrical current in the photoreceptors that a computer can graph onto a screen. Tests reveal the fastest fly records distinct responses to flickering up to 400 times per second, more than six times faster than our own rate.
The fastest vision of all is found in a species literally called a "killer fly". It's a tiny predatory species found in Europe that catches other flies out of the air with super-fast reactions. In her "fly lab" at Cambridge University, Dr Paloma Gonzales-Bellido demonstrates the killer flies' hunting behaviour by releasing fruit fly prey into a special filming box with a female killer fly.
Media caption,Some flies see six times faster than us, catching prey in mid-air in less than a second.
Paloma records the behaviour at 1,000 frames per second using slow motion video cameras with a recording buffer. The attached computer constantly saves the video, over-writing itself every twelve seconds. When the fly moves, Paloma clicks a button to permanently save the last 12 seconds.
"Our reaction time is so slow that if we were to stop it when we think something is happening it would have happened already," says Dr Gonzales-Bellido. Essentially, we can't even click a button before the behaviour has happened, it's that fast.
Fly vs fly
With the killer flies and their prey in the filming box, initially the killer fly just sat around motionless, but as one of the fruit flies flew about 7cm above it, there was a flash of movement and suddenly the killer fly was at the bottom of the box chomping into the quivering fruit fly.
Only looking at the slowed-down footage on the computer did it become clear what happened; the killer fly took off, circled the fruit fly three times as it tried to grab it repeatedly, before succeeding in capturing the elusive fruit fly with its front legs.
The whole behaviour from take-off to landing took just one second. It appears as a flash to our eyes, so conversely, the swatting hand of a human must appear at a snail's pace.