Flies can thank their quick, sophisticated eyesight and certain neural quirks for their ability to evade blows with such speed and agility.
A fly’s world is quite low resolution, as small heads can only hold a limited number of facets —
usually hundreds to thousands — and there’s no easy way to narrow down their blurry vision to the millions of pixels people actually see. But despite this coarse resolution, flies see and process fast movements very quickly.
We can infer how animals perceive fast movements from how quickly their photoreceptors can process light. Humans discern a maximum of
approximately 60 discrete flashes of light per second. Any faster speed usually appears as a steady light. The ability to see faint flashes depends on the lighting conditions and the part of the retina you are using.
Some LED lights, for example, emit discrete flashes of light fast enough that they appear as a constant light to humans – unless you turn your head. In your peripheral vision, you may notice a flicker. This is because your peripheral vision processes light faster, but at a lower resolution, like the vision of flies.
Remarkably, some flies can see up to
250 flashes per secondabout four times more flashes per second than people can perceive.
If you took one of these flies to the cineplex, the smooth 24-frame-per-second movie you watched would appear, on the fly, as a series of static images, like a slideshow. But this quick vision allows it to react quickly to prey, obstacles, competitors and your attempts to crush.
Our research show it flies
in dim light, losing some ability to see fast movements. This might seem like a good chance to crush them, but humans also lose their ability to see quick, sharp strokes in the dark. So you can be just as handicapped as your target.
When they fly in the dark, flies and mosquitoes
fly erratically, with twisty flight paths to evade swats. They can also rely on
non-visual cuessuch as information from small hairs on their bodies that detect changes in air currents as you move to strike.
neural stuff
But why do flies see more slowly in the dark? You may have noticed that your own vision becomes slow and blurry in the dark, and much less colorful. The process is similar for insects. Low light means
fewer photonsand just like cameras and telescopes, eyes depend on photons to create images.
But unlike a nice camera, which lets you upgrade to a bigger lens and collect more photons in dark environments, animals can’t swap the optics of their eyes. Instead, they rely on
additiona neural strategy that sums inputs from neighboring pixels, or increases the time they sample photons, to form an image.
Larger pixels and longer exposures capture more photons, but
at the cost of sharp images. Summing is equivalent to shooting with grainy film (higher ISO) or slow shutter speeds, which produce blurrier images, but avoid
underexpose your subjects. flies,
especially the little onescannot see quickly in the dark because, in a sense, they wait for enough photons to arrive until they are sure of what they are seeing.
Handling in flight
In addition to quickly perceiving imminent threats, flies must be able to take flight in a fraction of a second. This requires preparation for take-off and
fast flight maneuvers. After visually detecting an imminent threat, fruit flies, for example, adjust their posture by
a fifth of a second before taking off. Predatory flies, such as
killer fliescoordinate their legs, wings, and halteres—dumbbell-like remnants of wings used to sense rotations in the air—to quickly grab prey in midair.
How to best swat a fly
To outsmart a fly, you must strike faster than it can detect your approaching hand. With practice you can get better, but flies have perfected their evasions over hundreds of millions of years. So, instead of swatting, it is better to use other means of managing flies, such as setting fly traps and cleaning backyards.
You can attract some flies to a narrow-necked bottle filled with apple cider vinegar and beer. Placing a funnel in the neck of the bottle allows them easy entry but difficult to escape.
When it comes to mosquitoes, some commercial repellents may work, but eliminating standing water around the house – in some plants, pots, or any open container – will help.
eliminate their egg-laying sites and reduce the number of mosquitoes around from the start. Avoid insecticides, as they
harm beneficial insects like bees and butterflies.
Jamie Theobald is an associate professor of biological sciences at Florida International University. Ravindra Palavalli-Nettimi is a postdoctoral research associate at this university. Theobald receives funding from the National Science Foundation. Palavalli-Nettimi does not work for, consult, own stock or receive funding from any company or organization that would benefit from this article, and has not disclosed any relevant affiliation beyond a name university. Florida International University provides funding as a member of The Conversation US.
