The common sight of moths circling a porch light or a candle flame is a widespread natural phenomenon. The true scientific explanation for why these nocturnal insects approach light is far more complex than simple attraction. It does not involve a conscious desire for the light itself, but rather a malfunction in the insect’s ancient biological navigation system.
Attraction or Orientation Failure?
A moth flying toward a light is not true biological attraction, such as being drawn to a pheromone or nectar. Instead, this movement, known as positive phototaxis, is a disoriented flight resulting from a navigational error. The insect’s internal compass, designed to keep it flying straight, becomes confused by the proximity of a human-made light source. Moths are merely unable to process the visual information correctly.
How Moths Navigate Naturally
Nocturnal moths navigate across long distances using a reliable method known as transverse orientation, or a celestial compass. This system requires the moth to maintain a fixed, constant angular relationship between its body and a very distant light source. For millions of years, this distant light source has been either the moon or the stars. Since the moon or a star is so far away, the light rays reaching the moth are effectively parallel, no matter how far the insect flies.
By keeping the light source at a consistent angle to its eye, such as 30 degrees, the moth can fly a straight path across the night sky. The distance of celestial objects ensures the angle between the light and the moth remains unchanged, providing a stable navigational reference point. This innate mechanism allows the moth to travel efficiently in the dark. Some migratory species, like the Bogong moth, can also use the patterns of stars and the Earth’s magnetic field to orient themselves over vast distances.
The Geometry of the Spiral
When a moth encounters an artificial light source, such as a flame or a lamp, its transverse orientation system fails due to a geometric flaw. The moth instinctively attempts to apply the same fixed-angle rule it uses for the distant moon to this new, nearby light. Because the light source is close, its angle relative to the moth’s eye changes rapidly as the insect flies past it.
To correct for this change and maintain the constant angle, the moth continuously turns inward toward the light. This automatic course correction forces the moth into an ever-tightening flight path, which mathematically describes a logarithmic spiral. The moth is locked into attempting to correct a navigational error that cannot be resolved. This results in a collision with the light source or exhaustion from the spiraling flight pattern.
The specific type of light also exacerbates this disorientation. Moths are highly sensitive to ultraviolet (UV) light, a wavelength abundant in natural celestial light. UV light is also emitted strongly by many artificial sources, including flames and fluorescent bulbs. This strong visual signal overwhelms the moth, mimicking and amplifying the natural celestial cue, which hijacks its navigation system.