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Reflections On Golden Scarabs Donald B. Thomas, Ainsley Seago, and David C. Robacker “All that we see or seem, is but a dream within a dream” -Edgar Allen Poe I n Edgar Allen Poe’s The Gold Bug, the protagonist describes a species of “scarabaeus,” from South Carolina: “It is of a brilliant gold color – about the size of a hickory nut – with two jet black spots near one extremity and another longer one at the other.” In Poe’s story the golden beetle is dropped to the ground through the eyesocket of a human skull. By digging at the spot where the beetle landed, a treasure of real gold coins is found. Students of Poe’s writing suggest that the passage of a golden beetle through a human brain case is an allegory for the acquisition of knowledge. Interestingly, such a beetle, Pelidnota punctata (L.), is found in South Carolina and may have served as the inspiration for Poe’s story, though it is more shiny yellow than gold. Truly metallic gold-colored scarab beetles are found in the genus Chrysina (Fig. 1), whose very name is derived from chrysos, the Greek word for gold. The first society of entomologists, founded in London in 1745, was called the Aurelian Society from the Latin word for gold, aureolus. Chrysina beetles are also known as “jewel” scarabs (Cave and Hawks 2001), because in addition to the gold and silver species, many—actually, most—are emerald green (Fig. 2), with some spectacular species having combinations of green, gold and silver. They are prized among collectors to the point of an inordinate fondness, some would say obsession, known among aficionados as the “green fever.” Entomologists are truly treasure hunters! Apart from their extraordinary beauty (and market value), Chrysina beetles are of scientific interest as exemplars of the role of color in evolutionary adaptation and radiation. The genus is a diverse, and thus evolutionarily successful group, 224 Fig. 1. Chrysina aurigans (Rothschild & Jordan) is normally a shiny gold beetle, but this variant shows a red blush, perhaps attributable to an irregular distribution of uric acid in the exocuticle. Fig. 2. Chrysina gorda Delgado, named for the Sierra Gorda in Queretaro, Mexico. American Entomologist • Winter 2007 F emer varia with over one hundred species known (Hawks 2001) and new species still being discovered (Fig. 3). Although they are sometimes thought of as rare beetles, they are more properly regarded as restricted in distribution. Essentially, all of the montane forests from Arizona to Ecuador have one or more species, with each individual species typically restricted to a few and sometimes only one mountain range (Moron 1990). A curiosity is that among the green species, one not infrequently encounters an individual color variant, and even more curiously, these “sports” are almost always red (Fig. 4). Moreover, some species have distinct color morphs. Chrysina purulhensis (Monzon & Warner), originally discovered in the cloud forests around Purulha, Guatemala, is a lovely lavender hue. But the population in the Maya Mountains of Belize has about equal numbers of lavender and emerald green individuals (Fig. 5), leading us to wonder if there is some adaptive purpose driving the frequency of these color variants, or if it is a character that is subject to a higher mutation rate. In the eye of the beholder Like rainbows and mirages, color is an optical illusion. As appropriately suggested by Poe in his poem, “A dream within a dream,” color is a figment of our imagination, or more exactly, the psycho- Fig. 3. Chrysina alticola (of authors, not Bates), (above) a typical rald green form and, (below) a red variant. The puzzle is why the ants are almost always red and seldom (or never) yellow or blue. American Entomologist • Volume 53, Number 4 Fig. 4. The two color morphs of Chrysina purulhensis (Monzon & Warner) from the Maya Mountains of Belize. logical imagery formed by our brains when sensing the visible light phenomena detected by our eyes. Our brains perceive different wavelengths of light as color. Among mammals, only the tree dwellers—squirrels, tree shrews and primates (including the naked ape)—perceive trichromatic color. The retina on the inner surface of the lumen of our eyes has cone-shaped sensors of three types, which respectively detect (are stimulated by) wavelengths of light that correspond to the colors green, blue, and red. The varying proportion of stimulation among the respective types of cone induces our impression of the full spectrum of colors. If all three are stimulated in equal proportion, we perceive the color white. If none of the three are stimulated, we perceive the color black. In the absence of cones, we would see the world as most mammals or colorblind people do: in shades of gray. Interestingly, birds have four types of cones: three responding to the green, red, and blue wavelengths, but also another that detects wavelengths in the ultraviolet part of the non-visible spectrum—that is, non-visible to us. Who knows what color the bird brain images with this cone; perhaps a color that we can’t imagine! Or it may be that birds see the same spectrum as we do, but offset such that our blue is the bird’s green. Insects also perceive color, using analogs of our cones, specialized types of visual cells in their ommatidia (Carlson & Chi 1979). But whereas humans have trichromatic vision, insects have tetrachromatic or even pentachromatic vision, depending on the species (Srinivasarao 1999). Furthermore, and perhaps because of the size limitation of the facets in their compound eyes, insect vision is shifted to the shorter wavelengths of light, the ultraviolet part of the spectrum. Even more astounding, their eyes and brains are able 225 Fig. 6. Chrysina xalisteca (Moron), is green when viewed head-on but exhibits a blue sheen over its high curvature areas when viewed at an oblique angle. This is an optical effect known as Tyndall scattering. It is caused by impurities in the epicuticle and the fact that blue light (shorter wavelengths) is more easily scattered by particulates. Note also the contrasting black legs. Fig. 5. An undescribed species with purple legs from Nayarit in western Mexico.. to perceive polarized light whereas ours do not (Wehner 1976). Objects reflect light of different wavelengths from their surface, producing the optical quality that we perceive as color. An insect appears green to us if it reflects light of a wavelength around 550 nm. Most insects are black or brown because of a pigment called melanin that is embedded in the integument. Melanin absorbs light, and thus such insects appear black to us. Native chitin, the basic material of which insect integument is made, is colorless. It is as transparent as cellophane, a material chemically related to chitin. If it weren’t for the scales (modified hairs), a butterfly’s wings would be transparent, and some butterflies, such as the ithomiine nymphalids, do have clear wings. Light is transmitted through the chitin, not reflected or absorbed, and is thus perceived by us as colorless. The how and why of color The ability to see color increases our visual acuity—a very handy sensory capability. We can spot a red apple in a tree of leafy green, for example. Using color vision, an insect can spot objects like flowers more easily, but similarly, birds can use color to spot insects. Clearly, there is an evolutionary advantage in terms of an animal’s capacity to search the environment for food, mates, and natural enemies aided by color vision. But there is also an evolutionary disadvantage if an insect reflects light of a different wavelength from its immediate environment. Presumably the reason why so many insects are green or brown is for camouflage; they can match the color of the leaves they feed on or the branches they perch on. Many insects have color pigments embedded in their integument. Pigments produce color by selectively reflecting a short range of wavelengths 226 from the incoming sunlight and absorbing the rest. The commonest green pigment in insects is insectiverdin, a compound chromo-protein which includes blue biliverdin and yellow carotene. Insects acquire carotenes and flavinoids in their diet by eating plants and metabolizing them into the pigments which they sequester in their cuticle. Chrysina beetles never evolved the ability to make insectiverdin mainly because their larvae do not feed on the parts of the vegetation where they could acquire the right components (preferring a repast of rotten log). The green color in Chrysina and almost all other beetles is structural color (Crowson 1981). The cuticle of insects consists of three layers: from the outside in, these are the epicuticle, the exocuticle, and the endocuticle. The epicuticle is a waxy lipid and cuticulin layer which functions mainly in water balance, keeping the insect from dessicating. This layer is typically thin and transpar- Fig. 7. Schematic representation of a multilayer reflector, a stack of chitin layers, some with urate crystals incorporated to enhance reflection. American Entomologist • Winter 2007 ent. It also has some optical properties that scatter light, contributing to the shiny appearance of some beetles (Fig. 6). The endocuticle usually consists of sclerotized chitin; chitin tanned with polyphenols and protein residues. The endocuticle is a thick layer functioning as the structural exoskeleton, but also contributes some optical properties by acting as an absorber of residual light transmitted through the cuticle above. Between the epicuticle and the endocuticle is the exocuticle, which produces the structural color in beetles. In Chrysina the exocuticle is lamellar, consisting of many stacked layers of chitin. The layers alternate between thick sheets of transparent chitin and thinner sheets of chitin containing small quantities of uric acid (Fig. 7). Uric acid is the primary waste product from protein metabolism in insects and in its native state has a white color (yes, it’s the white stuff in bird poop!). The moiety incorporated into the chitin is probably the urate crystal form. The presence of the urate crystals increases the reflectivity by a factor of 20-fold (Caveney 1971). All of these chitin layers transmit light, that is, they allow most of the direct light to pass through to the layers below. But the layers with uric acid reflect some of the angular light back towards the surface. This arrangement is called a multilayer (or quarterwave stack) reflector. The incident light, that is, the incoming sunlight, is “white” light consisting of many wavelengths. As the incident light passes through the chitin layers, it is refracted: the light beams arriving at an angle to the cuticle surface are bent by the selective slowing of the beam as it passes through one layer to the next. Refraction is the phenomenon that causes a pencil to appear bent in a glass of water. It is the ray of light that is bent, not the pencil. A consequence of refraction is that the incident light also changes its color due to dispersion. An everyday example is the color of the sun. The sun appears white when it is directly overhead and refraction is minimal. But later in the afternoon, the sun appears yellow because the light now slows as it passes through the atmosphere at an angle relative to the interface, dispersing the shorter wavelengths at a higher angle and leaving the longer wavelengths of the light to reach our eyes. This stimulates the red receptors in our eyes more and the blue receptors less, causing our brains to perceive the light as yellow. The layers in the beetle cuticle work much the same way. In this case, natural selection has favored an optical thickness of the layers such that the reflected light is in the green wavelength. In order to reflect bright green light (constructive interference), the optical thickness of each chitin layer must be close to 150 nm, the wavelength divided by four, hence the term “quarter-wave.” Optical thickness is a combination of the physical thickness of the layer and the refractive index of its constituent material. Native transparent chitin has a refractive index about the same as window glass. By incorporating uric acid crystals into alternating chitin layers, the refractive index increases, and thus these layers American Entomologist • Volume 53, Number 4 have to be thinner so that their optical thickness is the same as the layers without the uric acid. By having all of the layers the same optical thickness, all of the layers reflect the same color, which in this case is green. The structural green of Chrysina beetles is much more intense than the pigment green of insects like grasshoppers because in pigmented insects, only a small portion of the total incoming light is reflected, but because of the constructive interference at each layer in the beetle cuticle, most of the incoming light is reflected (Fig. 8). Some Chrysina beetles are metallic silver in color. It takes only a simple morphological modification to produce this optical effect. A slight change in the thickness of the chitin layers would correspondingly increase or decrease the wavelength of the reflected light, so if the layers were thicker, the reflected light would be shifted toward the red end of the spectrum, or if thinner, the color would be shifted to the blue end of the spectrum. This perhaps explains the red color variant among the green species, but leaves us without a clear explanation of why blue or yellow variants are seldom found. In silver beetles, instead of having all of the layers of the same thickness, the layers successively decrease in thickness proceeding from outside in towards the endocuticle. This is called a “chirped” reflector. Instead of reflecting only one color, the cuticle can now reflect the full array of colors. This would produce a white color if the reflected light were scattered (certain weevils produce the most optically brilliant whiteness known, according to Vukusic et al. 2007). But because the reflected light is directional and coherent and not scattered, an effect called “specular” reflection is produced. The result is a mirror finish, like the reflection of light off the surface of a pond. Just as one can see the sky in a reflecting pool of water, one can see one’s Fig. 8. In a quarter wave stack, constructive interference at each layer reflects light of a wavelength four times the optical thickness. Structural colors are more intense because a greater proportion of the incident light is reflected compared to the light reflected by pigments. 227 Fig. 9. Chrysina chrysargyrea (Salle) has a metallic silver color because of a regular “chirped” stack reflector, as opposed to the irregular, chaotic stack in the scales of silver fish (vertebrates, not thysanura). own face in the elytra of a beetle like Chrysina chrysargyrea (Salle) (Fig. 9), and that is why we are careful not to be naked when photographing silver Chrysina beetles. Another simple modification produces gold instead of silver. If the thinner chitin layers are absent, there is no reflection of the blue light component, and just as in the case with the sun low on the horizon, the spectral shift in the reflected light is towards the yellow. As a consequence, instead of a silver beetle, the specular reflection is golden-hued (Parker et al. 1998). Why shine? While it is understandable that a beetle would have an evolutionary advantage to being green, why would a beetle be metallic silver or gold? Hinton (1973) suggested that the bright glare from the surface might temporarily blind a potential predator, enabling the beetle to escape. Alternatively, it is thought that a mirror finish enables a beetle to match whatever environment it finds itself in. Thus, hiding among the leaves, its shiny surface would mirror the green color of its environment, but during the dry season, it could also hide among the same leaves when they are brown. As elegant as this explanation is, it is not entirely satisfactory. One might predict that the silver and gold beetles should be most prevalent in seasonal environments, such as the tropical deciduous forests. In fact, all of the gold and silver species known are found in evergreen tropical cloud forests. Even the green species present yet another puzzle. If the bodies of Chrysina beetles are green to blend with their environment, why do so many of the species have contrastingly colored legs and tarsi (Fig. 10)? One suggestion is that the striking appendage color may function in conspecific mate recognition. Typically, Chrysina species occur in sympatric species assemblages and the need to distinguish one another may be driving character displacement in color pattern. 228 One of the most spectacular Chrysina species is C. gloriosa (LeConte), a beetle which combines a green dorsum with irregular stripes of silver. The host plant of C. gloriosa is the juniper tree, Juniperus monosperma (Engelmann). The foliage of this tree is green with white flecks and many of the insects that specialize on juniper, such as the stinkbug Banasa euchlora Stål and the larvae of the inchworm Semiothisa, are also green with white spots and stripes. So this striking beetle, gaudy in hand, is quite well camouflaged when tucked into the foliage of its host plant (Fig. 11). Some of the metallic Chrysina beetles, such as C. resplendens (Boucard) (Fig. 12) are brassy in color rather than a pure gold or silver. This species reveals some remarkable aspects of color Fig. 10. Chrysina macropus Francillon has a green body but striking red legs and blue tarsi. The male specimen in the photo also exhibits the sexual dimorphism of metafemoral enlargement. American Entomologist • Winter 2007 Fig. 12. A brassy Chrysina resplendens (Boucard), a species found in Panama. Fig. 11. Chrysina gloriosa (LeConte) blending with the foliage of its host plant, juniper. in Chrysina. Instead of having a single stacked reflector, C. resplendens has three. Chitin is a long-chain molecule, much like cellulose. Each layer of chitin is laid down as a sheet, and within this sheet the chitin molecules are parallel to one another. It thus has the ability to act as a polarizer of light (materials with long chain molecules are used to make polarized sunglasses). But even more remarkably, as the sheets of chitin are laid down, each is rotated slightly relative to the layer below it (Fig. 13). Hence, the chitin stack has a period, that is, the layers successively rotate the full 360º and continue through another turn in a helicoid arrangement. Optimal reflectivity is achieved when the periodicity, the pitch of the helix, matches the wavelength of the light. Consequently, the light reflected by Chrysina beetles is not just polarized, it is circularly polarized, a characteristic of the light produced by liquid crystals in the LED screen in laptop monitors (Neville and Caveney 1969). The sense of the rotation is always anticlockwise (looking down through the beetle), producing reflected light that is left circularly polarized. One consequence is that if one looks at a Chrysina beetle through a circular analyzer (a polarized filter and quarter wave plate), the color is extinguished (Fig. 14), whereas a grasshopper with insectiverdin, or other non-ruteline scarabs such as Phanaeus or Euphoria, will still be green (Kattawar 1994). In Chrysina resplendens, two of the stacked layers are helicoid in arrangement. But between the two helicoid stacks is a stack of layers that all lie in the same direction (Caveney 1971). This unidirectional stack acts as a retardation plate. Whereas in other Chrysina species the reflected light from the cuticle is only left circularly polarAmerican Entomologist • Volume 53, Number 4 ized, the retardation plate reverses the sense of the light reflected from the underlying helicoid stack, such that this beetle reflects both left and right circularly polarized light. Consequently, C. resplendens is able to reflect more of the incident polarized light than do the other species. The top stack of C. resplendens reflects primarily green wavelengths, while the lower stack reflects primarily in the orange wavelengths. The combination produces the brassy color. Hence, although C. resplendens does not appear to be as shiny to us humans as the gold or silver species, it is probably even shinier to other insects, because unlike us, insects have P-vision: they can see polarized light. It has thus been suggested that the optical property of the integument could function in intraspecific signaling (Vulinec 1997). The enhanced intensity in the reflection of polarized light might be a display that is important in mate recognition and stimulation, while not even visible to vertebrate predators. One can’t help but wonder what other optical effects occur in nature Fig. 13. The helicoidal arrangement of the chitin layers results in the reflection of circularly polarized light. 229 Fig. 14. A green Chrysina beetle viewed through a circular analyzer (a quarter wave plate and polarizing filter). Filtering of the polarized light depends on the orientation of the long-chain molecules in the filter; hence, a rotation of the filter by 90 degrees extinguishes the effect. that are beyond our visual capacity. Obviously, there is more to these beetles than meets the eye. Acknowledgements Karen Robacker executed the photographs for this article. David Hawks confirmed our species determinations. Mary Liz Jameson provided helpful comments on the manuscript. References Cited Carlson, S.D. & C. Chi. 1979. The functional morphology of the insect photoreceptor. Annu. Rev. Entomology 24: 379-416. Cave, R.D. & D.C. Hawks. 2001. Jewel Scarabs. National Geographic 199(2): 52-61. Caveney, S. 1971. Cuticle reflectivity and optical activity in scarab beetles: the role of uric acid. Proc. Royal Soc. London B. 178: 205-225. Crowson, R.A. 1981. The Biology of the Coleoptera. Academic Press, Glasgow. UK. Goldstein, D.H. 2005. Reflection properties of Scarabaeidae. Pp. 1-6, In: J.A. Shaw & J.S. Tyo (eds.). Polarization Science and Remote Sensing II. SPIE Proc. Vol. 5888, Bellingham WA. Hawks, D.C. 2001. Taxonomic and nomenclatural changes in Chrysina and a synonymic checklist of species (Scarabaeidae: Rutelinae). Occ. Pap. Consortium Coleopterorum 4(1): 1-8. Hinton, H.E. 1973. Some recent work on the colours of insects and their likely significance. Proceedings & Transactions British Entomological Society 6: 43-54. Kattawar, G.W. 1994. A search for circular polarization in nature. Optics & Photonics News Sept 1994, pp. 42-43. Moron, M.A. 1990. The Beetles of the World, Vol. 10: Rutelini 1. Sciences Nat. Venette, France, 145 pp. Neville, A.C. 1977. Metallic gold and silver colours 230 in some insect cuticles. J. Insect Physiol. 23: 12671274. Neville, A.C. & S. Caveney, 1969. Scarabaeid beetle exocuticle as an optical analogue of cholesteric liquid crystals. Biol. Rev. 44: 531-562. Parker, A.R., D.R. McKenzie & MC. Large. 1998. Multilayer reflectors in animals using green and gold beetles as contrasting examples. J. Exp. Biol. 201: 1307-1313. Srinivasarao, M. 1999. Nano-optics in the biological world: beetles, butterflies, birds, and moths. Chem. Rev. 99: 1935-1961. Vukusic, P., B. Hallam & J. Noyes. 2007. Brilliant whiteness in ultrathin beetle scales. Science 315: 348. Vulinec, K. 1997. Iridescent dung beetles: a different angle. Florida Entomologist 80: 132-141. Wehner, R. 1976. Polarized-light navigation by insects. Scientific American 23: 106-115. Donald B. Thomas is a USDA-ARS Research En- tomologist with expertise in insect ecology and quarantine issues concerning invasive species. Apart from a sideline describing new species of pentatomids, he is also President-Elect of the Coleopterists Society. Ainsley Seago is currently completing her graduate studies at the University of California, Berkeley. She is a staphylinoid beetle systematist who spends a disproportionate amount of time investigating the structure and function of iridescence mechanisms throughout Coleoptera. David C. Robacker is a Research Entomologist with the USDA-ARS with expertise in chemical ecology and behavior of Anastrepha fruit flies. He collaborates on websites featuring the biodiversity of tropical Lepidoptera and Chrysina beetles, requiring travels to remote areas of Mexico, Central and South America, searching for new records and undescribed species. 7 American Entomologist • Winter 2007