This article was originally Speech.
Sitting outside on a summer evening always sounds comforting until the flies and mosquitoes arrive – then the swatting begins. Despite having tiny eyes and a brain roughly 1 million times smaller than yours, flies can avoid nearly any swat.
Flies can thank their fast, enhanced eyesight and some neural quirks for their ability to evade swats with such speed and agility.
Our lab studies the flight and vision of insects to find out how such tiny creatures can process visual information to perform challenging behaviors such as running too fast from your mosquito net.
Flies have compound eyes. Rather than collecting light through a single lens that flies the entire image—the human eye’s strategy—images constructed from multiple directions create multiple individual lenses that focus incoming light onto clusters of photoreceptors, the light-sensing cells in their eyes. Essentially, each facet produces a separate pixel of the fly’s vision.
A fly’s world is pretty low resolution because small heads can only hold a limited number (usually hundreds to thousands) of faces, and there’s no easy way to sharpen their blurry vision down to the millions of pixels that humans can effectively see. But despite this rough resolution, flies see and process fast movements very quickly.
From how fast their photoreceptors can process light, we can infer how animals perceive fast motion. Humans perceive up to about 60 discrete flashes of light per second. Any speed usually appears as a steady light. The ability to see discrete flashes depends on lighting conditions and which part of the retina you are using.
For example, some LED lights emit separate flashes of light fast enough to appear as steady light to people unless you turn your head. You may notice a flicker in your peripheral vision. This is because your peripheral vision processes give light faster, but at a lower resolution, such as fly vision.
Remarkably, some flies can see up to 250 flashes per second, about four times more flashes per second than humans can.
If you took one of these flies to the movies, the smooth 24 frames per second movie you’re watching will instantly appear as a series of static images, like a slide show. But this quick vision allows it to react quickly to prey, obstacles, opponents and your swatting attempts.
Our research shows that in dim light, flies lose their ability to see fast movements. This may seem like a good opportunity to crush them, but people also lose their ability to see fast, sharp features in the dark. So you can be as disabled as your target.
When flying in the dark, flies and mosquitoes fly erratically with curvy flight paths to avoid swats. They may also rely on non-visual cues, such as information from tiny hairs on their bodies that detect changes in airflow as you move.
A mosquito flight. Source: Intellectual Initiatives.
So why do flies see slower in the dark? You may have noticed that your own vision becomes heavy and blurry in the dark and is much less colorful. The process is similar for insects. Low light means fewer photons, and just like cameras and telescopes, eyes depend on photons to create images.
But unlike a nice camera that lets you switch to a larger lens and collect more photons in dark environments, animals can’t change the optics of their eyes. Instead, they rely on summation, a neural strategy that combines inputs from neighboring pixels to create an image or increases the time it takes to sample photons.
Large pixels and longer exposures capture more photons, but this costs sharp images. Summary is the equivalent of taking pictures with grainy film (higher ISO) or slow shutter speeds that produce blurry images but prevent underexposure of your subjects. Flies, especially small ones, can’t see fast in the dark because, in a way, they wait for enough photons to arrive until they’re sure they can see it.
In addition to detecting rapidly approaching threats, flies need to fly away in a second. This requires preparation for takeoff and rapid flight maneuvers. For example, fruit flies adjust their stance in one-fifth of a second before takeoff after visually detecting an approaching threat. Predatory flies, such as killer flies, coordinate their legs, wings, and halter (dumbbell-shaped wing remnants used to sense spins in the air) to quickly capture prey mid-flight.
How best to crush a fly
To outrun a fly, you must hit your approaching hand faster than it can detect it. You can improve on this with practice, but flies have developed their escape over hundreds of millions of years. Therefore, it is a better bet to use other ways to manage flies, such as installing fly traps and cleaning backyards, rather than swatting.
You can attract some flies into a narrow-necked bottle filled with apple cider vinegar and beer. Placing a funnel in the bottle neck makes it easier for them to enter but harder for them to escape.
When it comes to mosquitoes, some commercial repellents can work, but eliminating stagnant water around the house—in some plants, pots, or any open container—will help eliminate spawning areas and reduce mosquito numbers from the get-go. Avoid insecticides as they also harm beneficial insects such as bees and butterflies.
Jamie Theobald receives funding from the National Science Foundation (IOS-1750833).