Night insects fly around the lamp because of the laws of geometrical optics

Night insects during flight guided by the moon and the stars. These light sources are far away from the Earth, and they emitted rays are virtually parallel. To fly in a straight line, the insect can withstand a constant angle between the direction of its movement and direction of the light. But the rays of light from artificial sources diverge in all directions. Therefore, keeping in relation to them a constant angle, the insect does not fly in a straight line, but on a logarithmic (isogonally) spiral. If the spiral rays intersect at an acute angle, it will shrink to a source of rays when an obtuse - to disperse. If the angle of the line, it will degenerate into a spiral circle. Indeed, following the flight of insects, you will notice that they usually do not fly straight to the source of light, and approaching him on a converging spiral or fly circles. In fact, insects tend to move right, but the laws of geometrical optics keep them near the lamp. Sometimes, though, they suddenly turned to the side, but also a new direction again spiraling. If an insect has chosen a course at an angle greater than a right, it will fly in a divergent spiral until he lost sight of the light source. Such moths around we do not see light.

via factroom.ru