Why are insects attracted to light?

Everyone has seen the enormous profusion of insects that eventually swarm around some lamp. The observation immediately brings up an explanation for such absurd behavior of so many different creatures.

Once, immersed in curiosity, I invented an explanation for this fact and, only after having constructed one, I discovered the existence of another, already accepted. I found the existing explanation wonderful, one of those that make me delirious, much more beautiful than my own. But as I observed the ever surprising phenomenon again, I realized that the existing explanation applied only to a tiny fraction of the myriad insects that behave in this apparently absurd manner.

To fly from home to a distant location and back, it is necessary to have developed a way to control the flight navigation. This can be done by remembering each location and retracing the return path based on that memory. A much more economical navigation system than this can be developed based on the direction of flight.

By computing the angle between the flight direction and the sun, as well as the distance traveled, one can subsequently retrace the path home. For the insect, this guidance system offers an enormous advantage over a system analogous to the one we use, because it requires a much smaller brain capacity. Instead of memorizing every frame, every image of the path taken, only the angle between the direction of flight and the sun is stored. A more effective brain, capable of computing each image, would have a much higher development cost, would need to gain volume and weight, and would end up providing the same efficiency for navigation, which is why the simpler system was imposed.

So, in order to be able to return to the starting point, the insect can simply go for food while maintaining a fixed angle towards the sun, and then return home by turning 180 degrees and maintaining the new angle relative to the sun.

When we turn on the lights, especially at night, when there is still a trace of the sun, we confuse the creatures, which start to trace their routes based on the lamps, taking them for the sun. It so happens that those who return home plotting routes directed more than 90 degrees to the sun will maintain this relative direction with each lamp they encounter, moving away from each one, being led aimlessly away until they are lost in the distance.

Those flying in directions relative to the sun of less than 90 degrees, while maintaining this direction relative to a lamp, will approach it by tracing a spiral path, so that all the insects that were flying at angles less than 90 degrees relative to the sun, end up caught in the trap consisting of a lamp, from which they cannot escape. The spiral path developed by the insect on its way to the lamp indicates that it has used the guidance mechanism described above.

This explanation, based on the flight navigation system, is older and much more ingenious than mine, I have to admit! I think it is beautiful. I believe it was originally developed for bees. But it is only valid for the relatively few diurnal insects that live in a house, to which they always return, as I realized when I observed the phenomenon again!

Why are nocturnal insects attracted to light?

Most of the insects we see around lamps, however, are nocturnal, the kind that wait until the sun goes down so that, veiled by darkness, they can roam the skies trying to avoid excessive exposure to voracious predators.

But with no sun at night, why would nocturnal insects do this? The obvious answer seems to be that they rely on the moon to plot their directions and return home. One problem with this solution is that, unlike the sun, which is always present during the day, the moon is absent for half of the nighttime period and, moreover, hidden between clouds for much of that time, making it much less suitable than the sun for basing a navigation system on.

Even so, in the absence of a better guide, perhaps the insects would have to make do with this one, during the night, stopping to avoid getting lost during “blind” journeys made when the moon was absent. I don’t like this hypothesis. Still, it could be plausible. The profusion of insects found in the lamps, however, has another explanation.

Many insects seek mating partners in what are called “nuptial flights”. During this activity, the females fly out, preferably hidden by darkness, exhaling an extremely attractive odor to the males of their species. Stimulated by the intense attraction, the males fly off in search of the source of this stimulus. The attraction eventually results in copulation between individuals, ensuring that those most able to follow the lead left by the females, as well as the most attractive females, will produce the next generation of the species, which will inherit the characteristics of their progenitors.

Now, let’s imagine the following scene: a multitude of beetles, for example, diluted in the immensity of the heights, piercing the skies full of sinister predators, flying at random, at great risk, in search of a mate, when, at a given moment, an attentive beetle realizes that he has crossed paths with an extremely attractive female, but that, flying in the opposite direction, is already lost in the distance. It is up to the male, driven mad by the enchanting odor, to make a turn and try to find the trail left by the female, who is already far away. Flying in a different direction, he must lose track of the female over and over again. If he is lucky, he will end up finding her before anyone else; if he is unlucky, he will be snatched by a predator before that.

This was probably the primitive search system, later replaced by a more efficient one. Suppose a male emerges with a privileged piece of information: the direction in which the females are moving! In possession of this information, the individual would greatly increase his probability of finding a mate for copulation and generating his offspring. While the other individuals fly about aimlessly, eventually crossing paths with females that pass like volleys in opposite directions, this individual, knowing the flight direction of the females, would adjust his direction to theirs, managing to keep track of any one that passed by him, without losing sight of her, like his adversaries. Such a male would therefore be much more likely than others to leave offspring, who would inherit his ability to find females.

The female who copulates with such an expert would also be very lucky, because, as a result, her offspring would tend to inherit the advantageous characteristic, the knowledge of the information about the direction of the females. By increasing the ability to generate offspring, the advantageous inheritable trait would quickly spread among the population, so that, in a few generations, all individuals would descend from the original mutant, knowledgeable about the behavior of females.

The chain of events triggered by the emergence of the male with the privileged information thus seems quite promising, but it has some flaws. If females fly at random, if the male adjusts his flight direction to one, he will lose the others, so that he will remain, like any other male in the group, adjusted only to the tiny portion of females that happen to fly in the same direction as him.

An even more relevant objection would concern the possibility of an individual knowing in advance the direction of others: what kind of magic could provide such significant information?

Both of the above objections seem to me to be quite strong, irresponsible, so that all the previous circumlocution seems untenable. Even if the appearance of a male carrying the privileged information were to trigger the succession of events described, it is not explained how the male in question could know, in advance, the behavior of the females, which invalidates the whole reasoning. But, let’s put all this information away.

Now suppose a strange and obstinate male emerges, an exotic type that, as it flies off in search of a mate, finds itself enchanted by the moonlight. We do not usually attribute poetic tendencies to insects, but we can admit that one of them prefers to fly toward the moon as if under enchantment. Let us consider this an innate compulsion that will differentiate it little from all the others; in the midst of the random swarm, it will seem to be flying at random, like all the others.

At first, this peculiarity will function only as a kind of mania of the individual in question, an idiosyncratic and innocuous behavior, just an oddity.

At the mercy of probabilities, like all the others, it is possible that such an individual will leave a numerous offspring, many of whom will inherit the peculiarity of their progenitor, the enchantment with light and the tendency originally expressed in him to direct his flight toward the moon, the great night light.

Let us suppose the existence of a contingent of insects descended from this original peculiar type, always flying toward the moon. Now, such types act as if they have inside information about the behavior of their relatives. Note that in fact they know nothing, they have no additional information that the others do not know, but only share the same “mania”, the enchantment, the compulsion to fly towards the light, that compels them all to fly in the same direction, as if they know that their partners do the same! Add to this that the moon appears only at certain times, causing the peculiar group of enchanted individuals to take to the skies synchronously, further increasing their probability of finding a partner 😉

Species that do not return home tend to lay large numbers of eggs in the wild, as opposed to those that care for their eggs and chicks in their homes, a fact that leaves them subject to huge statistical fluctuations. If some statistical fluctuation, or moon-induced synchronization, allows for an increase in the local density of individuals sharing the preferred direction, there will be an increasing advantage to sharing it. Let us remember the absurd individual with the insider information mentioned at the beginning of this story, and realize that all these “moon worshipers” act as if they know the behavior of potential partners, thus favoring their probability of leaving offspring. After a few generations, one can expect that all individuals of the species will share the same characteristic, no longer peculiar or strange, of flying toward the moon, preferably taking to the skies when it is there.

As for the other individuals, the atavistic types, oblivious to the moon, would have been extinct, for flying aimlessly, unaware of the privileged direction of all the others, subjecting themselves to predators for a much longer time in search of a partner, who will probably have already copulated with an adversary.

When we light a lamp, we confuse the creatures, enchanting them with the intense light, compelling them to exchange the moon for the most radiant visible light they can find. And this is why the profusion of flying insects is concentrated around light bulbs!

The issue is of enormous ecological importance, since there are several million species of insects, many of which are subject to this highly dangerous threat to them, all the more so because it makes it difficult for them to reproduce.