Why is it difficult to fly?

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Sitting outside on a summer evening always sounds cozy until the flies and mosquitoes come – then the swatting begins. Despite their tiny eyes and brains about 1 million times smaller than yours, flies can survive almost every swat.

Flies can thank their fast, sophisticated vision and some neural quirks for their ability to escape swats with such speed and agility.

Our lab investigates insect flight and vision, with the goal of discovering how such tiny creatures can process visual information to perform challenging behaviors like running away from your swatter so quickly.

quick sight

Flies have compound eyes. Instead of collecting light through a single lens to form an entire image—a strategy of the human eye—flies form images made up of multiple, individual lenses that focus incoming light onto clusters of photoreceptors, the light-sensing cells in their eyes. Essentially, each aspect produces an individual pixel of flight vision.

The world of flies is very low-resolution, because tiny heads can only hold a limited number of facets—typically hundreds to thousands—and there’s no easy way to sharpen their blurry vision to the millions of pixels that humans can effectively see. But despite this coarse resolution, flies see and process movements quickly.

We can infer how fast animals can process light with their photoreceptors. Humans perceive a maximum of 60 separate flashes of light per second. Anything faster is usually seen as a steady light. The ability to see individual flashes depends on the lighting conditions and which part of the retina you use.

Some LED lights, for example, emit discrete flashes of light so quickly that they appear to people as a steady light – until you turn your head. You may see flickering in your peripheral vision. That’s because your peripheral vision processes light faster, but at a lower resolution, like fly vision.

Remarkably, some flies can see up to 250 flashes per second, four times more flashes per second than humans can see.

If you take one of these flies to a Cineplex, the smooth movie you’re watching, made at 24 frames per second, will appear on the fly as a series of still images, like a slide show. But this quick vision allows it to react quickly to your attempts at hunting, obstacles, competitors, and swatting.

Our research shows that flies lose their ability to see rapid movements in dim light. This may seem like a good opportunity to swat them, but people also lose the ability to see sharp, sharp features in the dark. So you can be as handicapped as your target.

When they fly in the dark, flies and mosquitoes fly uncontrollably, with meandering flight paths to escape the swaths. They can also rely on non-visual cues, such as information from the tiny hairs on their bodies that sense changes in air currents as you strike.

A flight of mosquitoes. Source: Intellectual Enterprise.

Nervous movements

But why do flies see slowly in the dark? You may notice that your vision becomes dull and blurry in the dark, and becomes much less colorful. The process is similar for insects. Less light means fewer photons, and like cameras and telescopes, eyes rely on photons to form images.

But unlike a good camera, which allows you to switch to a bigger lens and collect more photons in darker settings, animals can’t swap their eye optics. Instead, they rely on summation, a neural strategy that adds the inputs of neighboring pixels together, or increases the time they sample photons to form an image.

Larger pixels and longer exposures capture more photons, but at the cost of sharper images. Summation is equivalent to shooting with grainy film (high ISO) or slow shutter speeds, which produce blurry images, but avoid underexposing your subjects. Flies, especially small ones, cannot see quickly in the dark because, in a sense, they are waiting for enough photons to arrive until they are sure what they are seeing.

Flight dynamics

In addition to quickly sensing emerging threats, flies need to be able to fly in a split second. This requires preparation for take off and quick flight maneuvers. For example, when the fruit appears to be in danger, adjust your posture a fifth of a second before takeoff. Predatory flies, such as killer flies, coordinate their legs, wings, and halteres–the dumbbell-shaped remnants of the wings used to detect turns in the wind–to quickly catch their prey midflight.

Flight of the fly. Notice how they adjust their posture before takeoff. Source: New York Times.

How best to swat flies

To chase the fly, you have to strike faster than you can detect the hand closest to you. With practice, you can improve on this, but flies have honed their escape skills over hundreds of millions of years. So, instead of using other methods to manage flies, installing fly traps and cleaning the backyard is a good bet.

Escape behavior of slow moving flies. Source: Florian Muijres et al, 2014 Science.

You can hide some flies in a narrow-necked bottle filled with apple cider vinegar and beer. Placing a funnel around the bottle neck makes it easier for them to get in, but harder to escape.

For mosquitoes, some commercial repellants can work, but removing standing water around the house—on some plants, pots, or any open containers—will help eliminate their egg-laying sites and reduce the number of mosquitoes around in the first place. Avoid pesticides, as they also harm beneficial insects such as bees and butterflies.

Jamie Theobald receives funding from the National Science Foundation (IOS-1750833).

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