Why is killing flies so difficult?

This article was originally presented on The conversation.

Sitting outside on a summer evening always sounds relaxing until the flies and mosquitoes arrive – then the pounding begins. Despite their tiny eyes and brains roughly 1 million times smaller than yours, flies can dodge almost any blow.

Flies can thank their quick, advanced vision and some nervous quirks for their ability to dodge strikes with such speed and agility.

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

Faster vision

Flies have compound eyes. Instead of gathering light through a single lens that makes the entire image—the strategy of human eyes—flies form images made up of multiple facets, many separate lenses that focus incoming light onto clusters of photoreceptors, the light-sensitive cells in their eyes. Essentially, each facet produces an individual pixel of the fly’s vision.

The world of flies is fairly low-resolution because tiny heads can only contain 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 effectively see. But despite this coarse resolution, flies see and process rapid movements very quickly.

We can infer how animals perceive fast motion from how quickly their photoreceptors can process light. Humans recognize a maximum of about 60 individual flashes of light per second. Anything faster usually appears as a steady light. The ability to see individual 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 to appear as a steady light to people—unless you turn your head. You may notice flickering in your peripheral vision. This is because your peripheral vision processes light faster but with lower resolution, like the vision of a fly.

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

If you take one of these flies to Cineplex, the smooth 24-frame-per-second movie you’ve been watching will immediately appear as a series of still images, like a slide show. But this quick vision allows him to react quickly to prey, obstacles, competitors and your strike attempts.

Our research shows that flies in low light lose some ability to see fast movements. This might sound like a good opportunity to hit them, but people also lose their ability to see quick, sharp features in the dark. So you can be just as disabled as your target.

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

Mosquito flight. 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 and much less colorful. The process is similar in insects. Dim light means fewer photons, and just like cameras and telescopes, eyes depend on photons to make images.

But unlike a nice camera that lets you switch to a bigger lens and collect more photons in dark settings, animals can’t change the optics of their eyes. Instead, they rely on summation, a neural strategy that adds together the inputs of neighboring pixels or increases the sampling time of photons to form an image.

Large pixels and longer exposures capture more photons, but at the cost of sharp images. Summing is equivalent to taking photos with grainy film (higher ISO) or slow shutter speeds, which create blurrier images but avoid underexposing your subjects. Flies, especially small ones, cannot see quickly in the dark because, in a sense, they wait for enough photons to arrive until they are sure of what they see.

Flight maneuverability

In addition to quickly perceiving impending threats, flies must be able to fly away in a fraction of a second. This requires take-off preparation and quick flight maneuvers. After visually detecting a looming threat, fruit flies, for example, adjust their posture in a fifth of a second before taking off. Predatory flies, such as assassin flies, coordinate their legs, wings, and quills—dumbbell-shaped remnants of wings used to sense rotation in midair—to quickly catch their prey in flight.

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

How best to kill a fly

To outsmart a fly, you have to hit faster than it can detect your approaching hand. With practice you might 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, like installing fly traps and cleaning up backyards, is a better bet.

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

You can lure certain flies into a bottle with a narrow neck filled with apple cider vinegar and beer. Putting a funnel in the neck of the bottle makes them easy to enter but difficult to escape.

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