The Benefits of Being Attracted to Light

      They used to be called ‘sun children’,^* flying creatures of the air that flutter here and there, apparently aimless yet self-sufficient, letting themselves be borne on the wind and seeking cover at the sight of the first clouds, since they cannot tolerate the shade. This romantic view is most likely still true for some species of butterfly. The world of Lepidoptera, however, in all its richness and diversity of species and behaviour, only really unfolds when twilight falls. Since we are diurnal, many of us would be surprised to know that there are more than ten times as many species of moth than there are of butterfly. It is only when moths ghost around the streetlights on a sultry summer night that we suddenly become aware of them.

Many caterpillars also prefer to feed in darkness. This protects them from the birds that hunt them by day. It is harder for the adult insects to escape bats, since they hunt at night. It is true that their ultrasound does not work as well in the thick undergrowth of leaves, twigs and branches. Moths that fly in the open air at night are much more likely to be caught. Certain species of moth have developed the quite extraordinary capacity to detect the ultrasonic pulses emitted by bats without having ‘heard’ them. Then, if they are struck by them, they let themselves fall into the grass or the bushes, as quick as lightning. This is quite hard for us to grasp. To offer a plausible comparison, if we had this capacity, we would be able to detect radar traps when driving and brake in time to avoid them, without being conscious of having done so. Night-flying moths are, above all, concerned with finding a flight path through the darkness without crashing into obstacles if, somewhere, perhaps several hundred metres away, a newly emerged female is exuding her own kind of sexual lure. The travelling males remain unscathed, which is surprising considering that they are practically flying blind with only the scent-sensors in their antennae to steer with. How they manage this never ceases to amaze me, since it is light that steers them off their course and causes them to bump into obstacles. ‘More light, worse sight’ is hard for us to comprehend, since we depend so much on vision.

      Night-flying moths avoid the day and pass the hours of light resting under cover. They start to come alive towards the end of dusk, when the weakening natural light forces us humans to go to sleep or to use artificial light. Yet it is that very light that attracts almost all moths, as if by some magical force. Certain wavelengths are particularly effective: above all, the ‘weak’ ultraviolet light (UV-A) that is invisible to us. Moths can see it, just like many other insects and the majority of birds, since they have an optical pigment that is sensitive to the wavelength of UV light. Birds that hunt at night, however, such as owls, do not use UV light, but the remaining daylight from the visible spectrum (as we would). Some owls, such as the barn owl, can hear the squeaking of a mouse that extends far into the ultrasound range, and can use this to locate that mouse and accurately swoop down to catch it, but only if there is sufficient residual light for them to do so: they avoid flying in complete darkness. The moths and night-flying butterflies evidently also need such residual light. But based on the current level of knowledge, we can barely even begin to speculate how they process this and how they use it to manage their often quite rapid flights, since, in terms of structure and function, their eyes are not significantly different from those of the butterflies and day-flying moths.

Like moths to a flame

      The best nights for ‘light-trapping’ are the darkest ones with no moon, thick cloud cover and a light (warm) rain. Clear moonlit nights, on the other hand, are not very good. One may assume from this that moths orientate themselves by the moon and thus manage to achieve a relatively straight flightpath, since they can maintain a fixed bearing on the moon’s distant light. But even if this were enough to maintain a particular direction, for example on a migration north or south, as is undertaken by some migrant moths and butterflies (for example, the death’s head hawk-moth and other hawk-moths), it is of little help in avoiding collisions with leaves and branches in the woods or the garden. The fact that they are attracted by artificial light is only partially explained with reference to the moon, if at all. If one watches the moths as they fly towards the light, one will rarely see a straight, goal-oriented approach. Even the alleged spiral, with which they are supposed to approach light sources, is the exception rather than the rule, if it exists at all. And when is the moon ever so readily visible?

      The fact is that light does attract a great many moths, but we do not yet understand why. On the other hand, we do know what type of light is particularly effective. Accordingly, the bulbs used in street lighting should be such that they emit light frequencies that either do not attract moths at all, or only barely – that is, above all ‘yellow’ and not ‘blue’ and in any event light that is as free of UV rays as possible. The much-maligned light pollution that occurs at night could at least be constituted more thoughtfully. We should not use any artificial light with the same spectral composition as sunlight, even where lighting is reasonable and necessary, such as in the illumination of roads for the safety of pedestrians and drivers. The same is true for floodlights used to light up historic buildings and other attractions and display windows. A more favourable spectral composition of artificial light would also benefit people, since we know that blue light is disturbing (as it should be, if it is used by police, fire engines and ambulances). Blue light disturbs moths too, and distracts them from their flights in search of animals of the same species for mating or the correct plants for egg-laying. We see this when moths swarm around lights on humid summer nights.

The red blindness of butterflies

      The UV component is most probably of cardinal importance. It is a form of high energy radiation that interacts with visual pigments as well as with protective pigments such as the melanin in our skin. We know that tanning is the simplest, most visible biological reaction of our bodies to light. The skin cells are stimulated by the UV to produce protective melanin. When the skin becomes brown, it regulates the extent to which light can affect the skin. In all probability, the UV light vision of insects and other creatures is based on a reaction of living cells to a process caused by light. Incidentally, the fact that we cannot see UV light is more likely to be due to a loss of an earlier, more primal ability.

      This limitation, and our severely reduced night vision, are the price we probably paid for developing trichromatic colour vision, through the capacity to recognize red and distinguish it from green. Lepidoptera cannot see red and red light does not attract them. Not only do they see very differently from us at night, but their whole view of the world, transmitted through the sensory organs, differs very significantly from ours. This is one of the more profound reasons why we find it more difficult to become attuned to the basic needs of butterflies and other insects than to those of birds. With their colour vision, birds are actually closer to us than most mammals, from whose world we have evolved. Dogs and cats are unimpressed by the beautiful red colour that we love. Together with green, it creates a mixed colour of inferior quality, of which people who are red-green colour blind could give the best description, inadequate though it would be, since red is not available as an experience to them. The alluring effect of UV on butterflies and moths is comparable to the attraction of red for most people.

We have this characteristic to thank for the best findings on the occurrence and abundance of many nocturnal insects, since they fly towards UV-rich light. What is so beautiful and appealing in individual butterflies lies in the eye of the beholder, that is, in our eyes. They do not see each other like this at all, and birds also recognize them in other ways. On their night-time hunts, bats estimate where they are using sonar imaging, which is different again. Yet over the thousands and millions of years of their existence, butterflies and moths must have learnt to deal continually with the visual ability of birds and the echoes produced by the ultrasound of bats. The challenges presented