Why is it so hard to catch a fly? Scientists provide a reasoned answer

Photo: Getty Images

While you read this text, the screen in front of you most likely flickers more than 240 times per second. However, you don't notice it – for a human, such flickering merges into continuous light.

But for a fly buzzing overhead, the screen would look like a real strobe light. This means it perceives more "frames" of reality in the same second than a human does.

For us, a ball flying at high speed might appear blurry. But dragonflies or pigeons see it in detail. Conversely, for snails, such movement is probably too fast to register at all.

To understand why this happens, researchers gathered and analyzed already published results of experiments measuring time perception in 237 species – from jellyfish to vertebrates.

These works used the same metric – the maximum critical flicker fusion rate. It indicates how many individual flashes of light the eye can distinguish in one second. This metric is measured in hertz (Hz).

To determine this limit, scientists gradually increase the flicker frequency of light and record the moment it starts to appear continuous. This threshold defines the speed of vision.

Analysis showed that the diversity in this metric is even greater than previously thought. For humans, the limit is around 65 Hz. Birds, such as the white-throated sparrow, see up to 138 flickers per second. Tsetse flies and dragonflies — up to 300.

With a rate of 65 hertz, humans appear quite respectable compared to many mammals: rats have around 47 Hz, dogs – around 84. However, there are also significantly "slower" eyes. The deep-sea fish escolar distinguishes only 12 flickers per second, while a starfish and a giant African snail — only about 0.7.

Why is that?

As the authors note, the differences are largely determined by the pace of life of a particular species. This idea is described by Autrum's hypothesis: high visual speed requires significant energy expenditure, so it is primarily established in species with a fast lifestyle – those that need an immediate reaction.

This is why flying species have the highest visual speed. On average, they perceive the world about twice as fast as non-flying animals. This metric is also higher in predators that pursue prey.

Researchers discovered an additional pattern in aquatic environments: smaller species there generally see faster than larger ones. Moreover, it was precisely in water that the connection between size and visual speed proved particularly clear. Why this happens is not yet precisely known, but perhaps water allows for instant changes in direction of movement.

In dark environments, visual speed decreases. To "catch" as much light as possible, eyes must work slower – similar to a camera with a long exposure. But this comes at a cost: fast motion becomes blurry.

Павялічыць

Photo: By Eve Livesey / Getty Images

How a fly sees you

What does this mean in practice? A fly constantly flies, rapidly changes direction, and is always at risk of being eaten. To survive, it must notice a threat before it becomes fatal. Therefore, its vision operates at a very high speed.

When you move your hand, for you it's one gesture. For a fly – a sequence of small phases. It receives more visual information in the same second and detects danger earlier. Therefore, when you think you've almost touched it, it has long since flown away.

Physically, a second is the same for all creatures. But how much of the world fits into that second depends on how fast you live.

• 13
• 0
• 0
• 1
• 2
• 0
• 0
• 0