moths and light -- 12/19/22

Today's selection-- from Cats vs Dogs. Why are moths attracted to bright lights?

"Moths are not exclusively nocturnal. Many species are active by day as well, and many are exclusively diurnal.

"Insect behaviour is almost entirely instinctive, as the famous nineteenth- and twentieth-century entomologist Jean-Henri Fabre showed us. He noted that a digger wasp of the genus Sphex would sting a caterpillar and carry it, paralysed, to the mouth of the burrow she had just excavated and deposit it at the entrance. She would then enter the burrow, presumably to check that no unwanted occupant had taken up residence there in her absence, emerge again, collect the caterpillar and take it inside to lay her egg and close the burrow. After observing this, Fabre began to move the caterpillar a little distance from the hole once the wasp was inside. The wasp would collect the caterpillar and then repeat the inspection process.

"Fabre was never able to break this routine. The wasp had evolved to behave this way and it was impossible for her to reason out anything different for this unusual set of circumstances. In the same vein, moths have undergone millions of years of evolution without ever having to deal with artificial lights at night. The few thousand years or so during which we have presented them with the problem is too short for them to have evolved appropriate behaviour.

Atlas moth

"Richard Dawkins, in his book The God Delusion (Bantam Press, 2006), presents us with the problem of moths drawn to a candle flame. His solution is the old glib explanation that the moths are trying to use the flame to navigate, mistaking it for the moon. The idea is that a moth sets its course according to the position of the light, so it will have to keep turning towards it to maintain the same relative heading, and the path it will take will be a spiral leading inevitably into the flame. This explanation does not tell us, however, why it is that in many species only males are thus attracted, and in a few, only females.

"What is more, if moths need to navigate, they must be from a migrating species. Yet most of the time such moths are not migrating. Indeed, most species do not migrate at all and thus have no need of navigation. Moreover, all groups of insects display the same behaviour: flies, wasps, hornets, mayflies and caddises are all drawn inexorably towards flames, although many of these insects are normally diurnal and the majority rarely, or never, migrate.

"So are they navigating to find food or a mate? At night in summer, male moths use scent to decide their heading, not light. Total cloud cover makes no difference to their behaviour. They move into and across the wind, hunting for telltale pheromones which will lead them to a female or for the scent of flowers to enable them to feed. Females, for their part, stay still until after mating and then go looking for the scent of plants that their larvae can feed on in order to lay their eggs on them. They don't need the moon, stars or candles to do this.

"I have spent thousands of hours sitting by light traps observing insect behaviour and I feel, for the most part, it is pure accident that they stumble upon the light. Many can be seen to fly straight past without deviating one iota. Others fly into the lighted area, land and stay still, as they would if it were daytime. Different species seem to have different sensitivities, the most sensitive ones alighting the furthest Still other moths circle the light, never bumping into it.

"Those that do fly towards the light often do so in a wild and confused manner. They seem disorientated and confused by a bright light rather than attracted to it. Just like Fabre's interference with the caterpillar, the light seems to trigger inappropriate behaviour, because the insects have no mechanism to deal with it.

"There is only one type of observed behaviour that seems appropriate, and that is moths moving towards a lighted window at night. Any insects trapped in a room will fly to the window'. Instinct tells them that in order to escape from a confined space they have to fly to where it is lightest, and even at night it is lighter outside than inside a cave or a cavity where the moth has been hidden during the day. The moth finds its way out by flying towards that light. So a moth would not be able to distinguish between a bright light source and an open space.

"It could well be this mechanism that is operating when moths fly to bright lights; however, the many other factors listed above seem to counter this view."



Cats vs Dogs: 99 Scientific Answers to Weird and Wonderful Questions about Animals


Nicholas Brealey Publishing


Copyright New Scientist 2020


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