Knowing Physics makes Biology Awesome

There is a belief knocking around somewhere in our cultural rhetoric, that “children have favorites,” and by the time we’re adults we have to have grown out of that silliness and show no ridiculous, unfounded partiality. Bollocks to that. My favorite color is purple, my favorite dinosaur is Euoplocephalus and my favorite butterfly is, without a doubt, the Blue Morpho.

The dorsal view of a large, iridescent blue Morpho butterfly

Morpho rhetenor – picture by Didier Descouens from Wikipedia

 

And why? Well the short answer is BECAUSE PHYSICS.

There’s a long answer too.

Blue Morpho is a common name that covers a number of butterflies in the Morpho genus. I’m not specific in my favoritism, although I will admit some aesthetic preferences. Morpho are predominately forest dwellers from South America, and are particularly well known for the iridescent colors of their wings. These colors may be for defense against predators (aposematism and Müllerian mimicry) – but as it’s a male feature, it’s probably for sexual selection, despite the similarity in human perception between the different species. The very best thing about Blue Morphos is – they’re not actually pigmented.

You remember how color works from school, right? White light is a mixture lightwaves of varying colors, that can be diffracted out under the right circumstances, such as atmospheric moisture or the use of a prism.

Light diffracted into a rainbow through a prism.

image credit: Adam-Hart Davis, taken from Starts With a Bang!

Most blue things look blue because they absorb non-blue light and reflect blue light. (Added complication, because of the way we perceive colors: blue pigments may either reflect blue light or reflect a combination of light colors that our eyes and brain perceive as blue. For my purposes, ‘reflect blue’ is fine.)

BUT it’s not that ‘some light is blue’ – blue and other colors are how we perceive light of different wave lengths. the longest wavelength visible to human perception is red, the shortest is violet.

spectrum

So – with most blue things, what is happening is the molecules of the object selectively absorbing appropriate wavelengths of light so only light with a pretty short wavelength is being reflected, and we perceive that as blue.

With me so far?

Why are Morphos different? Because their blue comes not from absorbing non-blue light, but by reflecting the light in multiple directions in such a way that it appears blue. This is called optical interference, and you’ve seen it on soap bubbles.

Iridescent soap bubbles

Image credit: wikipedia

A bubble consists of a volume of air enclosed by a thin film of a transparent material – soapy water – outside of which there is air again. There are therefore two surfaces from which light can be reflected – the outer air/water boundary, and the inner water/air boundary. The light that is reflected from the inner surface has to travel further than that reflected in the outer surface, but because the film is so thin, this difference is comparable to the  wavelength of visible light (390-700 nm). 

Last bit of physics, I promise!

Interference happens when two waves pass through the same point, and either combine to form a larger wave at that point (constructive interference), or cancel each other out (destructive interference.) It helps to think of the ‘peaks and troughs’ of sine-like wave here:

Screencap from Wikipedia

Screencap from Wikipedia

When white light is reflected off the inside and outside of a soap bubble, some wavelengths (colors) end up ‘in phase’ – peaks coincide because the thin film is an exact multiple of that wavelength – experience constructive interference, and therefore look brighter, and other wavelengths (colors) end up ‘out of phase’ – peaks coincide with troughs – experience destructive interference and therefore are blacked out altogether. The result is a rainbow of colors without the bubble actually absorbing any light. (Varying colors come from the curve of the bubble and a non-uniform thickness of the film.)

So: Blue Morphos!

The dorsal view of the iridescent blue Morpho peleides butterfly

Morpho peleides, from the ANSP - image credit Wikipedia

Lepidoptera – butterflies and moths – have wings that are covered in tiny scales. We learn very early on in our careers as tiny naturalists that it hurts a butterfly if you manhandle their wings, because these scales flake off relatively easily, leaving a powder like substance on our fingers. The iridescent quality of Morphos comes, as I said, not from any pigment, but from the microstructure of these scales.

A high-magnification image showing multiple branched structures in close proximity

Cross section of a scale from the wing of M. rhetenor. Scale=3 μm. Image credit: Vukusic et al. 1999

The scale bar on this image is 3μm, = 3000 nm, = 4 times the longest wavelength of visible red light, 7 times the wavelength of blue light.

Morpho scales are not flat surfaces, but are made up of lots of combed barbs on the micrometer scale, creating multiple surfaces from which light is reflected. Because the barbs are about 200nm apart, half the wavelength of blue light, it is blue light that undergoes constructive interference, and the longer wavelengths suffer destructive interference.

Simple, right?

Well, I simplified it. It is of course, like everything in the world ever, much more complicated than that. There’s a complex intersection of interference, and diffractive scattering. Some Morphos even have two layers of scales – a colorless layer of ‘glass scales’ overlaying their blue ‘ground scales’, which increases the angular range in which the color is visible.

Morpho iridescence is clearly different from that seen in many beetles, and in birds. It appears more limited in color, for example. But spectral analysis suggests that a greater diversity in the ultraviolet range, so butterflies that can see in that spectrum will have an easier time distinguishing species.

But – assuming that irridescent structures are more complex than producing pigment – why go to all the trouble just to look pretty? As far as sexual selection goes, there’s no upper limit on the extreme ridiculousness – ask birds. But Morphos have a much higher visibility range than pigmented butterflies – they can be seen from further away. Vukusic et al (1999) suggest that Morphos aren’t looking to attract females, so much as communicate to other males to keep rivals away. Like much communication, the longer the effective range, the more likely it is to succeed.

Also? they look really really pretty.

 

PHYSICS. Making butterflies better since the Eocene!*

 

*(Probably. I have no idea when Morpho made their first appearance.)

 

References

Kinoshita, S., Yoshioka, S., & Kawagoe, K. (2002). Mechanisms of structural colour in the Morpho butterfly: cooperation of regularity and irregularity in an iridescent scale Proceedings of the Royal Society B: Biological Sciences, 269 (1499), 1417-1421 DOI: 10.1098/rspb.2002.2019

Kinoshita, S., Yoshioka, S., Fujii, Y., & Okamoto, N. (2002). Photophysics of Structural Color in the Morpho Butterflies Forma, (17), 103–121

Vukusic, P., Sambles, J., Lawrence, C., & Wootton, R. (1999). Quantified interference and diffraction in single Morpho butterfly scales Proceedings of the Royal Society B: Biological Sciences, 266 (1427), 1403-1411 DOI: 10.1098/rspb.1999.0794

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