Richard Dawkins, in his recent 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 mostly 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 from the light. 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.
Terence Hollingworth, Toulouse, France
