This article was originally conversation.

Sitting outside on a summer night sounds relaxing until the flies and mosquitoes arrive. Despite having very small eyes and a brain about a million times smaller than a human, flies can fend off almost any attack.

Flies can thank their fast and sophisticated eyesight and some nervous quirks for their ability to escape swat with such speed and agility.

Our lab is investigating insect flight and vision, discovering how such tiny creatures process visual information to perform challenging behaviors, such as quickly escaping from a swatter. It aims to

faster vision

Flies have compound eyes. Rather than focusing light through a single lens to create an entire image (a human eye strategy), flies form an image built from multiple facets. This is a large number of individual lenses that focus incoming light onto clusters of photoreceptors, the light-sensing cells of the eye. Essentially, each facet produces an individual pixel of vision in flies.

The fly world is pretty low resolution. Because a small head can only accommodate a limited number of facets (usually hundreds to thousands), there is no easy way to sharpen the fly’s blurry vision to the millions of pixels effectively seen by humans. is. But despite this coarse resolution, flies recognize and process fast movements very quickly.

From how fast an animal’s photoreceptors can process light, we can infer how an animal perceives fast movement. Humans discern up to about 60 individual flashes per second. Faster velocities are usually seen as stationary light. The ability to see individual flashes depends on lighting conditions and the part of the retina used.

For example, some LED lights shoot discrete flashes so fast that they appear to be steady light to humans unless you turn your head. In peripheral vision, you may notice flicker. This is because peripheral vision processes light more quickly, but with less resolution, like fly vision.

Amazingly, some flies can see as many as 250 flashes per second, about four times as many flashes per second as humans can perceive.

Bring one of these flies to a cineplex and you’ll instantly see a smooth movie made up of 24 frames per second as a series of still images, like a slide show. However, this fast vision allows you to react quickly to prey, obstacles, competitors, and your swatting attempts.

Our research shows that flies in dim light lose some of their ability to see fast movement. You lose the ability to see sharp features quickly within.

When a fly or mosquito flies in the dark, it flies erratically and follows a tortuous flight path to escape the swat. They can also rely on non-visual cues, such as information from tiny hairs on their bodies that sense changes in airflow when you move to attack.

Mosquito flight. Source: Intellectual Ventures.

neural trick

But why do flies see more slowly in the dark? You may have noticed that your vision becomes dull, blurry and colorless in the dark. This process is similar to that of insects. Less light means less photons, and just like cameras and telescopes, our eyes rely on photons to create images.

However, unlike good cameras that can switch to larger lenses and collect more photons in dark environments, animals cannot swap the optics of their eyes. It relies on summation, which is a neural strategy that sums the input of the pixels to be processed or increases the time to sample the photons.

Larger pixels and longer exposures capture more photons, but at the expense of a sharper image. Addition is equivalent to taking pictures with grainy film (higher ISO) or slower shutter speeds . Flies, especially small flies, cannot see quickly in the dark. In a way, we’re waiting for enough photons to arrive until we’re sure what we’re looking at.

flight maneuverability

Flies need to be able to quickly sense an approaching threat, as well as fly away in an instant. This requires preparation for takeoff and rapid flight maneuvers. For example, after visually detecting an looming threat, a fruit fly adjusts its posture in five seconds before taking off. Predatory flies, such as killer whales, adjust their legs, wings, and halters (dumbbell-shaped wing remnants used to sense rotation in the air) to quickly catch prey in flight.

best way to swat a fly

To outsmart a fly, you have to attack faster than the fly can detect the approaching hand. With practice this could be improved, but flies have been honing escape routes for hundreds of millions of years. So instead of swattering the flies, you’re better off managing flies in other ways, such as setting up fly traps or cleaning your backyard.

Certain flies can be lured into narrow-necked bottles filled with apple cider vinegar and beer.

As for mosquitoes, some over-the-counter repellents may work, but removing stagnant water around your home can reduce mosquito populations in the first place, leaving them nowhere to lay eggs. Avoid pesticides as they also harm beneficial insects such as bees and butterflies.

Jamie Theobald is funded by the National Science Foundation (IOS-1750833).