To answer this question we will first have to review the physical properties of light and go over the basics of optics, which is the study of the relationship between sight and light.
Beetles and the Light
Visible light is a section of the electromagnetic spectrum, and it is the only portion that people can see. While electromagnetic radiation can be measured via its various prosperities, such as frequency or energy, it is most commonly categorized by wavelength. Taking one step back, radiation is the transfer of energy in the form of waves or particles. For electromagnetic radiation, wavelength is determined by measuring photons. Put simply, photons are massless particles that also exhibit the characteristics of waves. To measure wavelength a point is designated on one wave (usually a peak or trough) and the length between it and that same point on the next wave gives you the wavelength.
In terms of wavelength, the visible light spectrum ranges from about 700 nanometers (nm) at the extreme end of red to about 400 nm at the extreme end of violet. Longer wavelengths have proportionally lower frequencies and amounts of energy, while shorter wavelengths have proportionally higher frequencies and amounts of energy. Despite what one might assume given the differences in energy, all electromagnetic radiation moves at the same speed, about 3.0 * 10⁸ meters per second through a vacuum, also known as the speed of light. This is because all electromagnetic radiation, whether visible or not, is light.
Returning to the visible spectrum, Roy G. Biv is not just a useful pneumonic for remembering colors but is scientifically accurate as well. Conventional rainbows present these colors in this order due to the fact that the condensation which allows rainbows to form cumulatively act as a large prism that splits white light into its various colors. As for the rest of the electromagnetic spectrum, the most basic classification of types are as follows: just beyond violet is ultraviolet, then x-rays, and then possessing the shortest wavelengths are gamma rays, while just beyond red is infrared, then microwaves, and then possessing the longest wavelengths are radio waves. That we can only see a portion of this spectrum comes down to the physiology of our eyes.
Humans can see thanks to a combination of their eyes and their visual cortex. The eyes receive and encode visual stimuli (light), the optic nerves then pass this information along, and once the visual cortex receives this information, it and other parts of the occipital lobe decode and interpret it to give us our visual perception of the world around us. Each part of this group effort is far more complex than how it is summarized here, but to better understand shiny beetles only the first step needs to be interrogated further.
People are able to see the visible light spectrum due to the cones in their eyes. Wavelengths beyond the visible range cannot be seen because they are either too long or too short for our cones to detect. Both visible light and radio waves are light, it is simply that the tool we come naturally equipped with is unable perceive them optically. For a counter example, some beetles and snakes actually posses the ability to perceive parts of the infrared spectrum. As for us, light enters through the corneas of our eyes. The shape of our corneas make them natural convex lenses, which allow them to refract and focus light toward the pupil, the black opening in the center of the iris. Behind the pupil is the lens, which also refracts and focuses light, though this time onto the retina. The retina is where our photoreceptor (light sensing) cells lie.
There are two principle photoreceptor cells in the eye: rod cells and cone cells (there is technically a third, but for our summation here they are not important). Cones are what allow people to perceive color, whereas rods are relied upon in low light situations. Cones can be divided into three different types, and these are short (S), medium (M), and long (L). As their names suggest, each of these varieties is attuned to its own section of the visible spectrum. S cones receive shorter wavelengths (mostly blue, indigo, and violet), L cones are at the opposite end and receive longer wavelengths (mostly red, orange, and yellow, though some green as well), while M cones cover parts of the range between S cones and L cones. One might think of this as combining different types of radio receives (like AM and FM) to give oneself access to a wider range of the electromagnetic spectrum. As a side note, the various types of color blindness are caused by the lack of one or more of these cone types failing to function properly, leading to a partial or whole reduction of one’s ability to perceive color.
Now all of this explains how light enters the eye and is interpreted as color, but how do the objects being viewed come into play? When light is present and comes into contact with an object it can do at least three basic things: be absorbed, be reflected, or be refracted. Absorption and reflection are what we will focus on here, as these are central to how we perceive objects as having colors. Depending on the physical properties of an object it will absorb certain wavelengths of light (both visible and not) and those that it does not will either bounce off (reflection) or be redirected through it (refraction). Reflections are what allow us to perceive objects as having color, because when we see, for instance, an eggplant (aubergine) it is not intrinsically purple, but has absorbed all of the wavelengths (in the visible spectrum) other than those we perceive as purple and is reflecting those within the purple range. The cone cells we associate with purple are stimulated by this reflected light, causing our brain to interpret the eggplant as being a purple object. With all of that said, we can finally talk about the beetles.
Shiny Beetle Exoskeleton
Rather than calling them shiny beetles, a more technical term would be iridescent, though even this word does not apply to every type of “shiny” beetle. Instead of overapplying the term, it would be beneficial to get into the specifics of what causes the vibrant coloration of various beetles.
The short answer to this question is nanostructures. Certain configurations of nanostructures lead to what is called structural coloration. Optically, this is when perceived wavelengths interfere with one another, sometimes overlapping, and depending on the angle of observation they refract different wavelengths, which produces the rainbow-like iridescence one sees in a bubble or an oil slick. Structural coloration differs from pigments like melanin because interference causes the coloration rather than simple absorption and reflection. In beetles, there are at least three major categories of iridescence generating nanostructures: multilayer reflectors, three-dimensional photonic crystals, and diffraction gratings.
The majority of known beetles with iridescent qualities have multilayer reflectors to thank for this. Instead of skin, beetles rely on exoskeletons made up of chitin to form their outer protective layer. With some species this forms into tightly-packed layers that end up presenting different refractive indexes. The stacking of these small, compact layers with varying indices causes different wavelengths of light to reach different depths, so when these are reflected they present as a variety of colors rather than a uniform one.
Multilayer reflectors lend certain beetles iridescence by producing reflections similar to metals. This resemblance can be seen in scarab beetles, some of which appear silver, gold, or a metallic green. Among other beetles, multilayer reflectors can also be seen in jewel beetles (a type of woodborer) and some longhorn beetles.
Three-Dimensional Photonic Crystals
Far fewer beetles can credit their iridescence to three-dimensional photonic crystals. Most weevils gain their visual appearance through pigmentation or multilayer reflectors, but certain weevils belonging to the subfamily Entiminae posses a different set of nanostructures. While varying between Entiminae weevils, these crystalline arrangements are akin to those found in opals or diamonds with their three-dimensional lattices that bounce light around in the same manner as polished gemstones. These jewel-like reflections are not mathematically perfect in the way their mineral counterparts are, though they still give the Entiminae weevils possessing them a lustrous shine.
Somewhat similar to multilayer reflectors, diffraction gratings are compact, nanostructural arrays, though instead of depth and diffraction indices they are arranged into parallel lines. These lines function like a prism, splitting white light into its various wavelengths. This rainbow-like reflection is found in several different species of beetles, and while more common than three-dimensional photonic crystals, it is still rarer than multilayer reflectors. The spectral diffusion of diffraction grating can be found in certain water beetles, scarabs, and carrion beetles.
Evolutionary Advantages of Beetles Being Shiny
Why Be “Shiny”?
Now that we have gone over how some beetles get their iridescence, the next logical question is: Why? Scientists are not entirely sure, but the most educated guesses converge on survival of the fittest. Those beetles with the traits that make them the most fit to survive in their environments are likelier to live longer and thus have higher chances of mating and passing along their genes to future generations. So how does being iridescent help beetles survive longer?
One advantage to structural coloration is that it widens what colors beetles have access to beyond pigments. Green is a rare color among animals, so being green (even if that green is somewhat reflective) can allow a beetle to blend into foliage whereas another beetle with only pigments for coloration would lack this camouflage. It should also be noted that not all animals see exactly as we do. Just because something looks shiny to us does not mean it will be shiny to predators that eat beetles.
Beyond camouflage, iridescence can also aid in heat regulation. By having added reflective properties that a matte beetle would lack, iridescent beetles are able to remain cooler in hotter climates as well as those with less shade.
It is also possible that rather than being a primary driver of their ability to survive longer than their peers, the iridescence of certain beetles is incidental to what makes them more fit. In terms of protection, thicker shells provide an obvious benefit over thinner ones, and thicker shells are likelier to exhibit multilayer reflectors due to their increased number of layers. Perhaps being shiny is what allows some beetles to thrive, while for others it might just be a showy byproduct.
Works Cited: References
“Colours of light”. sciencelearn.org, Science Learning Hub, Apr 4 2012, http://www.sciencelearn.org.nz/resources/47-colours-of-light, Accessed Oct 26 2020
Grifantini, Kristina. “Beetles Use Nanostructures for Color”. MIT Technology Review, July 23 2009, http://www.technologyreview.com/2009/07/23/211402/beetles-use-nanostructures-for-color/, Accessed Oct 26 2020
Mukamal, Reena. “How Humans See In Color”. aao.org, American Academy of Ophthalmology, June 8 2017, http://www.aao.org/eye-health/tips-prevention/how-humans-see-in-color, Accessed Oct 26 2020
Seago, Ainsley E. et al. “Gold bugs and beyond: a review of iridescence and structural colour mechanisms in beetles (Coleoptera)”. Journal of the Royal Society, Apr 6 2009, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2586663/, Accessed Oct 26 2020
“Visible Light”. science.nasa.gov,NASA Science, Aug 10 2016, science.nasa.gov/ems/09_visiblelight#:~:text=The%20visible%20light%20spectrum%20is,from%20380%20to%20700%20nanometers., Accessed Oct 26 2020
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