Migratory desert locusts avoid reflective bodies of water
Feared as a biblical crop pest, African desert locusts (Schistocerca gregaria) appear to detect polarized reflections of light off large expanses of water and then try to avoid having to fly over them at low altitudes.
This is the finding of natural and experimental observations by a team of scientists led by Nadav Shashar from the Hebrew University in Eilat. The researchers became aware of this phenomenon when, in November 2004, during the insects' last intensive flight phase, a large swarm of locusts arrived in the Gulf of Aqaba from the Sinai desert. On the shore, the animals avoided flying out onto the water, instead turning ninety degrees north until they reached the tip of the gulf. Only when they got there did most of the swarm switch back east toward the Jordan.
It was already known that locusts are sensitive to linearly polarized light. Your compound eye in the rear area has specialized groups of individual eyes (omatidia), which are only intended to record and evaluate the pattern of polarized skylight in order to allow the animals to navigate in this way during migrations. In order to test the actual sensitivity of the insects to reflections, the entomologists caught some desert locusts, tied them with long ropes and released them again in the immediate vicinity of mirrors lying on the ground, while one of the researchers used a compass to determine the direction of flight monitored.
In the vast majority of cases, the animals avoided flying over the mirror altogether. Only when the take-off point was north of the test area was it crossed by a few specimens, which the researchers attribute to the prevailing winds, which came mainly from the south. So the animals had to choose between avoiding the mirror and flying against the wind, which they prefer to do. And in a second attempt, when the locusts had to choose between a polarizingly reflective surface and one that didn't reflect polarized light, they flew over the latter far more often. So they also appear to be able to see polarized light in their ventral visual field.
Through this special sensitivity to light, the insects recognize sources of danger, such as large areas of water. However, Schistocerca gregaria can also cross seas provided they are driven by strong winds and travel at altitudes of several hundred meters. In this way, the insects have already reached Italy or the Canary Islands.
