Why Do Flies Escape the Moment You Swing at Them?
– To a Fly, Your Hand Moves in Slow Motion
Wings are not the only reason flies are fast. Their eyes slice time much more finely than ours. In the time a human eye sees one frame, a fly's eye sees about four. So even a hand swung with all your strength looks to a fly like it is closing in slowly.
A fly is sitting on the kitchen table. You roll up a newspaper and raise it quietly.
You hold your breath and bring it down in one swing. But by the time the paper hits the table, the fly is already up near the ceiling.
You swung with everything you had, and you thought you had surprised it. So when did the fly notice?
There are just two reasons it gets away
A fly's eye can tell apart about 250 changes in a scene every second. For humans it is about 60. A fly sees the same movement in roughly 4 times as many frames.
A fly looks at the direction your hand is coming from and adjusts the position of its legs. It takes off only after it is posed to jump away from the danger.
So a fly does more than "flee fast". It notices early and flees in the right direction. Let's take these one at a time.
Even the eye has a limit of "frames per second"
Films and cartoons show about 24 still pictures per second. Even so, the motion looks smooth to us. That is because the human eye and brain cannot tell apart changes finer than that in time.
This limit can be measured with a flickering light. If you switch a light on and off fast enough, past a certain speed it looks as if it is on all the time. For humans in bright light, that speed is said to be around 60 times a second.
Run the same experiment on a fly, and it is said to see the flicker up to around 250 times a second (this varies with species and brightness). Look at Figure 1. It lines up the number of "frames" that a human eye and a fly's eye receive while a hand swings down.
For a human eye, it is over in a flash. But to a fly's eye, the approach of the hand arrives in dozens of separate frames. In human terms, an event lasting 1 second is stretched to about 4 seconds.
Why is a fly's eye so fast? Flies dart around on tiny bodies. If their eyes could not keep up with the speed at which the scene streams past in flight, they would crash into walls. The faster a creature flies, the faster its eyes need to be.
A fly picks its escape direction before it jumps
Fast eyes are not enough on their own, because it also takes time to go from noticing to taking off.
Researchers in the United States showed fruit flies (a group of small flies) a video of a black disc closing in, and filmed them with a high-speed camera. The flies changed the position of their legs and the tilt of their bodies to match the direction the disc came from. If it came from the front, they were set to jump backward. If it came from the side, they were set to jump to the opposite side.
From this preparation to takeoff took about 0.2 seconds, it has been reported. What a fly senses is not so much the "size" of the hand as how fast its image "swells" in the eye. An image that swells suddenly is judged to be something on a collision course.
Old fluorescent lamps varied in brightness 100 or 120 times a second, following the electric current. To humans they looked steadily on. But an eye that can tell apart up to 250 times a second could, by this calculation, pick out that flicker. To a fly, a human room may have been flickering all along.
In most cases a fly jumps away from the oncoming hand. If you swing from above, it escapes diagonally forward or backward. So researchers say it helps to aim not at where the fly sits, but at where it will jump. A slowly approaching hand makes the image swell slowly, and is said to be less likely to be noticed.
Summary
A fly can dodge your hand because its eyes slice one second about 4 times more finely than ours. It also reads the direction the hand is coming from, takes a pose for escape, and takes off in about 0.2 seconds. Even a hand swung at full strength looks to the fly like a large shadow slowly closing in.
It is the eyes that are fast, even before the wings.
A fly sees your one second as four.
How many particles of light the eye can count in the dark is covered in "How many particles of light does it take for the human eye to know it saw something?", and how long the eye takes to adjust to brightness is covered in "Why can't you see anything for a while when you go from a bright place into a dark room?". For why insects gather around lights at night, see this article.
- In front of a white smartphone or computer screen, wave your hand quickly from side to side with your fingers spread. If your fingers seem to split into many, your eyes are catching the screen's flicker.
- Next, wave your hand the same way in sunlight coming through a window. This time your fingers should just blur, and are less likely to split into many. Sunlight does not flicker.
- When you see a fly, compare the distance at which it takes off when you bring your hand close slowly and when you bring it close quickly (do not touch the fly; just observe).
The difference between 1 and 2 is whether you can see the flicker of the screen's light. It shows that the human eye also has a limit to how finely it slices time.
For those who want to know more – terms, formulas, and links to textbooksShows which level each topic is at, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school biology and physics
- High school+Advanced high-school material, or a textbook sidebar topic
- UniversityNot taught in high school; university specialist courses (sensory physiology, neuroethology)
- ResearchNot taught even at university as settled fact; researchers are studying it right now
Middle schoolTerms: this phenomenon has names
- Compound eye: An insect eye made of many small eyes (ommatidia) packed together. A housefly is said to have several thousand in each eye.
- Critical flicker fusion frequency: The number of flashes per second at which a flickering light starts to look steadily on instead of flickering. It is a guide to how finely an eye can tell apart time.
- Looming stimulus: An image that rapidly grows larger in the eye. Many animals flee in response to it, as a signal of an object on a collision course.
Middle schoolHigh schoolChecking with a formula: how many frames does a fly see before the hand arrives?
Suppose a hand swings down from 50 cm away. We calculate how many frames the human eye and the fly's eye each receive. Let's assume the hand moves at 2 m per second.
| In symbols | N ＝ f × t , t ＝ d ÷ v |
| In words | Frames seen ＝ number of changes the eye can tell apart per second × time until the hand arrives. Time is distance ÷ the hand's speed |
| Where it comes from | It combines the counting rule "times per second × seconds ＝ total times" with the relation "distance ＝ speed × time" |
| N | Number of frames the eye receives before the hand arrives (no unit) |
| f | Critical flicker fusion frequency (times/second) |
| t | Time until the hand arrives (seconds) |
| d, v | Distance the hand moves (m) and the hand's speed (m/second) |
| Human critical flicker fusion frequency (bright place) | said to be about 60 times/second |
| Fly's critical flicker fusion frequency | said to be about 250 times/second |
| Distance the hand moves (assumed) | 0.5 m |
| Speed of the hand (assumed) | 2 m per second |
| From the fly taking its pose to takeoff | reported as about 0.2 seconds |
| Time until the hand arrives | 0.5 ÷ 2 ＝ 0.25 seconds |
| Frames the human eye receives | 60 × 0.25 ＝ 15 frames |
| Frames the fly's eye receives | 250 × 0.25 ＝ 62.5 frames |
| How many times more frames the fly sees than a human | 250 ÷ 60 ≒ 4.2 times |
| Time to spare after subtracting the time to takeoff | 0.25 − 0.2 ＝ 0.05 seconds |
An event that ends in 15 frames for the human eye reaches the fly's eye split into a little over 60. An event of 1 second is stretched to about 4 seconds. And the fly still takes off with 0.05 seconds to spare. However, if the hand comes down from only about 10 cm away, the time shrinks to 0.05 seconds, and there is not enough time to take a pose.
High schoolHigh school+A fast eye costs a lot of energy
High schoolLight-receiving cells (photoreceptors) give off an electrical signal when light hits them. In the rod and cone cells of the human eye, many steps of chemical reactions sit in between, so it takes time for the signal to come out.
High school+A fly's photoreceptor is made of tens of thousands of fine hair-like projections (microvilli). Each projection responds quickly to a particle of light and shuts the response off right away. As a result, the cell can follow fast changes, but it uses a lot of energy. One estimate says a fly's retina uses close to a tenth of the energy used by the whole body at rest.
UniversityReading "time to collision" from the swelling of the image
Let θ be the apparent size (visual angle) of an approaching object. The value of θ divided by "the rate at which θ grows" is roughly equal to the time left before collision. This quantity is called "tau (τ)", and the psychologist Lee proposed it in 1976. The fly's brain has neurons that respond strongly to the swelling of an image, and their signal travels quickly through thick nerve fibres called the giant fibre system to the leg muscles used for jumping. A study comparing the critical flicker fusion frequency of various animals with their body size and metabolism reported that smaller animals with a higher metabolism tend to tell apart time more finely.
📖 Derivations and further reading: Explanation of critical flicker fusion frequency (English Wikipedia) ／ Compound eyes and simple eyes (Wikipedia)
ResearchWhat is still not clear
- Does a fly really experience "slow motion"? We can measure how finely the eye tells things apart. But how a fly experiences the flow of time cannot be checked from the outside.
- How many frames does it use to decide? How much of the information the eye receives goes into the decision to take off is still being investigated.
- Differences between species. Even among flies, it has been reported that the speed of the eye differs between fast-flying and slow-flying species. How this ties to their way of life is not yet fully understood.
So the content of this article is also "an explanation of what is known so far". The numbers vary with species, brightness and the way of measuring.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: stimulus and response, the insect body | Compound eyes, from the eye receiving to the body moving |
| High school | Biology: receptors and effectors; Physics: speed | How photoreceptors work, calculating the arrival time |
| High school+ | Biology, advanced (nerve information processing) | Fast photoreceptors and their energy cost |
| University | Sensory physiology, neuroethology | Critical flicker fusion frequency, time to collision, the giant fibre system |
| Research | Comparative neuroscience | How different animals sense time |
| ― | Links to daily life | Shooing flies away, flicker of screens and lights |
- Card G, Dickinson MH (2008) Visually mediated motor planning in the escape response of Drosophila. Current Biology 18.
- Healy K, McNally L, Ruxton GD, Cooper N, Jackson AL (2013) Metabolic rate and body size are linked with perception of temporal information. Animal Behaviour 86.
- Laughlin SB, de Ruyter van Steveninck RR, Anderson JC (1998) The metabolic cost of neural information. Nature Neuroscience 1.
- Lee DN (1976) A theory of visual control of braking based on information about time-to-collision. Perception 5.
- Wikipedia, "Flicker fusion threshold"
* This article is a science explainer for general readers. The numbers given are rough guides to help you understand how things work, and they vary with the species and conditions. If you use insecticides or similar products, follow the instructions on the product label.