Like a Moth to a Flame by Yunpeng (bob) Li

Like a Moth to a Flame

By Yunpeng (Bob) Li

The old saying “like a moth to a flame” perfectly describes the irresistible attraction from something extremely harmful, just like moths being attracted by a flame. People have long considered that insects, especially flying insects, have a natural instinct of flying toward light, yet the reason for that is unknown. This instinct was widely used for collecting insects as well as eliminating pests, and the theory of instinct was generally accepted by the majority of people, until one day the route of moths flying around light was observed.

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Moths, as seen in this picture, fly in circles around the light, instead of a straight line to the light. This raised questions to the widely accepted theory: if moths were naturally attracted by light, why would they fly in circles around it? Today it is still unclear why insects are attracted by light; a number of theories were proposed, and the most popular one suggests that this is related to the navigation system of insects.

After thousands of years of evolution, insects developed their own system of navigation using natural lights called transverse orientation. Because the sun, the moon, and stars are extremely far away from the earth, light beams coming from them are almost parallel; insects would be able to fly in a straight line if they follow a constant angle relative to the light beams. Changes in direction could be made through changing the degree of angle between the flying route and the light beams. This system provided insects with the best capability of saving energy, because flying in a straight line to a target has the least distance. However, the system that works perfectly in nature becomes problematic with artificial lights. Lights generated by human are all point-source lights, from which light beams are emitted in a radial pattern. Insects, following a constant angle with these beams, are not able to fly in a straight line, but instead, a spiral.

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It seems that this theory of transverse orientation perfectly explains how insects are attracted by light. However, it was later discovered that the navigation of insects is actually affected by much more factors than we previously thought, especially for those species that migrate over winters. People once believed that the migration of insects is totally a result of downwind displacement. They doubted insects’ ability to guide their way toward a compass direction, until analytical experiments demonstrated that insects actively orientate their direction of migration. Taking into account the composition of insects’ compound eyes and the nervous system in insects that converts skylight information into cues of direction, a complex model of the mechanism of insect migration was presented.

The migration of insects is performed with different purposes: some species migrate to a relatively fixed destination to spend the winter; others migrate to find the most favorable habitat. For those who migrate for the best-fit habitat, their mechanism of navigation is simply transverse orientation. Their migration direction shifts clockwise along with the movement of the sun throughout the day at a rate of approximately 15 degrees per hour, which allows them to cover the most area with the minimum amount of time and energy. For those who migrate to a certain destination, compensation for changes in the sun’s azimuth during the day is required in order to keep in an unchanged migration direction.