Nature: Stunning video reveals how butterfly wing scales grow during metamorphosis


A stunning video has revealed how butterflies grow and assemble together the tiny scales that make up their wings as they undergo the process of metamorphosis.

These scales — which number in the hundreds of thousands — both give butterfly wings their colour and shimmer, as well as act to protect against the elements.

Massachussets Institute of Technology experts used an advanced imaging technique to map out the scales on the wings of painted lady butterflies as they formed.

This is the very first time such a process has been captured in real-time — with experts previously having to rely on snapshots of moments in wing development.

Their findings, the team said, could serve as a blueprint for the development of new materials for applications including iridescent windows and waterproof textiles. 

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A stunning video has revealed how butterflies grow and assemble together the tiny scales that make up their wings as they undergo the process of metamorphosis. Pictured: overlapping scales seen in the process of development. Each scal is around 50 micrometres across

Scales — which number in the hundreds of thousands — both give butterfly wings their colour and shimmer, as well as act to protect against the elements. Pictured: a painted lady

Scales — which number in the hundreds of thousands — both give butterfly wings their colour and shimmer, as well as act to protect against the elements. Pictured: a painted lady

INSPIRING INSECTS

According to the team, the scales of butterfly wings confer special optical and structural properties.

Because of this, researchers have tried to use butterfly wings as inspiration for new materials and technologies.

These include, for example, solar cells, optical sensors, rain- and heat-resistant surfaces and even iridescent patterns to improve anti-counterfeit measures on paper currency. 

Understanding how scales form could help make these ideas reality, the researchers explained.

The study was undertaken by mechanical engineer Anthony McDougal and his colleagues at the Massachussets Institute of Technology.

‘Butterflies control many of their wing attributes by precisely forming the structural architecture of their wing scales,’ explained Dr McDougal.

Learning how to employing a similar strategy ‘might be used, for example, to give both colour and self-cleaning properties to automobiles and buildings.

‘Now we can learn from butterflies’ structural control of these complex, micro-nanostructured materials.’

To capture how wing scales are formed, the researchers first raised caterpillars in individual containers. They chose to study the painted lady (Vanessa cardui), a species whose wings have features that are common across most butterflies.

Once each insect had encased itself in a chrysalis and begun its metamorphosis, the team carefully cut into the paper-thin material, peeling away a small square — which they filled with a transparent window — letting them look in on the developing wing. 

Finally, the team used a type of imaging known as ‘speckle-correlation reflection phase microscopy’, which works by shining numerous small points of light across the wing and measuring how each is reflected back to create a map of the subject.

The advantage of this speckle field rather than a wide beam of light, the experts explained, is that it doesn’t risk harming the pupa’s delicate cells.

‘A speckled field is like thousands of fireflies that generate a field of illumination points,’ said paper author and biological imaging expert Peter So. 

‘Using this method, we can isolate the light coming from different layers, and can reconstruct the information to map efficiently a structure in 3D.’

Massachussets Institute of Technology experts used an advanced imaging technique to map out the scales (pictured) on the wings of painted lady butterflies as they formed

Massachussets Institute of Technology experts used an advanced imaging technique to map out the scales (pictured) on the wings of painted lady butterflies as they formed

To capture how wing scales are formed, the researchers first raised caterpillars in individual containers. They chose to study the painted lady (Vanessa cardui), a species whose wings have features that are common across most butterflies. Pictured: a painted lady caterpillar

To capture how wing scales are formed, the researchers first raised caterpillars in individual containers. They chose to study the painted lady (Vanessa cardui), a species whose wings have features that are common across most butterflies. Pictured: a painted lady caterpillar

Once each insect had encased itself in a chrysalis and begun its metamorphosis, the team cut into the paper-thin material (as depicted top), peeling away a small square — which they filled with a transparent window — letting them look in on the developing wing (bottom)

Once each insect had encased itself in a chrysalis and begun its metamorphosis, the team cut into the paper-thin material (as depicted top), peeling away a small square — which they filled with a transparent window — letting them look in on the developing wing (bottom)

Through their visualisations of the growing butterfly wing, the team were able to observe the formation of highly detailed structural features — including the nanometre-high rides on the surface of each micrometre-sized scale. 

Within days of the metamorphosis beginning, the researchers found, cells would quickly line up in rows, ultimately going on to divide up into an alternating pattern of pigmented ‘cover’ scales and structural ‘ground’ scales.

‘A lot of these stages were understood and seen before,’ explained Dr McDougal.

‘But now we can stitch them all together and watch continuously what’s happening, which gives us more information on the detail of how scales form.’

As each scale reached its final size, it was seen to grow long, thin ridges that resemble pieces of corrugated roofing, but it a way that was quite unexpected.

It had long been assumed that these grooves formed through compression — getting squeezed in as they grew, to end up resembling the bellows of an accordion. 

Instead, however, Dr McDougal and colleagues found that the scales continued to grow, rather than shrink, as the ridges appeared, suggesting that a different mechanism must be at play. The process, the team said, merits further investigation.

The ridges, the team explained, help not only to control the insect’s colourations, but also help wings to shed rain, moisture and excess heat.

This is the first time wing formation has been captured in real-time, with experts previously having to rely on snapshots of moments in wing development. Pictured: a comparison of butterfly scales seen under different imaging techniques — specifically, SEM (top right and background) vs speckle-correlation reflection phase microscopy (top left and bottom)

This is the first time wing formation has been captured in real-time, with experts previously having to rely on snapshots of moments in wing development. Pictured: a comparison of butterfly scales seen under different imaging techniques — specifically, SEM (top right and background) vs speckle-correlation reflection phase microscopy (top left and bottom)

‘This paper focuses on what’s on the surface of the butterfly wing,’ concluded Dr McDougal.

‘But underneath the surface, we can also see cells putting down roots like carrots, and sending out connections to other roots. There’s communication underneath the surface as cells organize.

‘And on the surface, scales are forming, along with features on the scales themselves. We can visualize all of it, which is really beautiful to see.’

The full findings of the study were published in the journal Proceedings of the National Academy of Sciences.

WHAT ARE THE PHOTONIC STRUCTURES IN THE WINGS OF INSECTS AND BIRDS?

Millions of years before humanity began to manipulate the flow of light, insets and animals had beaten us to the game.

The wings of butterflies, moths, birds and other creatures contain structures on the nanometre scale which can produce a range of striking optical effects.

They can produce iridescence, metallic colours, and other flashy effects that are important for a number of behavioural and ecological functions. 

Light from the sun bounces off these materials in different directions, with this refraction working like a prism, splitting the light into its component colours.

As the viewing angle changes, the refracted light becomes visible as a shimmering display.

Iridescent surfaces help animals to elude potential predators. 

When these creatures fly, the upper surface of their wings continually changes from bright to dull because the angle of the light striking the wing changes.

As they move their wings up and down during flight, they seem to disappear, and then reappear a short distance away. 

Dark undersides to their wings often strengthen this effect.

Combined with an undulating pattern of flight, this ability to change colour quickly makes them difficult for predators to pursue.

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