Insects gathering around artificial lights at night is a common observation. This attraction, however, is a complex interaction rooted in insect biology and evolutionary adaptations. Understanding why these creatures are drawn to light involves delving into their unique visual systems and their reliance on natural light sources for navigation and survival. These mechanisms reveal how human innovation can inadvertently impact insect behavior.
How Insects See Light
Insects possess a visual system distinct from humans, relying on compound eyes. These eyes are composed of numerous individual light-sensing units called ommatidia. Each ommatidium contains a lens and photoreceptor cells that detect light, providing a wide-angle, mosaic-like view.
Insects are sensitive to ultraviolet (UV) light, which is invisible to the human eye. Many insects have specialized photoreceptors tuned to the UV spectrum, particularly UVA and sometimes UVB. Some insects, like honeybees, have photoreceptors with peak sensitivity around 340-350 nm, in addition to blue and green light sensitivity.
The Evolutionary Purpose of Attraction
The primary theory explaining insect attraction to light centers on their reliance on celestial navigation. Nocturnal insects, such as moths, use distant natural light sources like the moon and stars for navigation. They maintain a constant angle to these far-off light sources to fly in a straight line, a behavior known as transverse orientation. Since the moon is extremely distant, its light rays are parallel, allowing insects to maintain a consistent angle as they fly.
Artificial lights, however, disrupt this natural navigational system. When an insect encounters a close artificial light source, the angle to the light changes significantly over short distances. The insect instinctively attempts to correct its flight path to maintain the constant angle it would use with a celestial body, resulting in a disoriented, spiraling flight pattern that draws it closer to the light. This behavior is often described as a “fatal attraction” because it can lead to exhaustion, predation, or collisions. Some researchers also propose that light may signal an open space or an escape route from predators, contributing to a positive phototactic response.
Beyond Just UV Light
While UV light is a strong attractant for many insects, it is not the sole factor drawing them to artificial illumination. Other characteristics of artificial light, including wavelengths within the visible spectrum, polarization, and heat, can also play a role. For instance, blue light, which is close to the UV spectrum, is particularly attractive to many insects, including moths, flies, and mosquitoes. This is because insect compound eyes are highly sensitive to shorter wavelengths.
Different artificial lights vary in their spectral output, influencing their attractiveness. Older incandescent bulbs, for example, emit heat in addition to light, which can attract some insects like mosquitoes. LEDs, while emitting less heat and UV light, can still attract insects depending on their wavelength output. Lights with a high blue spectrum or those that combine UV and blue light are highly attractive. Some insects can also detect polarized light, which is reflected off surfaces like water or leaves, and this can also influence their behavior.
Varied Insect Responses
Not all insects are attracted to UV light or light in general in the same manner; responses vary by species, physiology, and ecological niche. Many nocturnal insects, such as moths, flies, and mosquitoes, are strongly attracted to UV and blue light.
Conversely, some insects exhibit negative phototaxis, meaning they are repelled by light. Cockroaches, for example, move away from light sources and prefer dark spaces. Some beetles, like ground beetles, are active at night and may fly to lights, while others might avoid them. The statement that “all bugs are attracted to UV light” is an oversimplification, as different species have diverse visual adaptations and behavioral responses to light in their environment.