When the light reaching their eyes is not the same on both sides, one eye is getting more light than the other, they will turn toward that direction in an attempt to fly straight. However, no matter how they turn, the light will keep striking one eye more than the other, except when they are headed straight toward or away from the light. So they are very confused. Animals like moths have never adapted to fly at night in the presence of a bright light source such as a light bulb. They normally would fly at night with all areas of the sky illuminated pretty much equally, and would know "up" from "down" by the brighter light coming from the moon. That's why they have a hard time flying away from the light also. They basically circle it to exhaustion.