The unexpected reason behind insects circling lights at night. Since ancient times, humans have gathered around fires, noting how light at night can attract a swirling swarm of insects. This phenomenon has transcended into art, music, and literature, symbolizing irresistible yet perilous attractions. Observing these insects' frenzied flight patterns suggests they're ensnared rather than navigating for survival.
Despite centuries of observation, the reason behind this behavior remained elusive. As researchers focused on flight, vision, and evolution, we employed advanced tracking methods, detailed in our publication in Nature Communications, to unravel this mystery.
Numerous old theories attempting to explain this mesmerizing behavior have fallen short. One early idea suggested that insects were drawn to the heat of a flame, akin to pyrophilic species attracted to fire for survival. However, most insects attracted to light aren't of this category, and they're just as drawn to cooler light sources.
Another notion was phototaxis, where insects move towards light to escape darkness or confining environments. Yet, if this were the case, one would expect them to approach the light source directly, which doesn't account for their erratic circling patterns.
Another theory proposed was that insects could mistake a nearby light for the moon, relying on celestial navigation. Many insects use the moon to maintain their course at night.
This method hinges on the phenomenon where distant objects appear to remain stationary as you travel along a straight path. A steady moon indicates a steady course, while nearby objects seem to lag behind as you move.
According to the celestial navigation theory, insects attempt to keep the light source steady, leading to sharp turns in an unsuccessful effort to fly straight. While elegant, this model predicts that many flights would spiral inward towards a collision, which contradicts observed flight patterns. So, what's the actual explanation?
To delve into this inquiry, we and our colleagues conducted high-speed video recordings of insects around various light sources, meticulously analyzing their flight paths and body positions. Our investigations took place both in the controlled setting of Imperial College London's lab and at field sites in Costa Rica, including CIEE and the Estación Biológica.
What we discovered didn't align closely with existing models. Instead, a significant portion of insects consistently oriented their bodies with their backs towards the lights - a behavior known as the dorsal light response. In nature, where more light typically comes from above than below, this response aids insects in maintaining proper flight orientation.
However, positioning their backs towards nearby artificial lights led to altered flight trajectories. Much like airplanes banking to turn, these insects executed banking maneuvers when their backs faced a nearby light source. This banking motion caused them to loop around the light, circling without colliding.
These circular flight paths weren't the only behavior we observed. When insects flew directly beneath a light, they often arched upwards as the light passed behind them, maintaining their dorsal orientation until they eventually stalled and fell. Additionally, when flying directly over a light, insects tended to invert, momentarily turning their backs to the light before abruptly crashing.
Several insects circling round a street light
While light at night can disrupt other animals, such as causing migrating birds to veer off course into urban areas, larger animals typically maintain their vertical orientation. So why do insects, the oldest and most diverse group of flyers, rely on a response that leaves them so susceptible?
The answer may lie in their small size. Larger animals can directly sense gravity through sensory organs affected by its acceleration. For instance, humans use the vestibular system in our inner ear to maintain balance and discern direction.
However, insects possess only small sensory structures. During rapid flight maneuvers, they rely on acceleration, which provides a limited indication of their orientation. Instead, they seem to rely on the brightness of the sky.
Before the advent of modern lighting, the sky was consistently brighter than the ground, offering a reliable cue for small, active flyers to maintain their orientation. However, artificial lights disrupt this cue, leading insects to fly in circles - a behavior that has emerged only recently in evolutionary terms.
As technology advances, the proliferation of artificial lights is outpacing ever before, with the widespread adoption of inexpensive, bright, broad-spectrum LEDs. Many regions, particularly urban areas, now experience perpetual illumination, with dark nights becoming increasingly rare.
The impact of this light pollution extends beyond insects, disrupting the circadian rhythms and physiological processes of various organisms, including animals, plants, and humans, often leading to significant health ramifications.
However, insects trapped around artificial lights endure particularly dire consequences. Hindered from obtaining food, vulnerable to predators, and prone to exhaustion, many perish before daybreak.
In theory, addressing light pollution is relatively straightforward - often as simple as flipping a switch. By restricting outdoor lighting to functional, targeted warm sources, and minimizing brightness and duration, we can significantly enhance the health of nocturnal ecosystems. Furthermore, these practices not only benefit insects but also restore the visibility of the night sky; currently, over one-third of the world's population resides in areas where the Milky Way remains obscured.
While the spectacle of insects encircling a light is intriguing, it is undeniably preferable for both insects and the services they provide to humans to preserve the darkness of the night and allow them to carry out their essential activities under the natural starlit sky.