How do the insects we photograph create the extreme multicolored body surfaces that fascinate us so much? And why do they do it? In contrast to the other articles in this blog, this one does not deal with photographic techniques, devices or working methods, but with what most of us find so fascinating about living image motives: the colorfulness.
In the animal kingdom, we sometimes encounter extremely striking multi-colored images, which also do a lot to motivate people to take focus stacking shots. How do animals create this spectacle of color, and why do they do it?
A Chrysiridia rhipheus butterfly can be seen on a black background, and a tiny rectangular area of its right forewing, marked in red, is shown extremely magnified to the right in a focus-stacking image taken with a 20x microscope lens
We find many different colors in the animal kingdom, most of which are undoubtedly created by pigments. But evolution uses an ingenious physical trick to create color effects even without pigments. In butterflies, in addition to pigment colors (e.g. pterins, ommatins or papiliochromes and others), we also find colored areas created by what are known as structural colors, or physical colors. Nature also uses these visual color effects in other animal groups, e.g. in the body scales of extremely colorful beetles such as Eupholus or Pachyrhinchus species (both family Curculionidae) or the extremely colorful cuckoo wasps (family Chrysididae), but also in birds (e.g. peacock, hummingbird). These surfaces are not really colored, they have no pigments, although that is how they appear to us. They are colors that emerge through the scattering of light, similar in principle to a prism.
Blue, for example, is almost always a structural color that is not produced by pigments, but by physical effects, because blue pigments are extremely rare in inanimate nature and obviously difficult for a living organism to produce.
Seen in this way, the colors we see on a butterfly wing are not really there, they are to a certain extent an illusion based on a physical light phenomenon. This sounds difficult to understand at first, but if you look at a rainbow, the phenomenon becomes clear, or at a thin film of oil on water that you look at in the light, or at the surface of a soap bubble. Here you can see stripes of different colors that are reminiscent of a rainbow. And these are also colors that are not present as color pigments.
The key word is interference. This refers to the mutual influence of waves that overlap and either cancel each other out (destructive interference) or amplify each other (constructive interference). In the case of sound waves, we know this, for example, from headphones with “noise cancelling”, which neutralize sound waves from the environment with destructive interference.
Constructive and destructive interference
However, it also works with light waves, and here too we are dealing with constructive and destructive interference. What happens here? Colorless daylight consists of numerous rays of different wavelengths and light colors.
Let us first consider our normal perception of color: a color pigment on the surface of any object appears colored to us because it reflects a single spectral component of the light with a certain color effect to our eye and absorbs, i.e. swallows, all the other color components.
Interference, on the other hand, occurs when daylight is scattered by optically effective structures (such as a prism) and broken down into its individual spectral colors, which then influence each other.
Insect skin wings
The physical mechanisms at work here are somewhat different. Let's look at the wing of a Drosophila fruit fly. It is transparent and the penetrating mixed light is scattered within it so that the individual color components are separated from each other. At the upper boundary surface of this wing material (light entry) there is a reflection of light, and at the lower boundary surface (light exit) there is a second reflection. The individual spectral components of these two reflections now interfere with each other: certain components are extinguished depending on the direction and disappear, so that only their complementary colors remain. In this way, the seemingly colorless mixed light becomes radiation of a certain color, which we can only perceive to the maximum from a certain viewing direction, more precisely, at a certain angle between the direction of radiation and the viewing direction.
We find this phenomenon in the Drosophila wing as well as in the wings of many other insects, e.g. mosquitoes or flies. It is crucial that their material is not too thick or too thin for the light-physical effect, because the distance between the upper and lower boundary layer of this wing must be exactly the same as the coherence length of the radiation components. This is also the case with the thin layer of oil on water mentioned above, or with the soap bubble with colored stripes.
However, interference can also occur without the transparency of insect wings or the oil layer. Examples of this are the wing scales of butterflies, the wings of many beetles, whose iridescent green or blue we only perceive from a certain viewing direction, or the mantle lobes of giant clams of the genus Tridacna, which live in tropical coral reefs.
Opalescence
This phenomenon even occurs in inanimate materials, e.g. the precious stone opal. This is why it is also known as opalescence. The physical mechanism is similar: the penetrating mixed light is scattered by tiny nanostructures of the material, which act like mirrors, and these radiation components influence each other through constructive or destructive interference. Two radiation waves meet, and if they have the same deflection (amplitude) and the same frequency (repetition distance), they overlap, which adds their deflection (constructive interference) or cancels it out (destructive interference). This changes the color effect of the light and creates the color phenomena that we perceive in butterfly wings, for example. They are also very dependent on the viewing direction or the direction in which the light is emitted.
Many of these materials, which produce color effects in the animal kingdom via interference, are lattice structures that have cavities inside, e.g. the wing scales of butterflies. This lattice structure not only saves material, but also makes them extremely light, which is particularly important when flying. The body scales of many beetles also have such a lattice structure, and many of them consist of numerous individual elements with different interference effects, which leads to different color effects.
Why these colors?
But why do animal species invest so much in creating structures that make them look colorful? And why does evolution create such incredibly ingenious physical mechanisms? It almost seems as if these multi-colored animals are competing with each other. And competition does indeed play a role. But it's not about beauty, at least not in most cases. In certain bird species whose males compete for the favor of a female, the aesthetics of these color elements can be important. But in most cases, the benefit for the animals is completely different.
There are certainly many different reasons for the colorfulness of animals, and we humans probably only know a few of them. One, however, is seeing, being seen and not being seen. Mimesis is one of the explanations, the adaptation to an inanimate background. If I dissolve my body shape, I am more difficult for a predator to recognize, and whoever does this best lives the longest and has the most offspring. In this respect, selection is carried out by predators.
For example, insects who often spend time on the flower of a particular plant live longer if their bodies have similar colors or color combinations to the flower itself. This even works with nudibranchs in coral reefs, which feed on certain sponges and have adopted their colors. Here too, predator selection is the driving force of evolution.
Warnings to potential predators are also a reason for color phenomena that can prolong life: “Watch out, I'm poisonous!” Copycats also use the effectiveness of this color warning mechanism, e.g. the harmless hoverflies, which feign poisonousness and defensiveness with their striped pattern. This phenomenon is called mimicry. But the tiny predatory spiders that lie in wait for these hoverflies in flowers know this. And the hoverflies know that they cannot deceive the spiders, which is why they inspect each flower from all directions before hovering to make sure that none of the spiders are lurking inside.
And even these small predatory spiders benefit from the protective effect of body coloration – think of the variable crab spider (Misumena vatia), which can adapt its color to the flower in which it lives.
Deception is another advantage of color effects. If I create color elements on my body that imitate certain structures, I can deceive my predators. False eyes, also known as eye spots, on the wings of butterflies are one of these, because they signal to my predator that I am watching it with alert eyes and have long since discovered it, so that its attack is no longer worthwhile but would be a waste of energy. False eyes at the rear end of the body can also be deceptive about the expected direction of escape.
This species-dependent coloration can also help me as an animal to find a partner. It is important that I find a partner of the same species, because only then will reproduction work.
These two phenomena, mimesis and facilitated mate finding, explain, for example, the numerous coloration variants in closely related beetle species (e.g. genera Eupholus or Pachyrrhynchus), but also in butterflies, birds or butterfly fish in coral reefs.
On the one hand, the colour structures dissolve the body outlines in such a way that the animals are difficult to recognize by predators. The directional dependence of the color phenomenon of opalescence further supports this, because the insect, bird or giant clam suddenly disappears from view when the viewing direction changes.
And on the other hand, the species-specific color patterns make it possible to recognize when you are looking at a member of the same species that is worth either defending its territory against a rival in order to secure its food supply, or seeking a partnership in order to preserve the species through reproduction.
The fact that people enjoy the colorfulness of these animals and find them an aesthetic pleasure is an unintended side effect, similar to the colorfulness of flowers. Just take it as a gift from nature and enjoy the fact that it is a great opportunity to discover nature and its secrets with the photographic possibilities of our time.
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