No, there isn't any metal or metal oxide content in insect integument. The iridescent appearance of the integument of many insects is structural in origin. An excellent example is provided by the butterfly in my avatar., which possesses a stunning metallic blue iridescence across its wings, without requiring any metal or gemstone input.
In the case of the butterfly, the iridescence arises from the micro-structure of the scales upon the wings. Butterflies have small scales upon their wings (hence
Lepidoptera, which arises from the Greek λεπισ, a scale, and πτερον, a wing - literally, "scale wing"). When the scales of iridescent butterflies such as
Morpho rhetenor and allies are analysed microscopically, using high magnification electron microscopy, they are found to possess an interesting structure. Each scale possesses micro-structures that consist of a central support, rising from the scale surface, and attached to this central support is a stack of vanes, which, when seen in cross section, have a Christmas tree like arrangement and appearance. These vanes are separated from each other by a distance equal to that of the wavelength of blue light, and thus act as constructive interference amplifiers for blue light. If you recall from the basic physics of wave motion, that the principle of superposition applies to wave motions, then any time two waves that are in phase are added by superposition, the wave amplitude equals the sum of the amplitudes of the original two waves, and so, one way of producing a large amplitude wave is to add, by superposition, two smaller amplitude waves, a process known as
constructive interference. If the waves are out of phase, however, the result will be the cancelling out of the waves - a process known as
destructive interference. This diagram illustrates the principle nicely: the left hand shows what happens when two identical waves are added in phase, resulting in a composite added wave of twice the amplitude, and the right hand shows what happens when two identical waves are added 180° out of phase, resulting in zero signal:
Because the vanes in the butterfly scale microstructure are spaced apart at a distance that is equal to the wavelength of blue light, they cause light waves reflecting off the uppermost vanes to be amplified via constructive interference with the light waves reflected off the lower vanes. Arrange the vanes in series, and the wavelength of light in question is amplified several times. In the case of
Morpho rhetenor, the scales can have as many as 12 vanes in their "Christmas tree" structures, which means that the light signal undergoes a 12 × amplification in intensity. Consequently, the butterflies are visible up to a mile away in bright sunlight. If the vanes were spaced at a different setting, they would amplify a different wavelength of light, resulting in the butterfly appearing to be a different colour.
If you want to see all of this documented
properly, including the relevant electron micrographs of the scales, then there is a nice scientific paper you can download for free, courtesy of
this link. Namely this one:
Quantified Interference And Diffraction In Single Morpho Butterfly Scales by P. Vukusic, J. R. Sambles, C. R. Lawrence and R. J. Wootton,
Proceedings of the Royal Society of London Part B,
266: 1403-1411 (20th April 1999)
Vukusic [i]et al[/i], 1999 wrote:
Brilliant iridescent colouring in male butterflies enables long-range conspecific communication and it has long been accepted that microstructures, rather than pigments, are responsible for this coloration. Few studies, however, explicitly relate the intra-scale microstructures to overall butterfly visibility, both in terms of reflected and transmitted intensities and viewing angles.
Using a focused-laser technique, we investigated the absolute reflectivity and transmissivity associated with the single-scale microstructures of two species of Morpho butterfly and the mechanisms behind their remarkable wide-angle visibility. Measurements indicate that certain Morpho microstructures reflect up to 75% of the incident blue light over an angle range of greater than 100° in one plane and 15° in the other. We show that incorporation of a second layer of more transparent scales, above a layer of highly iridescent scales, leads to very strong diffraction, and we suggest this effect acts to increase further the angle range over which incident light is reflected.
Measurements using index-matching techniques yield the complex refractive index of the cuticle material comprising the single-scale microstructure to be n = (1.56±0.01 )+ (0.06±0.01)i. This figure is required for theoretical modelling of such microstructure systems.
Other examples of iridescent microstructure colouration exist in the animal kingdom, one currently living in one of my aquaria being the Cardinal Tetra,
Paracheirodon axelrodi, a Characin fish from the Rio Negro in South America, which has subcutaneous iridocytes containing crystals of the amino acid guanine, spaced in such a manner as to produce constructive interference of blue light when seen from the correct angle. In the case of the Cardinal Tetra, the colouration of the dorsal body stripe varies with viewing angle: seen from an angle looking around 45° down onto the fish, the colouration is more greenish than blue. When the observer's eye level is level with that of the fish (0° viewing angle), the colour is distinctly blue. If the fish is viewed in an aquarium located high above floor level, so that the fish can be seen at a negative angle of incidence, the colour of the dorsal stripe shifts a little way toward the violet. Here's a photograph of the fish in question:
As can be seen, the fish is strikingly iridescent, and a large shoal of these is a stunning spectacle in an aquarium. Iridescence of this sort is seen in several freshwater fishes, including several popular aquarium species, and the underlying mechanism is common to all of them, namely constructive intereference amplification via spaced guanine crystals in iridocytes. Fishes displaying particularly striking iridescence include:
Paracheirodon axelrodi (Cardinal Tetra) - blue
Paracheirodon innesi (Neon Tetra) - blue
Paracheirodon simulans (Green Neon or False Neon Tetra) - greenish-blue
Hemigrammus erythrozonus (Glowlight Tetra) - red
Rasbora pauciperforata (Glowlight Rasbora) - red
Danio choprai (Glowlight Danio) - red
Hemigrammus ocellifer (Head And Tail Light Tetra) - orange
Hemigrammus rodwayi (Brass Tetra, Gold Tetra) - brassy or gold (note: in this species, iridescence is partly due to trematode infestation, which appears to be harmless to the fish, and tank bred specimens are silver)
Moenkhausia pittieri (Diamond Tetra) - multicoloured, variable
Micralestes interruptus (Congo Tetra) - multicoloured, variable
Tanichthys albonubes (White Cloud Mountain Minnow) - greenish
Barbodes schwanenfeldi (Tinfoil Barb) - silver (the common name of this fish reflects, pardon the pun, its iridescent nature)
More on fish iridescence can be read
here.
There's also a nice scientific paper, which you can download courtesy of
this link. Namely:
Biogenic Guanine Crystals From The Skin Of Fish May Be Designed To Enhance Light Reflectance by Avital Levy-Lior, Boaz Pokroy, Berta Levavi-Sivan, Leslie Lieserowitz, Steve Weiner and Lia Addadi,
Crystal Growth & Design,
8(2): 507-511 (2008)
Levy-Lior [i]et al[/i], 2008 wrote:ABSTRACT: The metallic luster from the skin of fish is due to a photonic crystal system composed of multilayer stacks of cytoplasm and crystals. The crystals are described as thin (50-100 nm) plates of guanine, with no reference to their hydration state. We established through X-ray diffraction that their crystal structure is that of anhydrous guanine. We noted that their crystal structure-function relationship is exceptional compared to other purines with similar molecular stacking of the crystal structure. These elongate in the direction of molecular stacking, in contrast to the biogenic anhydrous guanine crystals whose smallest dimension is in the stacking direction. On the basis of the known crystal structure of anhydrous guanine, theoretical growth morphology was calculated. These calculations predict crystals elongated in the direction of the molecular stacking. The exposed molecular plane of the biogenic crystals is the (102) plane, which is composed of densely packed H-bonded guanine molecules. It is known that the in-plane polarizability of guanine molecules is significantly higher than the direction perpendicular to the molecular plane, most likely causing anisotropy of the crystals refractive index. It is therefore conceivable that the unique morphology observed in crystals from the skin of fish is designed to enhance their light reflective properties.
Incidentally, if you ever see adult specimens of the Diamond Tetra,
Moenkhausia pittieri, in a properly constructed aquarium setting, you'll see why they're called Diamond Tetras - the scales of these fishes possess a multicoloured sparkling that changes with the incidence of the ambient light, resembling the play of light upon the facets of cut diamonds. Another fish that exhibits dynamically changing rainbow iridescence, particularly when morning sunshine illuminates the aquarium, is the Congo Tetra,
Micralestes interruptus. Look out for both of these fishes in a sympathetic aquarium setting, and revel in the spectacle.
In the case of beetles, such as this fine member of the Chrysomelidae,
Chrysolina fastuousa, illustrated below:
structural details of the chitinous exoskeleton are responsible for the iridescence. A particularly fine shot of this species, that is too large to fit on a forum page, can be viewed
here. If you want a
really spectacular example of insect iridescence among the beetles, try the Golden Scarab Beetle,
Plusiotis resplendens, illustrated below:
You can see why its specific name is
resplendens, can't you?
You can obtain a similar effect to some of the Chrysomelid beetles by looking at the data surface of a CD-ROM at an angle, and looking at the iridescent colours produced by interference and diffraction from the fine structure of the CD-ROM data layer. Similar fine structure in the chitinous exoskeleton produces the rainbow hued iridescence of these beetles.
Oh, and in the case of the Golden Scarab Beetle, there's another scientific paper for you to read, and again, it's a free download (from
here). The paper in question is:
Imaging Polarimetry Of The Circularly Polarising Cuticle Of Scarab Beetles (Coleoptera, Rutelidae, Cetoniidae) by Ramón Hegedüs, Gyözö Szél and Gábor Horváth,
Vision Research,
46(17): 2786-2797 (September 2006)
Hegedüs [i]et al[/i], 2006 wrote:Abstract
The light reflected from the metallic-shiny regions of the cuticle of certain beetles belonging to the Scarabaeoidea is known since 1911 to be left-handed circularly polarized. Only photographs of a few selected species of scarabs, taken through left- and right-circular polarizers, have earlier been published. Through a right-circular polarizer these beetles appear more or less dark. This demonstration is, however, inadequate to quantitatively investigate the spatial distribution and the wavelength dependency of the circular polarization of light reflected from the scarab cuticle. In order to overcome this problem, we have developed a portable, rotating analyzer, linear/circular, digital, and imaging polarimeter. We describe here our polarimetric technique and present for the first time the linear and circular polarization patterns of the scarab species Chrysophora chrysochlora, Plusiotis resplendens (Rutelidae), and Cetonischema jousselini (Cetoniidae) in the red (650 nm), green (550 nm), and blue (450 nm) parts of the spectrum. We found the wavelength- and species-dependent circular polarization patterns in scarabs to be of a rather complex nature. These patterns are worthy of further studies.
You can anticipate that microstructures of this sort, or related thereto, will be found to be present in Comma pupae.
