A polarization-sensitive response of the flying water bugNotonecta glauca to UV light

Summary

Weak UV light emitted by a horizontal surface below flying backswimmers can cause the animals to turn their flight paths vertically downward, bringing them to the horizontal surface. In the laboratory this response could be elicited even though a second surface above the horizontal surface emitted much more intense UV light (Figs. 1, 2).

The response is polarization-sensitive. Polarized UV light from below oscillating at right angles to the median plane of the approaching animal is more effective than UV light with e-vector parallel to the median plane (Fig. 3).

With the horizontal surface emitting polarized UV light having the e-vector perpendicular to the median plane, 50% of the animals flying over that surface landed on it when the intensity of the light it emitted was only 4.5% of the intensity of the light emitted by the upper surface (Fig. 3). With UV-free light from both surfaces, 50% of the animals landed on the lower surface only when the intensities of upper and lower surfaces were equal (Fig. 4). When the upper surface emitted UV light and the lower surface UV-free light, none of the animals flew downward. Under these conditions the reaction is UV-specific.

The mechanisms eliciting the descent response are adapted to the reflection properties of water surfaces in nature.