Tar Babies

Adult mayflies have only a few hours in which to find a mate and reproduce. Worse still, reflected light is deceiving them into laying their eggs on roads instead of in rivers, Hungarian researchers have found.

Direct sunlight reflected off the surface of water is strongly polarised in the horizontal plane. Many water-dwelling insects use this polarised light to identify open stretches of water where they can lay their eggs during their brief mating period.

Entomologist Sándor Andrikovics of Eszterházy Teachers' Training College in Eger, Hungary, noticed swarms of mayflies laying eggs on the surface of local roads, where their broods quickly dried up and perished. Now, together with biophysicists György Kriska and Gábor Horváth of Eötvös University in Budapest, Andrikovics has found that light reflected by dry asphalt roads is also horizontally polarised, possibly explaining their allure.

To confirm this, the researchers offered mayflies various surfaces for laying their eggs, and measured how strongly each one polarised the light it reflected. They found that the insects swarmed more frequently above a shiny black plastic sheet than other shiny and matt surfaces. According to the researchers' report in The Journal of Experimental Biology (vol 201, p 2273), so many mayflies were mating and landing on the shiny black sheet that it sounded like "raindrops rattling on the plastic".

The light reflected from the shiny black plastic was more strongly polarised than light from the other test surfaces. But when the scientists mounted the plastic vertically, rotating the plane of polarisation, the mayflies were no longer attracted to it.

Author: Jon Copley

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Contact: Claire Bowles
claire.bowles@rbi.co.uk
44 171 331 2751
New Scientist
22-Jul-1998