Moths are a common type of nocturnal insect that seem mysteriously drawn towards light sources like lamps, candles, and flashlights. This phenomenon has become so well-known that the phrase "like a moth to a flame" refers to someone irresistibly attracted to something that may be harmful to them. But why exactly are moths so captivated by artificial light given that it often leads them to their demise?
Scientists have proposed a few major theories over the years to explain moths' attraction to light. One is that moths use natural light from the moon and stars for navigation during flight. Artificial lights can disrupt this, leading moths to become disoriented. Another possibility is that moths are searching for food around light sources, since their prey is also drawn to light. Additionally, moths may perceive light as floral nectar guides that help lead them to potential mates. While the exact mechanisms are still being researched, these theories provide insight into moths' light-seeking behaviors.
Navigation by moonlight
Moths are able to navigate at night by using visual cues from the moon and stars. Their eyes are specially adapted to detect dim light, allowing them to see well under low light conditions. As they fly, moths keep the moon and celestial objects at a constant angle to maintain their heading. This behavior is called "transverse orientation."
Artificial lights, such as street lamps or porch lights, can disrupt this navigation system. The bright lights overwhelm their sensitive eyes, obscuring the more subtle moonlight. Drawn to the artificial light source, moths end up spiraling around it in loops rather than flying in a straight path. So while we may think of moths mindlessly bashing themselves against light bulbs, they are actually just becoming hopelessly disoriented.
Moth Eyes
Moths have compound eyes that are made up of thousands of tiny lenses called ommatidia. These ommatidia allow moths to see in low light conditions. Each ommatidium is connected to a single photoreceptor cell. The ommatidia combine to provide a mosaic image.
A key feature of moth eyes is that they are able to detect UV light. While humans can only see light in the visible spectrum (400-700 nm wavelength), moths can see light ranging from 300-400 nm in the ultraviolet spectrum. This allows them to find flowers that reflect UV patterns to attract pollinators. It also enables them to navigate effectively at night by the light of the moon, which emits UV light.
Some researchers hypothesize that moths may mistake artificial lights for flowers reflecting UV light or for the moon. This could explain why they are attracted to artificial lights even when those lights do not seem to resemble flowers. The UV wavelengths emitted by artificial lights attract moths that are seeking nectar sources.
Feeding Behavior
Moths are primarily nocturnal creatures that do most of their feeding at night. They are drawn to flowers like jasmine and honeysuckle that produce strong fragrances after dusk. Many flowers and fruits reflect UV light, which helps attract night-flying moths that use UV vision to locate food sources.
Since moths cannot see color very well in low light, they use their sense of smell and vision tuned to UV wavelengths to zero in on plants, fruits, and flowers opening up at night. Certain species like the hummingbird moth have long tube-like tongues perfectly designed to sip nectar from night blooming flowers. Moths act as important pollinators that carry pollen from one flower to another as they move from plant to plant in search of nutrition.
Transverse orientation
Moths have an innate ability to navigate by maintaining a constant angle relative to a distant light source, a behavior known as transverse orientation. This helps them travel in a straight line as they fly at night.
Moths orient themselves by using visual cues to set their internal compass. Their eyes contain light-sensitive receptors that allow them to optically detect light sources and hold a flight path at a fixed angle to those light sources. This prevents them from flying in circles around the light.
Even though moths have compound eyes, they have specialized receptors that point upward to detect light from the moon and stars. This gives them a visual compass reference point to maintain while in flight. By locking onto light sources like the moon, they can keep traveling in their desired direction.
Transverse orientation provides moths with a survival advantage at night. It allows them to efficiently reach feeding grounds, find mates, and migrate over long distances while compensating for drift caused by wind. This hard-wired behavior evolved to take advantage of natural light cues like the moon for nocturnal navigation.
Fatal attraction
Artificial lights at night can prove fatally attractive for many moth species. Their transverse orientation navigation system that relies on maintaining a constant angle to the moonlight can be disrupted by unnatural light sources. Street lights, flood lights, and other outdoor electric lighting attract and disorient night-flying moths. They end up spiraling around the artificial light sources until they eventually exhaust themselves and die.
Some major light sources like skyscrapers take an enormous toll on local moth populations. One study in Germany estimated a single skyscraper was killing 1.6 million moths per year as they became trapped in its fatal artificial light beams.
For moths that rely on moonlight navigation for seasonal migration, light pollution in cities and suburbs can be devastating. Major moth migration routes have declined sharply in recent decades as artificial light sources have proliferated. Conservationists are now promoting "lights out" initiatives during peak migration seasons to reduce the fatal impacts of artificial lights on moth populations.
Magnetic sensitivity
Some research indicates that moths are able to use the Earth's natural magnetic field to orient themselves and navigate at night. The magnetic field provides cues that help moths maintain their directional bearings. This magnetic sense appears to be light-dependent and mediated by crystals of magnetite in the moths' bodies.
Artificial light at night can disrupt the magnetic orientation abilities of moths. In experiments, many moth species lose their sense of direction when exposed to light pollution or artificial lighting. The light fields created by man-made lights distort the natural magnetic field cues that moths rely on for nocturnal navigation. As a result, moths can become disoriented and attracted to the artificial light sources.
The light-induced magnetic disruption offers one plausible explanation for why moths congregate around lights. The artificial light overwhelms their magnetic orientation, leaving them unable to navigate effectively. This causes them to gravitate toward the light sources as navigational beacons. So while moths do not intend to fly directly into lights, the lights interfere with their magnetic guidance system and lead them astray.
Evolutionary perspectives
Attraction to light may be an evolved response that manifests differently across moth species depending on their natural habitats and behaviors. Evolutionary biologists hypothesize that moths navigate using celestial cues from the moon and stars, and their flight paths may have evolved to maintain a transverse orientation to moonlight in order to fly straight. However, artificial lights on the ground disrupt this evolved programming, leading moths to spiral into the light source. This fatal attraction suggests moths haven't adapted quickly enough to evolutionary novelties like human-made lighting.
Additionally, some researchers propose the attraction to light stems back even further evolutionarily - that moths are drawn to light the same way their ancestors were drawn to forest fires and bioluminescent fungi as sources of heat and food. From this view, light attraction is an ancient impulse that served moths well in nature but now leads them astray in human environments. More research is needed to understand the evolutionary origins of this behavior and why certain moth species are more strongly attracted to light than others. Overall, the matter highlights the complex interplay between moth sensory systems, navigation strategies, and evolutionary adaptations that may not match their current environments.
Migratory moths
Some moths use moonlight and other cues to migrate long distances. The agriocule maggots are one species that migrate up to 1,000 miles between Central America and the northern United States every spring and autumn. These moths use the moon's position in the sky as a compass to maintain a straight flight path, allowing them to travel directly between their winter and summer homes.
Researchers have found that the agriocule maggots keep the moon at a constant angle to their bodies as they fly through the night sky. On overcast nights when the moon is obscured by clouds, the moths are able to use faint light from the obscured moon to navigate. Other celestial objects like stars may also assist with orientation.
This moon compass navigation allows the moths to travel quickly and directly between seasonal sites. While artificial lights can disrupt their flight paths, agriocule maggots demonstrate an impressive ability to use natural nighttime light cues to migrate enormous distances. Their seasonal travels are on par with some of the most epic migrations in the animal kingdom.
Conclusion
Moths are drawn to light sources at night for several reasons that are not entirely understood. The leading theories suggest it may relate to navigation, feeding behaviors, and evolution.
Moths use the moon for navigation and transverse orientation at night. Their eyes allow them to see in low light conditions. Artificial lights can disrupt their natural orientation, causing them to spiral towards the light source. This phototactic behavior likely evolved as moths sought out faint flowers and other feeding grounds.
Light sources that mimic the moon used to guide nocturnal moths can fatally attract them. For migratory species, this attraction can waste energy and threaten their migration patterns. There is still much to uncover around moths' sensitivity to different light wavelengths and magnetic fields.
Understanding moth light attraction has ecological importance. Artificial light pollution affects moth reproduction, feeding, and migration. It also impacts moth predators like bats. Further research could help address threats to moth populations from human activity. Overall, the complex moth behaviors we observe around light provide a window into their nighttime adaptations and ongoing evolution.
