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How horseflies find blood meals in dark environments

Last year Susanne Åkesson, biologist at Lund University in Sweden, was awarded the Ig Nobel Prize together with colleagues from Hungary. They received the award for showing that dark horses suffer more than white ones from horseflies. Now the scientists know how horseflies find dark-coated animals even in a dark environment.
Horseflies.
Horseflies. Photo: Susanne Åkesson

When the sun shines on black and dark brown horses and cattle their coat reflects polarized light. Female horse flies need blood meals for producing eggs, and horseflies (tabanidae) are attracted by the polarized light. The answer why light-coated animals gain a certain protection from being bitten lies in their coat, that does not reflect polarized light. These findings were rewarded with the Ig Nobel Prize last year.

Susanne Åkesson, professor at the Department of Biology in Lund, together with her colleagues from Hungary, have taken their research one step further. Now they can show why it is easy for the horseflies to find dark-coated host animals even when these animals stand in front of vegetation with very dark patches.

Dark-coated animals reflect a lot of polarized light. Dark leaves and other vegetation does not reflect polarized light to the same degree.

”In this study we show that the polarization from a dark-coated animal is different compared to polarized light from other dark patches in the surrounding,” says Susanne Åkesson.

”Previously we have shown that horseflies see and are attracted by polarized light. But we have not shown how the polarized light from a dark-coated animal differs from other polarized light.”

black cows on a meadow.
The polarization from a dark-coated animal differs from other polarized light in the surroundings. That is why horseflies easily find dark cows and horses in a dark environment. Photo: Susanne Åkesson

The researchers have used a dark model cow and placed it in front of different sorts of vegetation and various light conditions. They have measured the polarized light from the vegetation as well as the model cow. The results show that when the sun shines on the cow the degree of polarized light is higher. This makes it easy for the horseflies to identify the cow among all other dark patches they can see. The result is the same when the researchers measure polarization from a living cow under the same conditions.

Thus, the most effective way for a female horsefly to find a host animal is to use its vision and sensitivity for polarized light in order to find sunlit, dark patches that reflect a lot of polarized light. It is a big chance that such patches represent animals that the horsefly can feast on.

”A dark cow in bright sunshine makes it easy for the horsefly,” Susanne Åkesson says.

The results have recently been published online in an article in Royal Society Open Science.

Jan Olsson

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