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Scales: On the Wings of Butterflies and Moths


[clock] 6-MINUTE READ ISSUE:  Spring 2006

Ever since, as a boy, I got interested in insects, I wondered why butterflies and moths, the so-called lepidopterans, were covered with scales. You couldn’t help noticing this. When you handled butterflies or moths the scales tended to rub off, leaving your fingers coated with a fine powder. With a hand lens you could see the scales on a lepidopteran. Like tiny shingles on a roof, they were neatly arranged in rows, overlapping in part, each individually anchored in a socket. Scales cover not just the wings of lepidopterans, but the legs and full length of the body as well. In moths the scale cover tends to be fluffier and more loosely attached.

As a budding evolutionist, I was certain that lepidopteran scales had “survival value.” As products of natural selection, I thought, they had to have a function. But what might that function be? I asked around but got no answers. It was not until I became a professional entomologist and delved into the scientific literature that I learned that others had given thought to the question. One hypothesis that I liked was that the scales served to trap a layer of “dead air” next to the wing surface, thereby providing the airborne insect with added lift. This made sense. But was it the whole story? It turned out that it wasn’t. I was eventually myself to discover that lepidopteran scales also play a defensive role, that they protect butterflies and moths against orb-weaving spiders.

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Insects get trapped in spider webs because they get stuck to the glue-coated silken strands of the web. Lepidopterans tend to avoid capture because they are not really detained by the web. As they fly into a web and attempt to flutter free, they simply lose scales to the points of contact with the web’s sticky strands and make their getaway. Moths, because of the looseness of their investiture, are less likely to get trapped than butterflies. They are also in greater need of protection, given that spider webs are more plentiful at night than in the daytime. Most orb weavers spin their webs at dusk and keep them up for the night only, taking them down and eating them at dawn.

There was something else about lepidopteran scales that I thought needed explanation. Scales in butterflies, typically, come in a variety of colors, while those of moths, as a rule, do not. This accounts for a basic difference between butterflies and moths. Butterflies tend to be gaudy, while moths, more often than not, are drab. There are exceptions to the rule, provided for instance by moths that fly by day and are colorful like butterflies, or that rest by day and depend on color for camouflage. But on the whole the difference holds.

Interesting work has been done on how color is generated in butterflies. You can look at a butterfly with a microscope and easily confirm that it is the scales that are colored. Some of the colors, we now know, are generated by pigments, which have been characterized chemically in some instances. Other colors, such as the iridescent blues typical of certain tropical butterflies, are generated physically, by structural elaboration of the scales. Butterflies even have ultraviolet markings, generated either by pigments or by physical means, which they themselves can see but to which we are totally blind.

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A question that I thought had never been properly answered was the evolutionary “why” for butterfly coloration. What do butterflies gain by being gaudy? With some species, where males and females are differently colored, it seemed clear that the difference in appearance could provide the basis for sex recognition in courtship. And indeed, it does, as has been demonstrated for some species by experiment.

But coloration could play a more fundamental role in butterflies. Think of it. Butterflies are the most conspicuous of insects. Given away by their gaudiness and the broadness of their wings, they are recognizable for what they are from further away than any other insects. How is it that butterflies can risk detection, when there are predators around such as birds that could capitalize upon such detectability? The fact is that butterflies are not at all easy to catch. They are erratic fliers whose aerial trajectory is all but linear. To scoop them from the air, as any butterfly collector knows, takes a big net. Birds have no such net and may in fact, as a group, ignore butterflies. They might learn from experience that gaudy insects detectable from afar are not worth chasing, or they might be programmed genetically to forgo butterfly pursuit altogether. Butterflies as a group could thus be thought of as constituting a gigantic mimetic assemblage, in which gaudiness and elusiveness are the defining traits and safety from birds is the payoff. There are, of course, birds that do eat butterflies (and it is these that one could imagine to have been instrumental in forging the evolution of detailed mimetic resemblances, such as that of the monarch butterfly by the viceroy butterfly), but as regards insectivorous birds as a whole, the evidence indicates that most simply don’t go after butterflies.

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Butterflies are generally believed to have evolved from moths, and it is tempting to speculate how their derivation from such ancestry might have unfolded. First, it is clear that evasive flight is not something that was “invented” by butterflies. Rather, it seems to have evolved already in moths, most probably under selective pressure imposed by bats, the primary aerial enemies faced by moths in the dark. Scales doubtless also had an early origin, their acquisition having been driven by the dual aerodynamic and defensive advantage conveyed by their possession. Erratic flight in moths can be envisioned to have played a “preparatory” role, in that it might have enabled butterflies to face up to birds from the very outset of their “invasion” of the birds’ diurnal world. Color, too, although exploited to the full only after the shift to diurnality, could already early on have been put to use by moths, as for the achievement of camouflage.

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There was an unexpected spin-off to all these studies, a consequence of my having examined so many butterfly wings close-up. I had almost forgotten the sheer splendor that is embodied in the lepidopteran wing. The diversity of color and pattern, and of scale arrangements and shapes, was stunning. There was a world of hidden dimensions in these images, a treasury of abstract art, unique in execution and gripping in its impact. There was proof in these images that science and art, while dwelling separately in the confines of our consciousness, do merge in that vague domain of the subconscious that guides us in our enthusiasms. And there was a reminder, infinitely gratifying, that science, in tangible, perceptible ways, can lead to an appreciation of beauty.
 

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