Moths – Why do they always fly toward artificial light?

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Moths and other flying insects will naturally gravitate toward a source of light.  Sadly, if this source of light is in the form of a flame, rather than a bulb, it usually means certain death to boot.  So why do they do it?

Moths and their behaviour towards light have fascinated me for a long time, so I’ve been reading up on the subject.  However, it has not been an easy topic to resolve, by a long, long way.

Theory 1 – Transverse Orientation

Transverse Orientation is a popular theory on the internet.  Moths using the light of the moon to navigate is an old theory.  By keeping the lunar glow in the same position, relative to themselves as they fly, moths can keep a pretty straight line.  You have to bear in mind that relative to the moon and it’s movements, moths are insignificant.  As far as the moth is concerned, the moon is a fixed object in the sky.

The Transverse Orientation model goes on to suggest that the problem this causes for moths and other insects, is the new distance to the light sources they are finding.  The moon is a long way away [citation needed], whereas artificial light sources are not [citation: my monitor is only 20″ from my face].

Let’s say that a moth keeps the moon on the left when flying.  Even better, let’s say that a moth is finding a straight path by keeping itself perpendicular to the moon, but then finds an artificial light source.  What will happen?

Here comes the merry go round.

A little bit of Newton

swingballAs analogies go, this isn’t great, but I’m going to try it like this.  Newton’s first law of motion states [broadly] that an object travelling in a given direction, will continue to do so unless another force acts upon it.  Imagine then a ball on a piece of string being swung around and around above your head.  If the string were to disappear at any given point, the ball would disappear off into the distance.  No longer would it be a ball in a circular motion, it would head off in a straight line.  The reason it doesn’t while the string is attached is that the string is providing a force which is keeping the ball perpendicular to the strings point of origin.  In this particular case, the root point is your head.

I would also like to apologise for the terrible stock photo demonstrating a ball on a string in circular motion.  It’s amazingly hard to find a stock photo of somebody swinging a ball around their head.  If anybody would like to take one and send it in, I will be happy to replace this one.

Well, we have our moth who is trying to fly in a straight line.  The moth sees an artificial light and moves toward it [see theory 3: phototaxis].  Now, to fly in a straight line, the moth has to keep the light 90 degrees to its left.  This flight pattern works until the moth has flown a tiny amount. However, then the light source shifts angle because it’s close, relative to the moth, so more adjustments are needed.  Ok, the moth thinks, I need to bear left slightly to maintain my 90-degree angle to the light.  These adjustments work for another tiny amount of flight, and the same happens again.

We could continue this into the thousands of words, but by now you should see what is happening.  The moth is permanently adjusting its coordination and flight path, which to the moth is a straight line.  However, looking on from outside this frame of reference, we see a moth flying around and around the light in a continuous loop.

You can sometimes see the circular motions getting bigger and smaller as the moth experiments with different patterns to attempt to get this right.  These experimental motions often result in the moth running into the light source.

Running into the light source is OK if its a lightbulb, but not so OK if it’s a candle or fire.  Poor moth.

Theory 2 – Infrared Sex

The infrared sex theory is a puzzling one.  Personally, given all I have read, I don’t have a great deal of confidence in this theory.

It’s all about the wavelength.

Just as a sidenote, I’m really pleased that everything in the whole world, including biology is explained by some branch of physics.  Chemistry can now be explained at a quantum level, as too can biology.  This demonstrates that physics, is by far the best, most interesting and most relevent branch of science.

There are a handful of studies that suggest that the wavelength of certain lights, especially infrared (as emitted by an open flame), matches certain wavelengths emitted by the female moth’s sex hormones.  This theory is used to provide evidence of why male moths zoom straight over to these lights and jump on them.  I’m unsure about this though, as ultraviolet light is far more attractive to moths than infrared.

If it is true though that the males are trying to pounce onto these light sources because they think they are females, it could be potentially harmless.  That’s if it’s a lightbulb.

If it’s a candle or a fire.  Poor moth.

Theory 3 – Phototaxis

I like phototaxis as a theory.  Transverse Orientation does a pretty decent job at explaining the circular motion of the moths around light sources.  Sometimes, though, simpler is better.

Imagine that you are a moth.  Then imagine you are happily sitting on a branch in your bush or tree when a bird swoops down and lands.  You, being a moth, are naturally going to want to escape your place of resting as quickly as possible.

The problem; its nighttime and dark (we’re pretending you are also thousands of years into the past and there are no artificial lights).  So what do you do?  You need to make a split second decision about where you are heading.  Hello, Phototaxis.

Phototactic animals fall into two categories.  Positively phototactic and negatively phototactic.

I’m going to try again with an analogy, but have to say, its a little on the tenous side.

If there were such a thing a heat-taxis, then humans would be negatively heat-tactic.  If you put your hand onto something hot, your immediate instinct is to pull your arm away.  There is no conscious decision made here; it’s just something you do.  This is kind of how phototaxis works.  There is no conscious decision made; it’s just something that happened instinctively.  Phototaxis is an instinctive reaction to light.

Cockroaches are a great example of an animal that is negatively phototactic.  While scurrying around, eating the leftover nibbles from your floor in the dark is fun, somebody walking in an turning a light on, is not.  As soon as the light comes on, Cockroaches will instinctively head for the dark, and quickly.  Phototaxis also applies, but to a lesser degree, to earthworms.  This trait is a useful aid to worm farmers who can stop their worms from escaping by shining a bright light on top of the worm beds at night.

Moths are the opposite.  Moths are positively phototactic, meaning that in a dangerous situation, they will instinctively do the opposite to cockroaches and fly toward the light.

Back to our moth in a tree.  You are suddenly startled and need to flee.  If you flee left, right or down, you will likely hit something.  So, the only way to go is up.  But which way is instinctively up?  Well at night, with no other artificial light sources around, up, is towards the moon and stars.  Any other direction is dark.

Conclusion:

Having looked at multiple articles on this subject, I cannot conclude that any of these theories are correct.  Nobody else can either.

If I had to make a choice, I would conclude that as moths have had millions of years with no artificial light to evolve, it would seem logical they would need the light for escape and navigation.  Moths are nocturnal after all.  So my final answer is, it is a mix of both transverse orientation for one aspect of their lives, and phototaxis for their instinctive survivability.

If anybody can give me some more ideas about moths artificial light fetish, then please get in touch.  You can do this via the contact page.

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2 Comments
  • Marek
    Sep 21,2018 at 9:50 am

    I read somewhere that moths don’t really go for orange light, is that true? If it is, why go for the orange flame?

    • Fluffy
      Sep 21,2018 at 1:14 pm

      I’m pretty sure it yellow that is undesirable to moths. With regards to the camp firs and candles producing orange, it’s not so much the colour as the infrared produced by the heat they are attracted to.

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