Why is swatting flies so hard?

This article was originally published in the conversation

Sitting outside on a summer evening always sounds relaxing until the flies and mosquitoes arrive, and then the whack starts. Despite their tiny eyes and a brain about 1 million times smaller than yours, flies can evade almost every blow.

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

Our lab investigates insect flight and vision, with the goal of finding out how these tiny creatures can process visual information to perform difficult behaviors, like escaping your killer so quickly.

Faster vision

Flies have compound eyes. Instead of collecting light through a single lens that makes the whole image, the strategy of human eyes, flies form images built from multiple facets, many individual lenses that focus incoming light into clusters of photoreceptors , the sensory cells of their eyes. Essentially, each facet produces an individual pixel of the fly’s view.

A fly’s world is fairly low-resolution, because tiny heads can only accommodate a limited number of facets, typically hundreds to thousands, and there’s no easy way to fine-tune their blurry vision down to millions of pixels. that people actually see. But despite this coarse resolution, flies see and process fast movements very quickly.

We can infer how animals perceive fast movement from how quickly their photoreceptors can process light. Humans discern a maximum of about 60 discrete flashes of light per second. Anything faster usually appears as a steady light. The ability to see discrete flashes depends on the lighting conditions and which part of the retina you are using.

Some LED lights, for example, emit discrete flashes of light fast enough that humans appear as steady light unless you turn your head. You may notice a flicker in your peripheral vision. This is because your peripheral vision processes light faster, but at a lower resolution, like fly vision.

Amazingly, some flies can see up to 250 flashes per second, about four times more flashes per second than people can perceive.

If you took one of these flies to the cineplex, the fluid movie you watched, consisting of 24 frames per second, would appear as a series of static images, like a slide show. But this quick vision allows him to react quickly to prey, obstacles, competitors and your attempts to strike.

Our research shows that flies in low light lose some ability to see fast movements. It may seem like a good opportunity to hit them, but humans also lose their ability to see fast and sharp features in the dark. So you may be as disabled as your target.

When flying in the dark, flies and mosquitoes fly erratically, with winding flight paths to escape being hit. They may also rely on non-visual cues, such as information from tiny hairs on their body that detect changes in air currents when you move to strike.

Flight of a mosquito. Source: Intellectual Ventures.

Neural tricks

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, let alone colorful. The process is similar for insects. Low light means fewer photons, and just like cameras and telescopes, eyes rely on photons to make images.

But unlike a good camera, which lets you switch to a bigger 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 adds inputs from neighboring pixels, or adds up the time that photons are sampled, to form an image.

Larger pixels and longer exposures capture more photons, but at the expense of sharp images. The sum equates to shooting with grainy film (higher ISO) or slow shutter speeds, which produce blurrier images but avoid underexposing 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 of what they see.

Maneuverability of flight

In addition to quickly perceiving imminent threats, flies must be able to fly in a split second. This requires take-off preparation and quick flight maneuvers. After visually detecting an impending threat, fruit flies, for example, adjust their posture in a fifth of a second before takeoff. Predatory flies, such as assassin flies, coordinate their legs, wings, and halters (dumbbell-like wing remnants used to detect mid-air rotations) to quickly catch their prey in mid-flight.

Flight of a fly Watch how they adjust their posture before take off. Source: The New York Times.

The best way to swat a fly

To outrun a fly, you have to hit faster than your oncoming hand can detect. With practice, you can get better at this, but flies have perfected their escapes over hundreds of millions of years. So instead of swiping, using other ways to manage flies, such as installing fly traps and cleaning up backyards, is a better bet.

Escape behavior of a fly in slow motion. Source: Florian Muijres et al, 2014 Science.

You can attract certain flies to a narrow-necked bottle filled with apple cider vinegar and beer. Placing a funnel on the neck of the bottle makes it easy for them to get in, but difficult for them to escape.

As for mosquitoes, some commercial repellants may work, but removing standing water from the house (on some plants, pots, or any open containers) will help eliminate their egg-laying sites and reduce the number of mosquitoes around from the start. Avoid insecticides, 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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