Mirrors on the wings – story of Gulf Fritillary

On my usual walk outside with kids; I was met with a visual so stunningly beautiful. 


I rubbed my eyes to believe what I had in front of me. An almost motionless butterfly glistening in the sun; shinning with mirrors on its wings. Let me repeat; yes “mirrors on the wings”. 


To my advantage, the butterfly was alive but almost motionless. Probably at the end of its life-span; it did not resist me picking it up gently by its wings. The first thing I did; is share it with my kids. 


After some quick pings with my entomology friends; it was clear I had a fantastic butterfly at hand – this was “Gulf Fritillary” or passion butterfly (Thanks @Arnaud – who is going to crisper this eventually). 

The first thing you notice is spectacular silver shine on the wings; because of the individual “mirror like” scales on the butterfly. Now; many previous Foldscope enthusiast have explored butterfly wing scales – see these posts: 

Butterfly beauty!

Butterfly Wings – A World of Color

Butterfly Wing Structure – Transparency Edition

Wing Scales & Structural vs Pigment Color

Here we will explore the scales on passion butterfly. Using a regular Foldscope – the kind you all have in your own hands. We will also use this post to actually test the resolution of the 140x Foldscope lens. So let’s get started. 


First, let’s look at scales on a regular glass slide. Here, I created a low density transfer of scales. 


I noticed; using angular illumination – I saw some striking colors (akin to diffraction gratings). 


Video1: angular illumination, trans illumination 

Video2: angular illumination, trans illumination 

Once I saw this – it was clear I wanted to study optical properties of these scales suspended in air (with no backing surface of glass slide). Since I am interested in reflectance; I can’t have a glass surface reflecting some of the light back. 

In this post, I will do something new. I will demonstrate that it is possible to measure the transmission and reflected light properties of an “individual” suspended wing scale – moving towards quantitative analysis of interaction of wing scales with light. 

This is work in progress; but here I describe some Foldscope tricks for users to measure the transmission and reflection spectra of single butterfly wings. And I will use an object that comes in very handy – a human hair. 

In order for a single scale, which can be as thin as a few 100nm; to reflect almost all the light back; it’s got to be a very very good mirror. So I was quiet puzzled and clearly “structural” properties of individual scales have to be at play here. 

First challenge is to mount a “single” scale on something that can be suspended – so true optical properties of this object can be studied. I solve this simply by using a piece of my hair; and rub it against a specific location on the wing. With this, I can reliably pick single scales from anywhere on the wing. Here I first pick “mirror scales”. 


I put the slide half way in; to image individual scales. 


Here is another view of these scales in transmitted light mode. Notice the parallel lines on the scale. Each of those lines are roughly 1.5 to 2 microns spaced apart (I know this from some electron microscope data on these scales). So the resolution of the current setup is at least 2 microns. 


Now; here are the same scales in angular illumination. Notice the beautiful spectra of colors that these scales reflect when oblique white light is incident on the same. This does demonstrate some properties of diffraction gratings – surfaces that can separate the colors of incident light. 


Now, I image the same scales in reflected light mode (where light is coming from the same side as the imaging camera). The reflected mode does indicate that a portion of light bounces back (like a mirror); but it’s clear that some light passes through (otherwise I could not image the scale behind another scale). It gives me relief that these “mirrors” are not 100% efficient – since remember , they are only sub-micron thickness. 


Now; I turn my attention to a non-mirror scale; picked with the same technique to see if I can differentiate light interaction with the same. The first image is in transmission light mode. 


It is hard to distinguish characteristics that are different in transmitted mode. It’s clear that non-mirror scales are smaller. 

Next, I image the same in angular light and reflected light; respectively. 



As you can tell above; the images look dramatically different between mirror scales and non-mirror scales. 

This does show that our prediction that these scales act as mirrors is correct. But why characteristics of a diffraction grating. One clue might lie in the linear groves that are quiet visible. In my next series of experiments, I will also explore if I can modulate the reflective properties by coating the scales with thin film of oils/liquids. 

Until next time, enjoy these close ups of the butterfly. As you can tell; the next post will explore the optics of these incredible eyes. 


Happy new year, everyone. 

Cheers 

Manu 

Ps: I am curious to see if you can employ these “hairs as micro-tweezers” tricks in your own experiments. 

2 Comments Add yours

  1. mohitmojo says:

    That is……spectacular

  2. laksiyer says:

    What a fantastic post Manu. The great diversity of lepidopteran scale modifications for adaptation is fascinating. I suspect it would be controlled only by a few genes, allowing rapid variation in butterflies. Wonder if people have looked into the protein composition of the transparent scales? (I suspect they must have).

    If wing modifications are used to distract or frighten predators, I wonder what these wings are perceived as by predators? Further since they are at the underside, I wonder they work like the metalmark butterflies that have wing patterns to fool ants . I think this is Aaron’s or his advisor’s work! (http://news.nationalgeographic.com/2016/06/amazon-butterflies-ants-insects-stealing/). Diff. gratings at times, transparent at others, for this we need to put on insect-eye goggles :).

    Would be also very interesting to study the sonar properties of such scales in comparison to the other.

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