Having found that the insects responded strongly to polarized light, the team next tested the midges' preferences under more natural circumstances. Knowing that cloudy water reflects much more polarized light than clear tap water, they offered midges four more choices of bright and dark water, this time varying the degree of reflected polarization by using either tap or cloudy pond water. The results were even more clear cut. Virtually no midges laid their eggs in the unpolarized tubs of clear water, while the number of eggs laid in the tubs of cloudy water reflecting polarized light was proportional to the percentage of polarized light reflected, regardless of the intensity.
So why are midges so strongly attracted to polarized reflections? According to Lerner, polarization is a reliable cue at sunset. This is particularly important for short-lived female midges that only have a matter of hours to find water and lay their eggs when the light is fading.
And there could be another reason for the midges' polarization preference. Lerner explains that the reflections from cloudy water are highly polarized. Could a high level of polarization in reflections be related to the amount of nutritious organic matter in the water? By measuring the polarization of reflections from increasingly cloudy water samples, it was clear that the cloudiest water produced the most polarized reflections, suggesting that the water offers the best start in life to the midges' larvae, and their cholera bacteria hitchhikers.
The team are also keen to point out that their discovery could help control midge numbers and minimise cholera transmission. They suggest that by reducing the proportion of polarized light reflected by water supplies and offering midges cloudy, polarized light reflecting alternatives to lay their eggs in, we could limit the spread of future cholera epidemics.
|Contact: Kathryn Phillips|
The Company of Biologists