Millions of years passed, and life on Earth, including tiny life forms such as bacteria, adapted to the change. But when the amount of UV radiation increases dramatically, they can't stand it, and the amount of UV radiation shatters their structures. Finally the human also deciphered this secret, hence the hospital USES ultraviolet ray sterilization. This is something that humans learned two billion years later than plants.

Moonlight or sunlight is basically parallel light, and the insect's flight path is straight as long as it makes sure that the Angle between its flight direction and the light is a fixed value. For eons, insects have used this principle to navigate for themselves.

But human, point light source is more and more in the night, point light source the light is not parallel light, insects don't know, they also thought it was the stars in the sky, to continue in accordance with the "flight direction and Angle of light for a fixed value" navigation, flight path is no longer a straight line, and became a logarithmic spiral, swirling toward light source.