Why is it so difficult to kill a fly? (2023)

Try to kill a fly and it will soon become clear that it is faster than you. Much faster. But how the heck can these tiny creatures with their tiny brains fool us so easily?

You've probably pondered this after chasing a fly through your house and smacking your shoe with repeated, unsuccessful smacks. How is he moving so fast? Can he read my mind?

It was the question askedBBC World Service CrowdScienceteam for our latest episode, which explores the apparent superpowers of little animals. The answer is that flies essentially see the world in slow motion compared to you and me.

To illustrate, consider a clock with the hands ticking. As a human, you see the clock running at a certain speed. But for a tortoise, it seems to run twice as fast. On most fly species, each tick would crawl about four times as slowly. In fact, the speed of time differs depending on the species.

This is because animals see the world around them as one continuous video. But in reality they collect images sent from the eyes to the brain in separate flashes a certain number of times per second. Humans flash an average of 60 times per second, turtles 15 and flies 250.

The speed at which these images are processed by the brain is called the "scintillation fusion rate". In general, the smaller the species, the faster the critical scintillation rate melts — and flies in particular put us to shame.

Professor Roger Hardie from the University of Cambridge is studying how flies' eyes work and is conducting an experiment to determine their scintillation melting rate.

"The scintillation melting rate is simply how quickly a light must turn on and off before it is perceived or seen as just continuous light," says Professor Hardie.

Roger inserts tiny glass electrodes into the light-sensitive living cells in his eyes - photoreceptors - before LED lights flash at an increasingly rapid rate. Each LED flash creates a small electrical current in photoreceptors that a computer can graph on a screen. Tests show that the fastest flight logs record clear responses to vibrations up to 400 times per second, more than six times faster than our own rate.

The fastest sight of all is found in a species literally dubbed the "killer fly." It is a small predatory species found in Europe that uses super-fast reactions to catch other flies in the air. In her "fly laboratory" at the University of Cambridge, Dr. Paloma Gonzales-Bellido demonstrates killer fly hunting behavior by releasing fruit fly prey in a special film box with a female killer fly.

Paloma records behavior at 1,000 frames per second using slow-motion video cameras with a record buffer. The connected computer constantly saves the video and overwrites 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 stopped when we thought something was going to happen, it would have happened already," says Dr. Gonzales Bellido. Essentially we can't even click a button before the behavior occurs, it's too fast.

With the killer flies and their prey in the film box, the killer fly just lay still at first, but when one of the fruit flies flew about 7cm above there was a flash of movement and suddenly the killer fly was down the box chewing on the trembling fruit fly.

Just looking at the slow footage on the computer made it clear what had happened; The killer fly took off, circling the fruit fly three times while repeatedly attempting to grab it before managing to capture the elusive fruit fly with its front paws.

The entire behavior from takeoff to landing took just one second. To our eyes it looks like lightning, conversely, a human's striking hand should appear at a snail's pace.

To enable the killer fly's incredible speed, which is even faster than other fly species, the light-sensing cells in the killer fly's eyes contain far more mitochondria (the "battery" of biological cells) than are present in the killer fly's eyes. cells from other flies.

These are the cell's batteries, so fast vision must use more energy than slow vision, which explains why all eyes aren't simply tuned to the highest scintillation fusion rate.

The killer fly's carnivorous diet provides the large amounts of energy needed to fuel these high-energy cells. But even if we had the same number of mitochondria in cells or in our own eyes, we wouldn't have the same speed of vision because the light-sensitive cells of flies have a completely different design than those of vertebrates.

Behind the structural differences in fly eyes lies their evolutionary origin. Arthropods and vertebrates, the groups that flies and humans live in, evolved their completely separate eyes about 700 to 750 million years ago.

Flies' eyes evolved to capture light with a series of tiny thread-like structures that lie horizontally to the path that light travels through the eye. These structures respond mechanically to light, while vertebrates have long, tubular, light-facing cells with light-reactive chemicals at the base.

Roger examines this structure in the fly's eye in his laboratory. "It's more sensitive to the ability to give a large signal to the smallest amount of light, and it can also respond faster than the rods and cones in the vertebrate eye," he explains.

There are a few reasons for this increased sensitivity, but what Professor Hardie found is that they respond to light mechanically, as opposed to chemically like rods and cones.

Mechanical responses enable faster neural signals. In addition, the speed at which neural impulses can propagate is limited, and the shorter neural distances from the fly's eye to the fly's brain speed up processing compared to larger vertebrates.

Some vertebrates see much faster than we do. Whether the species can fly appears to correlate with faster vision and small scale. This may be because small flying animals have to react so quickly in flight to avoid approaching obstacles.

The fastest eyesight of all is found in species that catch flies in the air.

Getting back to the vertebrates, while studying the eyesight of the flycatcher, a small perching bird that catches flies in flight, scientists at Uppsala University in Sweden found that it was able to distinguish a light blinking 146 times per second from a continuous one Identify light source.

Birds were trained to associate a flashing light source with a tasty treat, and they would accurately identify the flashing light up to that rate, setting their scintillation fusion rate at 146. That's about twice as fast as humans can see, but still not as fast as the average fly.

This means that birds, like flies, experience every tick of the clock more slowly than humans.

Flycatchers are under evolutionary pressure to witness the ticking of the clock as slowly as possible in order to outsmart their nimble prey. Over the course of evolution, birds that experienced "slower ticking" were able to respond more quickly to their prey, allowing them to eat more, raise more young, and pass on this quick insight to future generations.

Flies that have been chased by short-sighted birds develop quicker escape responses. Creation of an evolutionary arms race that lasted longer than the existence of birds. Prey flies have evolved faster vision and reactions to escape predator flies like the killer fly since they evolved flight.

The next time you try and fail to kill a fly, don't get too discouraged. Their heavy slow-motion pushes are thwarted by hundreds of millions of years of natural selection, allowing the flies to watch their attempts in slow-motion.

Between you and the fly, time seems relative.

Ouça 'CrowdScience'on the BBC World Service, the program whose listeners inspired this article, and send your science questions to 'CrowdScience@bbc.co.uk':