Why do moths fly into candles? Moths fly into candles because they are trying to keep their back to the light so that they don’t fly upside down and they fly into the candle by accident. There are several theories and the most prevalent theory is that they mistake the light for the light of the moon, which they use to navigate, but this doesn’t seem to be the case.
If you light a candle in your garden, or switch on a porch light at night, it will very quickly become a source for many of the local insects. Some will land on it, often burning up, and many will just fly round and round it until they get exhausted, spiral into it, or you blow it out. Why is this? Well, whatever the actual reason, moths, like many other insects, are positively phototactic, which means they are attracted to light. Many other insects and creatures, like cockroaches and worms, are negatively phototactic, which means they run away from light. Earthworms have evolved to do this because light usually means the sun, which means they will dry out.
There do appear to be several theories why moths fly into candles. The first and the most common is that they mistake the light for the light of the moon. The second is that they mistake the light for the light of the sun. The third is that the light blinds them and they cannot see to get away from it. The fourth is that they think the light means open space. And the fifth is that they think they might be heading towards a potential mate. Let’s look at these in order and then look at why the really do it.
The most common theory is that moths use the moon to navigate by and they mistake the light of a candle or a porch light for the moon. The theory is that moths have evolved to always keep the moon at a certain angle so that they can find where they are going, but the angles with which they fly around light sources make this unlikely. They would also have to constantly adjust their angle as the moon moved through the sky.
The second theory is that they are running away from the light because they think it is the sun. Moths are generally nocturnal and when the sun rises they need to hide from predators. When the sun comes up, they fly down to hide. The theory is that they mistake the artificial light for the sun and fly down to hide. However, if this were true, they wouldn’t keep coming back up again and circling the light.
The third theory is that the brightness of the light overloads their eyes and they are too blind to see. Our eyes adjust to the dark relatively quickly and our night vision comes on in about twenty minutes. It takes a lot longer for a moth to adjust, which means they might just not be able to see.
The fourth reason is that they believe the light source signifies an open space and they want to fly towards it. There is no real evidence for this. And the fifth reason is that they think they are flying toward a potential mate. Female moths release pheromones to attract male moths and some of those pheromones give off a very faint light. The theory is that the male moths mistake the light source for these pheromones. However, if that were true, only male moths would fly into candles, which is not the case.
So, what is the real reason? The problem has always been that it is very difficult to observe the moths and see why they do what they do. However, a team of entomologists managed to use advanced motion capture technology and two high-speed cameras to map the flight paths of the moths as they flew around the light source. What they realized when they analyzed the data was that no matter where they were flying, the moths tried to keep their backs to the light. Moths and many other insects have a dorsal light response. The dorsal part of an insect is its back and the dorsal light response is an evolved response for insects to keep the light on their backs. They do this so that they fly the right way up. The sky is almost always brighter than the ground and if they can keep the brighter side on their backs, they will always be orientated the right way. You might ask why they can’t just sense which way is down and it is because they are too light. We know which way is down because our bodies pull that way due to the force of gravity. However, if you put us in water, it becomes harder to know which way is down because we are not pulled so much. Insects are too light to be able to work out down from the force of gravity alone, so they have evolved the dorsal light response to tell them. And that worked fine for millions of years until we came along and made the ground brighter than the sky. And now insects can’t tell which way is up and which way is down, which is why they fly around and sometimes into candles. And this is what I learned today.
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Sources
https://www.nature.com/articles/s41467-024-44785-3
https://www.sciencedirect.com/topics/immunology-and-microbiology/ground-reaction-force
https://www.bbc.com/future/article/20240215-why-insects-are-drawn-to-artificial-light-at-night
https://www.bbc.co.uk/newsround/68165122
https://www.terminix.com/blog/bug-facts/why-are-bugs-attracted-to-light
https://animals.howstuffworks.com/insects/question675.htm
Photo by Caio : https://www.pexels.com/photo/lighted-outdoor-lantern-198288/