Spiralling into a fiery death

Have you ever wondered why moths spiral into a fiery death? You know their ways: there’s a candle flame, or a bonfire, and the little furry fliers make a corkscrew-shaped moth-line right in there.

I have often idly wondered why this may be so. The answer is to be found in evolution; Darwinian natural selection, to be precise. I’m reading The God Delusion at the moment, and it’s bloody brilliant. Within its pages is an explanation as to why moths apparently commit suicide (he’s working up to asking why religious beliefs survive in the face of so much evidence to the contrary – there must have been an evolutionary advantage somewhere in our ancestry. But that’s another article).

Why did natural selection favour this apparent self-immolation behaviour? The answer is, obviously, that it doesn’t. This behaviour is almost certainly a by-product – a misfiring, as Dawkins puts it – of a useful behaviour.

Moths navigate using night lights – the moon and the stars. Because they are so far away from Earth, they are at optical infinity and rays of light emanating from them are parallel.


Light rays entering an eye from objects at different distances

Because of the parallel nature of the light rays, the insects can use them as a compass to steer accurately in a straight line. Returning home after an excursion, they simply reverse the system to find their way.

Natural selection has favoured the development of an insect nervous system that can use the night lights, and set up temporary rules to do so. The example that Dawkins gives is:

“Steer a course such that the rays of light hit your eye at an angle of 30 degrees.”

Insects have compound eyes made up of straight tubes radiating out from the centre of the eye. Think: hedgehog, and you’re close.


Look into my compound eyes…

The ability to steer may be something as simple as the moth keeping the light in one particular tube. As long as the light is in that one tube, the moth is steering in the right direction – because all the rays of light are arriving at the tube in a parallel manner. This is the key to the spiralling behaviour.

A moth’s navigation system relies critically on the light being at optical infinity. If it isn’t, the rays are not parallel, but diverge (see image above). In this case, applying the 30-degree rule of thumb to a light source closer to home will steer the moth, via a spiral trajectory, into the flame.


Logarithmic spirals occur throughout nature. Look out for a blog on this very subject soon…

Although encounters with candles end in fiery death for the moth, the general rule of navigation is still a good one, and so it endures. Moths rarely meet candles; thousands of them do, however, successfully navigate every night using the moon, a bright star, or even the light from a distant city.

Dawkins points out that we often ask the wrong questions, such as: “Why are all these moths killing themselves in fires and candle flames?” when we should be asking why their nervous systems steer by a method that ends, as far as we can see, in death.

There are all kinds of strange behaviours that can be explained using Darwinian natural selection; what was the original, and useful, behaviour that morphed into the misfiring by-product? If we find that, we’ll find the answer. Not so much of a mystery after all then.

Digging deeper into life’s little mysteries is incredibly rewarding. How can one not want to learn more about the world we inhabit? Thank goodness for the curious. For they shall inherit the Earth.


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