Have you ever been driven to frustration by a fly buzzing around your home, seemingly impossible to swat? It’s a common experience, swatting wildly with a rolled-up newspaper or fly swatter, only to have the fly effortlessly evade your clumsy attempts. You’re left wondering: how do these tiny creatures with brains the size of a grain of rice outmaneuver us so easily? The answer lies in their perception of time and their incredibly fast vision.
This very question of insect agility and speed was posed to the BBC World Service CrowdScience team, delving into the fascinating “superpowers” of miniature animals. The revelation? Flies essentially experience the world in slow motion compared to humans. This difference in temporal perception is the key to their seemingly superhuman reactions.
To understand this concept, imagine observing a clock with a ticking second hand. To a human, the ticks occur at a normal pace. However, for a slow-moving creature like a turtle, those ticks would seem to pass much faster. Conversely, for a fly, time stretches out. Each second on our clock is perceived as significantly longer, approximately four times slower for many fly species. In essence, the speed of time itself is relative, differing from species to species based on how they process visual information.
Animals perceive their surroundings as a continuous stream of motion, much like a video. However, this perception is actually constructed from a series of distinct images transmitted from the eye to the brain in rapid succession. Humans process around 60 of these “flashes” per second. Turtles, with their slower pace of life, manage only about 15. Flies, on the other hand, operate on a vastly different scale, processing a staggering 250 flashes per second.
The Flicker Fusion Rate: Time in Slow Motion
This rate at which the brain processes these visual images is known as the “flicker fusion rate.” Generally, a smaller animal size correlates with a faster critical flicker fusion rate, and flies are exceptional in this regard, putting human vision to shame in terms of speed.
Professor Roger Hardie, a researcher at the University of Cambridge specializing in fly eye function, conducts experiments to precisely measure their flicker fusion rate.
Alt text: A hand wielding a fly swatter attempts to swat a housefly, illustrating the common struggle to catch these agile insects.
“The flicker fusion rate,” explains Professor Hardie, “is simply the frequency at which a flashing light appears to become a continuous, steady light.” To measure this in flies, he employs incredibly delicate techniques, inserting minuscule glass electrodes into the living, light-sensitive cells (photoreceptors) of their eyes. He then exposes these photoreceptors to LED lights flashing at increasingly rapid speeds. Each light flash induces a tiny electrical current in the photoreceptors, which is then recorded and graphed by a computer.
These experiments reveal that the fastest flies can register distinct responses to light flickering at an astonishing rate of up to 400 times per second. This is more than six times faster than the human flicker fusion rate, highlighting the dramatically different visual world experienced by flies.
The pinnacle of visual speed in the insect world is found in a species appropriately named the “killer fly.” This diminutive predatory fly, native to Europe, is an aerial hunter, catching other flies mid-flight with lightning-fast reflexes. In her dedicated “fly lab” at Cambridge University, Dr. Paloma Gonzales-Bellido studies the hunting prowess of these killer flies. She recreates their natural hunting environment by releasing fruit flies into a specialized filming enclosure containing a female killer fly.
Dr. Gonzales-Bellido utilizes high-speed video cameras capable of recording at 1,000 frames per second, coupled with a recording buffer system. This sophisticated setup constantly captures video, overwriting itself every twelve seconds. When a fly exhibits movement, Dr. Gonzales-Bellido triggers a save command, preserving the preceding twelve seconds of footage for detailed analysis.
“Our own reaction time is so sluggish,” Dr. Gonzales-Bellido points out, “that if we were to react and try to stop the recording when we perceive something happening, the event would have already concluded.” The speed of these flies is so extreme that human reaction times are simply too slow to capture the initial moments of their movements in real-time.
Killer Fly vs. Prey: A Mid-Air Ballet of Speed
In the filming enclosure, the killer fly initially remains motionless. However, as a fruit fly ventures approximately 7cm above, a sudden burst of motion erupts. In a flash, the killer fly is at the bottom of the enclosure, feasting on the now-motionless fruit fly.
Only by meticulously reviewing the slowed-down footage frame by frame can the intricate details of the hunt be revealed. The killer fly launches into the air, circles the fruit fly multiple times, attempting to seize it repeatedly before finally succeeding in capturing its elusive prey with its front legs.
The entire hunting sequence, from take-off to capture, unfolds in a mere second. To our human eyes, it appears as an almost instantaneous blur. Conversely, the approaching hand of a human wielding a swatter must appear to a fly to be moving in extreme slow motion, giving them ample time to react and escape.
Alt text: A close-up photograph showcases the distinct features of a killer fly, highlighting its adaptations for high-speed vision and predatory lifestyle.
Alt text: Dr. Paloma Gonzales-Bellido is pictured in her lab, demonstrating the experimental setup used to study the exceptional vision and hunting techniques of killer flies.
The extraordinary speed of the killer fly, surpassing even that of other fly species, is attributed to specialized adaptations in their visual system. The light-detecting cells within the killer fly’s eyes contain a significantly higher concentration of mitochondria – the cellular “powerhouses” responsible for energy production – compared to the same cells in other fly species.
These mitochondria are the energy batteries of the cell. High-speed vision demands a greater energy expenditure than slower vision, explaining why not all species have evolved to possess the highest possible flicker fusion rate.
The killer fly’s carnivorous diet provides the substantial energy reserves necessary to fuel these energy-intensive cells. However, even if human eye cells were packed with the same density of mitochondria, we would still not achieve the same visual speed as flies. This is because fly light-sensitive cells exhibit a fundamentally different structural design compared to those of vertebrates like humans.
The distinct eye structures of flies and humans are a consequence of their distant evolutionary origins. Arthropods (the group encompassing flies) and vertebrates diverged evolutionarily approximately 700-750 million years ago, and their eyes evolved independently.
Evolutionary Design: Strings vs. Tubes
Fly eyes are structured to capture light using a series of minute, string-like structures oriented horizontally relative to the path of light entering the eye. These structures respond to light through mechanical processes. In contrast, vertebrate eyes utilize long, tube-shaped cells positioned directly facing the incoming light, employing chemical reactions at their base to detect light.
Professor Hardie’s research lab delves into the intricacies of this string-like structure in fly eyes. “It exhibits enhanced sensitivity, capable of generating a strong signal even from minuscule amounts of light,” he explains. “Furthermore, it can respond more rapidly than the rods and cones found in vertebrate eyes.”
Alt text: Professor Roger Hardie, a leading expert in fly vision, is shown in his lab, surrounded by equipment used to study the intricacies of the fly visual system.
Several factors contribute to this heightened sensitivity. Crucially, these structures respond mechanically to light, unlike the chemical responses in vertebrate rods and cones. Mechanical responses facilitate faster neural signaling. Additionally, the speed of neural impulse transmission has inherent limitations. The shorter nerve pathways from the fly eye to its brain contribute to faster processing speeds compared to larger vertebrates with longer neural distances.
Within the vertebrate world, certain species exhibit significantly faster vision than humans. A correlation appears to exist between flight capability and faster vision, as well as smaller body size. This may be because small, flying animals require rapid reactions during flight to navigate and avoid obstacles effectively.
Slow Motion Swats: An Evolutionary Arms Race
The fastest vision among all species is often found in predators that hunt flies in mid-air.
Returning to vertebrates, research on the pied flycatcher, a small perching bird that specializes in catching flies during flight, conducted at Uppsala University in Sweden, revealed remarkable visual acuity. These birds could distinguish a light flashing on and off 146 times per second from a continuous light source.
The researchers trained the birds to associate a flashing light with a tasty treat. The birds accurately identified the flashing light up to this rate, establishing their flicker fusion rate at 146 flashes per second. While double the human rate, it still falls short of the average fly’s visual speed.
This implies that pied flycatchers, similar to flies, experience time at a slower pace than humans. Evolutionary pressure drives flycatchers to perceive time as slowly as possible to effectively outmaneuver their fast-flying prey. Over evolutionary timescales, birds with “slower ticking” internal clocks could react more swiftly to their prey, leading to greater hunting success, increased offspring, and the propagation of this rapid vision to subsequent generations.
Conversely, flies that are preyed upon by fast-sighted birds are under evolutionary pressure to develop even faster reactions to escape. This creates an ongoing evolutionary arms race that predates the existence of birds themselves. Prey flies have been evolving faster vision and reactions to evade predatory flies, such as the killer fly, since the very dawn of insect flight.
So, the next time you find yourself futilely swatting at a fly, take solace in the fact that your seemingly sluggish, slow-motion attempts are being thwarted by hundreds of millions of years of natural selection. Flies are essentially watching your efforts unfold in slow motion.
Between humans and flies, the perception of time itself appears to be remarkably relative.
This article is inspired by discussions on the BBC World Service program ‘CrowdScience’.
