Round & round the straight line

If you’ve ever switched a light on in the dark you’ll have no doubt noticed the rather strange effect it has on moths, which are soon attracted to the light and begin to spiral round it for hours. Many people wonder what causes this bizarre behaviour and, at the moment, there is no definitive answer as even entomologists (the scientists who study insects) find it confusing. 

This isn’t to say that they don’t have theories regarding this behaviour however, and there is one main explanation that is generally accepted among the entomological community that seems to have some scientific evidence. This theory is surprisingly simple and basically works off the principle that lepidopterists (the family of butterflies and moths) use light to navigate when they are flying. 

Like many insects, moths have very poor vision that is mainly used just to detect light and movement.  Most of the information about their surroundings actually comes from their highly developed antennae, which provide them with an incredibly sensitive sense of smell. In fact, the antenna of male moths (pictured above) are so sensitive in some species that they can detect a single molecule of a female moth's sex pheromone in 1 cubic yard of air - allowing them to smell the moth from 11 kilometres away!

So, to start at the basics, there are two fundamental responses that all types of life (that are capable of detecting photons) have in response to light – they either respond positively to it and move towards the source (positive phototaxis) or negatively and flee from it (negative phototaxis). Lepidopterists are known to be the former, which explains why they converge on sources of light (such as bulbs). 

And while this appears to make sense so far, it is actually confusing to many scientists – why would an insect that is vulnerable to predators move towards a light source and make itself more visible? In fact, logic suggests that moths should actually show negative phototaxis and head towards the darkest areas they can find – they do have drab colours for camouflage afterall! 

However, the idea that lepidopterists use light for navigation helps to explain this and gives a plausible reason why they are in fact attracted by ambiance rather than repelled by it. The idea is simple and suggests that lepidopterists use the brightness of the lights in the sky (i.e. the sun, stars and moon) to calculate how high they are flying and use the angles that these lights hit their eyes to determine their direction. Thus, they think that because the lights are getting brighter, they are actually getting higher in the sky (which generally makes them safer from many of their predators, such as spiders, which live amidst foliage). 

So although this might seem like a bizarre explanation for why moths are attracted to light, remember that moths have evolved over millions of years in an environment where the brightness of the night sky has scarcely changed. It is only recently that humans have invented and built all of these streetlights and exterior lamps that are confusing them! Essentially, all the light we produce at night is hijacking their complex and highly evolved navigating systems because they now get much close to sources of light than they are expecting to! 

This concept also explains why moths end up spiralling round bulbs for hours at a time and, basically, because the stars and the moon are so far away from us, their light hits moth eyes in parallel to the horizontal axis of flight. Thus, moths have evolved a system where they use the information that this light provides to work out whether they are turning or travelling in a straight line. (Think of a cross where the flat line represents direction and the vertical one represents height).

This system is actually fairly simple and works well, until of course they become too close to a light. Once this happens, the angle the light strikes the eye at is steep enough to make the insect think that it is turning so it constantly has to compensate and turn itself to ‘return’ back to a straight line of flight. Thus, while we can see that the moth is actually flying in circles around the light, the disorientated moth thinks that it is flying in a straight line! 

And if this isn’t enough for the poor moths to contend with, many lepidopterists also believe that because moths are nocturnal (sleep during the day), being close to such a bright light actually makes them sleepy. When this happens, they are believed to enter a ‘rest mode’ and attempt to sleep, which is why they often try to land on (or nearby) the light – making it even harder for them to escape its clutches!