We've all had this question in our lives: Why do insects swarm toward artificial lights at night? Now, scientists have found the answer by using high-speed cameras and motion capture technology to map the insects' flight trajectories in three dimensions.
Many explanations have been proposed for this behavior of insects - the insects may mistake the lights for the moon, flowers, or gaps in leaves they can walk through. Maybe they were blinded by the bright light and lost their way. They may simply like the light and warmth as a refuge from cold nights. They may also be attracted to other insects there to mate and/or feed. It could also be a mixture of the above.
But without asking the exhausted moths on the porch, how do we know which statement is true? To find out once and for all, researchers at Florida International University (FIU) and Imperial College London used high-speed three-dimensional cameras in the laboratory and in the jungles of Costa Rica to unravel the insects' flight paths around artificial light.
It turns out that insects themselves are not "attracted" to artificial light. The lights on your porch are just messing with their navigational system honed over tens of millions of years of evolution. Basically, because insects perform aerial maneuvers that would make even the most experienced jet fighter pilots nauseous, they cannot rely on their usual sense of gravity to tell which is up and which is down. So they learn to let the sky - nature's brightest nocturnal light - at their backs to help them stay upright.
The problem, of course, is that the sky is no longer the brightest light. This means that when a bug flies past a streetlight or other light bulb, it will instinctively adjust its direction so that the light hits its back. As they fly by, they constantly adjust their flight path to maintain the same angle, causing them to circle around the bulb. Eventually, they'll get dizzy and start running chaotically into the light, and the impact, heat, or just plain exhaustion will take their toll.
The scientists behind the new study first identified this explanation in laboratory tests, attaching motion capture tags to moths and dragonflies to reconstruct how they flew around lights, including their rolling and rotational movements.
"In one of the first experiments, I let a large yellow underwing moth take off from my hand and fly directly over a UV light bulb, and it immediately did a somersault," said study author Sam Fabian. "But we didn't know at the time whether the behaviors we saw and measured in the lab would also occur in the wild."
To find out, they went to Costa Rica, one of the countries with the most diverse insect species in the world, and set up a light in the jungle surrounded by high-speed camera equipment to capture insect activity in a similar way. Soon the area was abuzz with insects including moths, flies, dragonflies, beetles and even a praying mantis.
The researchers captured 477 videos of 10 different types of insects interacting with light. What is certain is that all species of insects turn their backs to the light, flipping over as they fly over the light source.
"This is a problem that goes back to prehistoric times," said study author Jamie Theobold. In the earliest writings, people noticed this phenomenon around fire. It turns out that all our guesses about why this phenomenon was happening were wrong, so this is definitely the coolest project I've ever been a part of. "
Next, the researchers plan to investigate whether cool or warm lighting affects insects differently and explore ways to potentially minimize disturbance.
The research was published in the journal Nature Communications.