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Tuesday, January 25, 2022

When scientists filmed the inner sanctum of a chrysalis

They found a speckled illuminated field, like of thousand fireflies, on the wings of the butterfly

Written by Ranjit Lal |
December 22, 2021 10:00:50 am
ranjit lalThe Common Peacock - the magic of iridescence (Source: Ranjit Lal)

Every child knows that if you pick up a butterfly and let it go, it will leave a faint calling card of the finest dust imaginable on your fingers: That ‘dust’ is actually a magic dust, and under powerful microscopes have reveled itself to be scales, one of the most important components of the butterfly’s wings and beauty. (The scientific name of the family of butterflies and moths is Lepidoptera, meaning ‘scale-wing’.) Researchers also figured out mostly what happened when a caterpillar turned into a chrysalis and what happened inside the chrysalis: the caterpillar dissolved into a gooey ‘soup’ and with the help of beautifully choreographed hormones and enzymes taking their cues precisely, the dormant butterfly cells, deliberately held back from maturing, now began to develop into the various parts of the insect, nourished by the rich caterpillar soup.

Here inside the chrysalis, the beautiful wings of the butterfly developed, and when looked at under a microscope, researchers for long have compared them to the fine tiling on a paper thin roof. But while they had stills of various stages in the formation of the wing and its scales, frustratingly they didn’t have a continuous timeline as to how exactly these scales grew and developed, until recently. They had a slide show, but they wanted the video! With all the disingenuousness of nerdy Peeping Toms, researchers at MIT got to work. First, with surgical precision, they cut out small squares from the cuticle (outer layer) of the chrysalis of one of the most popular butterflies, the Painted Lady, and peeled it off. They sealed the tiny ‘window’ with thin glass panes affixed with a bio-adhesive. But they couldn’t just shine a wide beam of light at their subjects and peer down the microscope just yet: that would damage their delicate subject. So they resorted to what they call, ‘speckle correlation reflection phase microscopy’.

They used many tiny needle-points of light to illuminate their subject. These reflected off the various points of the wing’s structure, and the researchers were able to construct a detailed three-dimensional image of the wing’s structure by figuring out which speck of light had bounced off which part of the structure by isolating the light bouncing off different layers. As one of the researchers rather poetically put it ‘a speckled field is like thousands of fireflies that generate a field of illumination points.’

When they finally put their eyes to their microscopes and pried, what they saw was astonishing.

They saw that as the wing formed, the cells, (made of chitin, a sugar molecule; the same as what the exoskeletons of insects comprise of) on its surface – both on the top and bottom; dorsal and ventral, began lining up neatly and quickly like cadets on parade. Then they began to differentiate: alternately, they grew on the top and bottom surfaces (called cover and ground cells), overlapping each other like shingles on a roof. As the scales grew to full size, they developed irregular ridges, growing in neat parallel lines, all along their length, rather like rain gutters – and partially serving much the same purpose – letting water run off, and controlling temperature (thermoregulation). The researchers thought that as the scales grew they would squeeze against one another rather like the pleats of an accordion but that didn’t happen; the scales continued to grow in size as the ridges appeared. Apparently some other mechanism was at work, which they haven’t quite figured out yet.

These ridges were also responsible for much of the butterfly’s beauty: In addition to pigments, which were responsible for reds, browns and blacks, which were present in the wing membranes, these irregular ridges bounced and scattered light prismatically giving rise to wonderful iridescent sheens and shades: blues, emeralds, violets and mauves and a million tints in between. These colours mix, merge and change as the butterfly opens and closes its wings. A wing may have both, pigmented colours and prismatic ones. Apart from beauty, scales – being so detachable – also enable a butterfly to escape from spiders’ webs – they just shed their scales, which stick to the web and flee. (But no, lost scales do not re-grow). Scales also help the butterfly to generate lift during gliding flight. By changing colour and patterns as the butterfly opens and closes its wings, they help in camouflaging the insect from predators – or scaring them off (those startling ‘owls’ eyes on the wings of some butterflies for example).

It is truly amazing to watch the way these delicate wing scales form; working in perfect step with one another as they grow. It is also remarkable how researchers found a way to sneak a peek into and film the goings-on in the innermost sanctums of the chrysalis. And of course, we’re already thinking up uses of this wondrous technology: from iridescent windows and solar cells, to waterproof textiles, rain and heat resistant surfaces and hard-to-counterfeit currency. But what’s most amazing of all is how this astonishing procedure evolved in the first place: how many millions of years did it take until Mother Nature figured it all out, one miniscule step at a time. When was the first step in this direction taken? Today we can only hope to adopt it for our own purposes. The process of evolution is certainly the best engineering system in existence on our planet and all we need to do is to study and observe it in detail – not recklessly destroy its products. So, the next time you reach out to catch a butterfly (maybe for your child), pause. That fine, faintly coloured, ultra hi-tech powder it will leave on your fingertips is best left on the insect it was made to serve.

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