Zebra Stripes, Horseflies, and Optical Illusions

Zebra stripes appear to create optical illusions for horseflies and might cause them to avoid approaching altogether.
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zebra stripes
Zebra stripes appear to create optical illusions for horseflies and might cause them to avoid approaching altogether. | Photo: iStock
When it comes to zebra stripes and flies, it’s all about optical illusions. Researchers know that zebra stripes deter flies and probably affect their vision, too. But what flies are—or aren’t—seeing when they spot a zebra still remains a mystery.

It appears, however, that researchers have eliminated one hypothesis from the list of possibilities: They recently determined that it’s unlikely that polarization is what confuses flies, said Kenneth Britten, PhD, of the Department of Neurobiology, Physiology, and Behavior at the University of California, Davis (UC Davis).

Polarization is a physics concept that—very simply put—describes the way the electric vectors in light waves move in certain distinct patterns. While humans can’t detect that polarization, horseflies (“tabanids”) can. Considered “polarotactic,” horseflies are very attracted to certain kinds of polarization. The horizontal polarization of water-reflected light, for example, leads them to water sources where they can mate, reproduce, and rehydrate. Linear polarization leads blood-sucking female horseflies to food sources such as equids and cattle.

For years, many researchers believed zebra stripes affected light polarization in a way that repelled flies. But scientists are now shedding a new angle of light on that theory

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Passionate about horses and science from the time she was riding her first Shetland Pony in Texas, Christa Lesté-Lasserre writes about scientific research that contributes to a better understanding of all equids. After undergrad studies in science, journalism, and literature, she received a master’s degree in creative writing. Now based in France, she aims to present the most fascinating aspect of equine science: the story it creates. Follow Lesté-Lasserre on Twitter @christalestelas.

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