Insects' Phototaxis

By Sylvia Yang '20

Have you ever wondered why insects are attracted to bright light? Did you think they fly straight to the light source? Actually, you misread the insect phototaxis! Especially for moths, we think that insects like to chase the light; in fact, that was a misunderstanding for thousands of years.

In the darkness, moths can't see their surroundings. If no suitable reference material can be found, how can it find a faster and safer way to get to its destination? In reality, insects such as moths that are active at night have been guided by moonlight and starlight for hundreds of millions of years. Because it is a very distant light source, light can be seen as parallel light to the ground and can be used as a reference to make a straight flight. As shown in the figure below, note that as long as the moth is flying at a fixed angle, it can fly a straight line, and straight lines are the most energy-saving. With slight adjustments, it can fly straight to another object.

But since humans learned to use fire, these artificial light sources were so close that the light became central and radial, and the poor moth began to have unlucky journeys. The moth thinks that flying at a fixed angle with the light is a straight line, but since the light source is so close, the result is that it flies in an equiangular spiral into the fire. This phenomenon is also known as insects' phototaxis.