PINK GLASSWING BUTTERFLY
This is a photo of a pink glasswing butterfly (Cithaerias merolina) from Peru. The wings of this butterfly are for the most part transparent. However, there is a limited range of viewing angles where the wing membrane reflects iridescent-like colors, as shown in the photos below.
Approximate Photo Location (Underside)
Field of view: ~1/4” x 3/8” (6.0mm x 9.0mm)
Images in focus stack: 48
Butterflies in the Cithaerias genus can be found in the rainforests of Mexico and Central and South America (1). They typically live in the understory, close to the forest floor (1). (Some photos of them in the field are available here.) The wing transparency in these butterflies likely serves as a camouflage, making it harder for predatory birds to track their movements during flight (3). The hair-like structures on the wing are piliform scales. These help the butterfly shed water and may be used for defensive purposes (3).
In the animal world, transparency is a characteristic that is commonly seen in invertebrate sea life in the top 500 meters of the ocean (5). Surrounded by such a featureless environment, these animals rely on it to hide from predators and stalk their prey (4, 5, 6). It is more uncommon, however, to find transparent animals on land (3, 4, 5). Several reasons for this are: 1) the refractive index of living tissues is more similar to that of water, than of air (i); 2) land animals need pigments to protect themselves from UV radiation; and 3) land animals cannot use buoyancy to support their bodies, but rather must use additional anatomical structures that are often opaque (3, 4). Some land animals that do have relatively large transparent features are glasswing butterflies, clear-winged moths, glass frogs (transparent abdomens), and cave-dwelling snails (translucent shells)(ii). Scientists studying the Greta oto butterfly (a glasswing, pictured above) found that the transparency in their wings is caused by tiny nanostructures that cover both sides of the wing membrane (3). These structures have thin pillar-shaped bodies with cone-shaped bases. Their height, radius, and location on the wing are irregular and appear random (3). The pillars create a relatively smooth change in the refractive index from the air to the base of the wing membrane and reduce the level of reflected light (3). Using optical modeling, the team found that the level of reflection was higher, and the change in refractive index more abrupt, when the pillars had: 1) random heights and no bases, 2) the same height with bases, and 3) short or thin bases (3). While the index of refraction for the wing membrane and a material like glass are quite close, the wings (under normal incident angles) reflect four times less visible light (2% vs. 8%)(3). This is true for a wide range of angles (up to 65 degrees), making the transparency nearly omni-directional (3). Inspired by these forms, a team at MIT prototyped a similar material for solar energy cells (8). Typically, bare silicon cells reflect up to 30% of light, reducing the amount that can be captured and turned into energy (8). The team replicated the pillar-shaped structures on a transparent coating and was able to increase the transmission (i.e., passage) of light by 2% to 8% for angles of 25 degrees, 45 degrees, and 65 degrees (8, 9).
i. An easy to understand introduction to refractive index can be found here. If, for example, we hold a pyrex stirring rod in the air we can easily see that it is transparent but not invisible. This is because the light is reflecting off the surface and refracting (or bending) through the glass. (If there is an image or pattern behind the glass rod, the refraction is more obvious as the image will appear distorted.) The index of refraction tells us the extent of the refraction, so here we have light passing from air (n=1.0003) to pyrex (n=1.470). (N is the speed of light in a vacuum divided by the speed of light in the material. The higher N is, the more the light slows down and refracts. What we are interested in here is the amount of change between materials.) If you fill a pyrex container with Wesson vegetable oil (n=1.474), and then submerge the rod into the oil, the rod will now look almost invisible. This is because most of the refraction now occurs at the container wall, and the light passes through the rod largely unchanged (7). Going back to the description above, it is easier for an organism to limit the amount of reflected/refracted light in water because the refractive index of water is 1.33 and living tissues are around 1.35 to 1.55 (3, 4). Even if there is a small amount of refraction, the featureless nature of the open sea makes it harder to visually detect it (i.e., there is no pattern behind the glass rod)(4).
ii. Of course the wings of many insects like wasps, flies, cockroaches, cicadas, dragonflies, and others can have transparent structures, some of which also interestingly have colorful interference patterns.
1. Penz, C. M., Alexander, L. G., & Devries, P. G. (2014). Revised species definitions and nomenclature of the rose colored Cithaerias butterflies (Lepidoptera, Nymphalidae, Satyrinae). Zootaxa, 3873(5), pp. 541–559. Retrieved from ResearchGate.
2. Smithsonian Tropical Research Institute (STRI). Cithaerias pireta (C. & R. Felder, 1862). Retrieved from the STRI Symbiota Portal.
3. Siddique, R. H., Gomard, G., & Holscher, H. (2015). The role of random nanostructures for the omnidirectional anti-reflection properties of the glasswing butterfly. Nature Communications, 6(6909). Retrieved from Nature.
4. Ruxton, G. D., Sherratt, T. N., & Speed, M. P. (2004). Avoiding attack: The evolutionary ecology of crypsis, warning signals and mimicry [Chapter 4]. Oxford: Oxford University Press. Retrieved from Google Books.
5. Johnsen, S. (2014). Hide and seek in the open sea: Pelagic camouflage and visual countermeasures. Annual Review of Marine Science, 6, pp. 369-392. Retrieved from Semantic Scholar.
6. Johnsen, S. (2000). Transparent animals. Scientific American, 282(2), pp. 80-89. Retrieved from the Johnsen Lab.
7. Exploratorium teacher institute. (n.d.). Science snacks: Disappearing glass rods. Retrieved from the Exploratorium.
8. Matheson, R. (2015). Biomimetic non-reflective coating for solar cells wins MADMEC. Retrieved from MIT News.
9. Sourakov, A. A., & Al-Obeidi, A. (2019). Biomimetic non-uniform nanostructures reduce broadband reflectivity in transparent substrates [Abstract]. MRS Communications, 9(2), pp. 637-643. Retrieved from Cambridge Core.
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