The glass plate had an extremely thin, fragile coating, one that could easily be rubbed off by hand, leaving a plain, transparent glass plate. The coating was obviously responsible for the metallic-like colouration. It is quite literally a ‘thin film’, which may seem a fairly broad description. But in the world of optics this term means much more than that. And again it was Newton (or possibly Robert Hooke) who first realised that thin films also occur in nature, when he deciphered the cause of the peacock’s iridescence. Newton, in fact, gained his ideas from thin flakes of glass.
The following few pages are devoted to describing the commonest structural colours in nature. The mechanism behind these metallic-like colours can be interesting if only because they explain the paradox of their transparent foundations. But more incisively, it will demonstrate that physical structures really can cause colour - an understanding which will turn out to be invaluable later in the book.
Unlike chemical pigments, physical structures are preserved in the pickled collections of natural history museums. So one can study their cause and diversity without requiring access to living specimens; that can be very useful. But, as revealed in the previous chapter, physical structures have been preserved in animals other than those living today. In order to help paint a more informative picture, it is worth persevering with a short lesson in optics.
Simply put, a thin film is a thin layer of material. It only has upper and lower surfaces. In terms of its effect on light, the material acts as a different medium to air. Descartes demonstrated that light reflects from the outer and inner surfaces of a water droplet, and Fermat explained this in that light travels at different speeds in different media. A thin film of transparent material also acts like a water droplet - light reflects from its upper and lower surfaces. Maybe about 4 per cent of the rays in the original beam reflect from each surface of the thin film, and 92 per cent pass through the film.
Figure 3.2 Light rays affected by a thin film, such as a fly’s wing, in air. The film is shown in cross-section; the light-ray paths and wave profiles are illustrated as solid lines (incoming light) and dashed lines (reflected light).
When the reflected rays are out of phase, they cancel each other out like the ripples caused by Leonardo’s stones. In this case, a beam does not exist. But when the wave profiles superimpose, they are said to be in phase and a light beam does exist. In which case there is a 4 + 4 per cent, or 8 per cent, reflection. This may seem trivial, but remember pigments appear as less than 1 per cent reflection because their reflection covers a hemisphere, and we see only a tiny segment of that hemisphere. The 8 per cent reflection from the thin film, on the other hand, travels in a single direction. So if we view the film in that direction, we see the full 8 per cent. Consequently the thin film appears much brighter than a pigmented material under the same illumination (although not that much brighter because the eye is a logarithmic detector). This situation exists when the thickness of the thin film is about a quarter of the wavelength of light.
By introducing a change in medium, Newton’s prism caused different wavelengths, or colours in white light, to travel in different directions. If we apply this concept to our thin film model, we also get different colours reflected in different directions. In one direction the reflected waves for only one colour can be in phase, and the others will be out of phase and will not oscillate any more in this direction. So the colour will appear different as we view the film from different directions. And that is the effect we get from the Roman plate and the housefly’s wing. Soap bubbles and oil slicks are also thin films.
I have attributed the 4 per cent surface reflection of our thin film to a change in media. I didn’t mention, however, that different media cause different reflectivities. The 4 per cent condition occurs between glass and air, but this is reduced when the glass is placed in water. This has happy consequences for the transparent jellyfish, with a skin of similar optical properties to glass - reflections from their body in water will be slight, much less than in air. And it is the floating relatives of the jellyfish, like the Portuguese man-of-war, that suffer most from the pitfalls of reflectivity. Parts of their bodies are naturally exposed to air.
Previously I have explained the workings of the jellyfishes’ dream - chromatophores, colour cells that cause the changes in skin hues originally thought to result from chemical reactions. Although an erroneous explanation, the chemical reaction scenario was a theoretical possibility. And there is yet another possibility.
I once attended a public lecture on liquid crystals. Liquid crystals contain helical molecules, slotting together like a row of tiny springs. And I do mean tiny - side on, each complete turn of the coil measures only half a wavelength of light. That is, half the wavelength of the light that is reflected from the structure - liquid crystals can appear strongly coloured. We can buy toys or thermometers containing liquid crystals - those that change colour with a change of temperature, as when they are touched.
The colour of a liquid crystal derives from light waves that reflect from each half turn of the helical molecules. This can be best understood by considering the whole structure as a stack of thin layers, where the materials of alternate layers have different optical properties.
Each half turn of the helical molecules covers a quarter of a wavelength in distance or ‘thickness’, and is now equivalent to a single thin film. So the whole molecule from top to bottom approximates many thin films piled up. As a stack of thin films, the liquid crystal molecules can reflect a greater portion of the light. The 92 per cent of light that passes through a single layer meets with another layer, and another 4 per cent reflects. Eventually, with enough turns of the helical molecules, all the light will be reflected and none will pass through the system. Now we have 100 per cent reflection - the brightest possible effect, if you are looking in the right direction.
Figure 3.3 A cross-section through a liquid crystal (left), showing individual helical molecules, and its approximation as a stack of thin layers and effect on light (right). Reflected light rays are in phase when the layers are approximately a quarter of their wavelength in thickness.
During the question time that followed the liquid crystal lecture, a member of the audience enquired, ‘Does the chameleon change its colours using liquid crystals?’ A very nice thought. In fact, it was so nice that the lecturer responded with an enthusiastic, ‘Yes, you could be right!’ The lecturer was a chemist and could be forgiven for not knowing the real cause of the chameleon’s guise, but the question demonstrated an excellent understanding of the whole lecture. The lecturer had succeeded in getting her message across and was clearly so delighted that a negative response would have seemed inappropriate. Liquid crystals, nonetheless, can be found in the literature on animal colours.
Down House, in south London, was the home of Charles Darwin. His original microscope, desk and bookshelves are preserved intact, along with some of his specimens. Although barnacles were Darwin’s chosen group, beetles seem to have followed him everywhere. And within Down House, as within Oxford’s Huxley Room, shine the metallic effects of liquid crystals, naturally embedded with the exoskeletons of Darwin’s beetle collections.
Beetles are often so spectacularly coloured that they end up adorning the costumes of New Guinea tribal chiefs or are sold as the earrings in jewellery shops. Their metallic colours appear to be more diverse in the tropics, which is not so surprising because here the sun’s rays are stronger. The lack of cloud cover results in up to twice the luminance of temperate regions. So the selective pressures for bright colouration are stronger in the tropics, and evolution has responded accordingly.
Although liquid crystals approximate multilayer reflectors, true multilayers also exist in beetles, particularly tropical species. Observe the broken wing case of a Thai flea beetle in an electron microscope and a clearly defined stack of thin layers becomes evident. But then spongy structures can sometimes be found in the wing cases of beetles, which, like the liquid crystal, work in a sim
ilar way to the true multilayer reflectors.
H. E. Hinton was an entomologist at Bristol University in England with a strong interest in colour. In 1971 he was collecting insects in Venezuela. But his most exciting find came not from his trap samples, but while he was filling his car with petrol. A male Hercules beetle, the second largest insect in the world, flew into the strip lighting of the petrol station and fell to the ground. This would have been quite a sight - in flight, this beetle looks like an armoured bird. Hinton got to the stunned beetle first and quickly placed it inside a sock from his luggage. The beetle’s horns became stuck in the sock, which turned out to be the perfect prison. Hinton, however, was more than curious about the specimen. ‘At odd moments I used to take it out and play with it,’ he admitted. But of more interest to science, he added, ‘In due course I became aware that its elytra [wing cases] would change to greenish yellow and back again to black.’
The wing covers of the Hercules beetle contain a spongy layer above a black pigmented layer. The holes in the sponge act as alternate layers of a multilayer reflector, and this can account for the greenish yellow colour Hinton observed. But what about the intermittent black colouration?
The above multilayer condition is satisfied when the holes in the sponge are filled with air. In such a case, light effectively recognises a difference in media and the thin layered effect emerges. But that effect disappears when the holes are filled with water, a medium with optical properties similar to that of the beetle’s wing cases in this instance. Now, light recognises no boundaries as it passes through the spongy structure, and is stopped in its tracks only by the black pigment.
Hinton’s beetle was observed under different conditions of humidity. Under high humidity levels, the spongy layer of the wing covers filled with water and they appeared black, from the pigment. Under low humidity levels, the air spaces were restored and the yellow and green wavelengths were reflected before they reached the underlying black pigment. The physical structure, and consequently the colour, was altering. So it’s not surprising that liquid crystals and chameleons were mentioned in the same sentence in the lecture theatre. But, like the chromatophores of chameleons, do structural colours have a biological function?
Because of their behavioural effects, structural colours are easier to define than pigments. They are the brightest colours found in nature, and their visual effect must always have a function - where they occur on parts visible in the environment, that is. Structural colours do derive from physical structures, so potentially they may have another function. A broken mammoth tusk, for instance, reveals a stack of concertinaed layers internally, which lend strength to the whole tusk. The layers in this case, however, are much thicker than the size of light waves, and they do not cause colour. So change the dimensions of a reflector’s structure, and the colour, but not the strength property, can disappear. This change may be slight. Consequently a minor mutation is all that’s needed to put an end to a structural colour. And considering its powerful visual effect, selective pressures acting on a redundant structural colour would be strong. Redundant structural colours do occur in nature, but only on parts not visible in the natural environment. Many shells opt for an alternative to changing their internal layer thicknesses. They have shiny, structurally coloured internal surfaces, but this visual effect is masked from the mollusc’s environment by an outer layer of absorbing pigment. In Darwin’s statement at the head of this chapter, the phrase ‘whenever colour has been modified for some special purpose’ refers only to pigments. I suggest that structural colours entering the environment are always functional. They can’t afford not to be.
Unfortunately, lack of space in this chapter precludes mention of some fascinating alternative mechanisms for producing structural colours, although some of these will appear in subsequent chapters. I must also omit detailed reference to the large glass cabinet to be found in Down House, stuffed with hummingbirds and birds of paradise with magnificent structural colours. Also, I have stopped before things start to get really complicated. This is where biology becomes a minor subject and complex electromagnetic scattering theory, deep within optical physics, takes central stage. The purpose of this chapter was not to explain the complete workings of natural colours, rather to hint at the range and sophistication of colour in the natural world.
More specifically, in this chapter I aimed to generate thought about how animals have adapted to light in general. This, as I have suggested, involves not only colour, but also shape and behaviour. Evolution has resulted in the refined adaptation to sunlight wherever we look. I have provided examples; it is now down to the reader to look around and complete the picture. If we can fully appreciate this great adaptation, we will have discovered a major clue to help us understand the Cambrian enigma. Thoughts assembled in this chapter will become moulded into something firmer as this book progresses, and eventually all will become clear. Crystal clear.
Light certainly is a force to be reckoned with today . . . where sunlight exists, that is.
4
When Darkness Descends
Blessed is your rising in the horizon of heaven, living Sun, you who were first at the beginning of things. Your rays embrace the lands to the limits of all that you have made
Hymn of Akhenaten, pharaoh of Egypt (1,000 BC)
. . . Well, almost to the limits.
In the second half of the eighteenth century, before the declaration of evolution, the Reverend Gilbert White wrote many letters to Thomas Pennant and Daines Barrington, acquaintances who shared his interest in the natural history of Britain. White lived in the Hampshire village of Selborne, and used the wildlife of his parish to encourage the zoological curiosity of his fellows. In 1788, more than a hundred of his letters were gathered into a single volume. The Natural History of Selborne became the fourth most published book in the English language.
White, Pennant and Barrington described the wildlife of Selborne, and some of the nature encountered during their expeditions around Europe, as they saw it. They painted a vivid picture, one in existence only under daylight. But did they acknowledge life at night? And did Darwin observe the fields and woodland surrounding Down House as the sun went down? The answers are ‘no’ and ‘no’ again. The previous chapter begs the question ‘What about nocturnal animals?’ Well, I had a reason for overlooking this subject. Night-time on terrestrial Earth is a grey area. It is neither bright nor completely dark.
Darwin, faced with a mountain to climb in any case, ventured only into the world he saw with clarity. Humans have adapted to the visual world of daytime. But a letter from Thomas Pennant to Gilbert White indicates that there also exists a visual world at night. During a tour of Scotland, Pennant noted his sighting of an eagle owl.
I once spotted an eagle owl in the heart of England. Driving home in the dark, my headlights picked out the sign for my home village. All seemed perfectly normal, except that an eagle owl was perched on the sign. Wait a minute. An eagle owl, in England? I must have been mad or drunk. But I knew I hadn’t been drinking. Maybe the eagle owl, over 2 feet tall, was a figment of my imagination. I was unaware of Thomas Pennant’s sighting at the time, but I knew my owls. And eagle owls do not live in Britain.
I decided to forget about my apparition . . . until I turned on the radio the following morning. Concluding the regional news was a story about an Egyptian eagle owl - one that had escaped from the local wildlife park. Suddenly I chose to recall my apparition of the previous evening. And the most memorable part of that bird was its eyes - its huge eyes.
What Thomas Pennant saw in the eighteenth century is no longer relevant today. Although they once lived in Britain, eagle owls reside elsewhere now. But where they do exist, they are active at night. And to catch their prey they use sound . . . and light.
In the previous chapter we learnt that through larger eyes pigments would appear brighter, because big eyes sample a larger segment of the pigments’ multidirectional reflectance. At night the Earth is lit b
y moonlight - the sun’s rays reflected from the moon. Humans cannot efficiently detect these rays and often fall short of the visual frontier at night.
Now Darwin’s exclusions and the eagle owl’s eyes become interesting. What Darwin could not see, the eagle owl can. The theme of this chapter is darkness, and what happens to wildlife that is deprived of light. But on a journey into total darkness it is worth adjusting our eyes via intermediate cases, beginning with the first step.
Night-time on land
Without the aid of night-vision equipment, it is not surprising that Victorian and earlier naturalists concentrated their efforts on daytime. But while they gazed into their perceived darkness, nocturnal rodents scurried in front of their eyes, and owls were watching them.
Mammals were never going to be champions of camouflaged shapes. Their highly sophisticated machinery, particularly their warm blood, calls for a generous volume compared to surface area - they must be roundish. Still, they try their best to be camouflaged, as with the lioness hiding itself in the grass. They have succeeded with background-matching colours, but sometimes that is not enough, in which case they are compelled to evolve in darkness.
It is interesting that on land the same physical environment exists at night as it does during the day. Trees and rocks continue to provide nooks and crannies . . . but no longer areas of brightness and shade. And the evolutionary outcome? There are considerably fewer species active at night compared with the day. There really are fewer niches - ‘ways of life’ - available at night.
The reduction in niches caused by the lack of light is central to this outcome. And then comes the secondary factor - feeding. Ripples travel down the whole food pyramid. Fewer niches lead to fewer species near the base of the pyramid. This in turn narrows the whole pyramid, where at the top there are fewer predators. But the night-time pyramid occupies the same physical space as that of the day-time pyramid. So the food web becomes stretched and offers less opportunity for tangling, or for evolution to cross lines. Evolution maintains a comparably low diversity at night.
In The Blink Of An Eye Page 14
