Tag Archives: #butterfly

How Butterflies Make Transparent Wings: MBL Scientists See The Invisible (Biology)

Many animals have evolved camouflage tactics for self-defense, but some butterflies and moths have taken it even further: They’ve developed transparent wings, making them almost invisible to predators.

A team led by Marine Biological Laboratory (MBL) scientists studied the development of one such species, the glasswing butterfly, Greta oto, to see through the secrets of this natural stealth technology. Their work was published in the Journal of Experimental Biology.

Although transparent structures in animals are well established, they appear far more often in aquatic organisms. “It’s an interesting biological question because there just aren’t that many transparent organisms on land,” notes lead author Aaron Pomerantz, a PhD candidate in Integrative Biology at the University of California, Berkeley. “So we asked the question, what is the actual developmental basis of how they create their transparent wings?”

Butterfly wings are known for their colorful patterns, created by tiny, overlapping, chitinous scales that reflect or absorb various wavelengths of light to produce colors. Pomerantz says that although scale coloration has been intensively studied, investigating the developmental origins of transparency in land-based butterflies hadn’t been done before. “Transparency is sort of the opposite of color,” he says.

Haetera species of clearwing butterfly. © Aaron Pomerantz

Pomerantz and his co-authors, including his PhD advisor and MBL Director Nipam Patel, were inspired by the work of students in MBL’s Embryology course, in which Patel teaches. “I decided to bring some of the transparent butterfly and moth species I had in my collection, which I never really looked at in detail, to the course and present it as a challenge for the students to look at how these wings were transparent,” Patel says. “A group of students took that on by imaging the wings with various microscopes. And they realized that pretty much any way you could think to make the wing transparent, some butterfly or moth had figured out how to do it. That’s what got us looking in more detail at the development of transparency.”

Building on that work, the researchers used confocal and scanning electron microscopy to construct a developmental time scale of how transparency emerges in Greta oto, from the pupal stage to adulthood. They found that the glasswing butterfly’s wings develop differently than opaque species, with a lower density of precursor scale cells in the areas that will later develop as transparent. At a very early stage, scale growth and morphologies differed, with thin, bristle-like scales developing in transparent regions and flat, round scale morphologies within opaque regions.

“What Greta oto does is to make fewer scales and to make them in these very different, bristle-like shapes,” Patel explains. “But getting the scales out of the way is only part of the problem of creating transparency. Aaron also made a series of observations about nanostructures on the wing that prevent glare in bright sunlight. When light hits these little arrays of nanostructures, it doesn’t reflect off — it goes straight through. So that gives much better transparency,” he says.

“As humans, we think we’re so brilliant because we figured out how to put anti-glare coating on glass, but butterflies basically figured that out tens of millions of years ago,” Patel says.

Unusual wing scales and nanostructures are only part of the story. A second layer of waxy hydrocarbon nanopillars lies atop the wing surface, providing further anti-reflective properties. The researchers examined the reflectivity of the wings before and after removing the waxy layer with hexane.

“We measured the amount of light that reflected off the wing,” says Pomerantz. “Those experiments demonstrated that that upper layer was very important for helping to reduce that glare.” Biochemical analysis showed that the waxy layer is mostly composed of long chain n-alkanes, similar to those found in other insect species. “They’re primarily thought of as something that helps prevent an insect from drying out or desiccating. But in this case, it seems like they’re used for these anti-glare properties as well.”

Future research directions may include delving more deeply into the how these transparent structures evolved. Pomerantz points out that “if we can learn more about how nature creates new types of nanostructures, that can be very informative for human applications.” The work is making the secrets of natural transparency considerably less opaque.

Featured image: A glasswing butterfly feeding at flowers in Costa Rica. The remarkable transparency of these butterflies allows them to be “invisible”, and the antiglare coating of their wings helps to prevent them from being given away by any glare of sunlight off their wings. © Nipam Patel


Reference: Aaron F. Pomerantz, Radwanul H. Siddique, Elizabeth I. Cash, Yuriko Kishi, Charline Pinna, Kasia Hammar, Doris Gomez, Marianne Elias, Nipam H. Patel; Developmental, cellular and biochemical basis of transparency in clearwing butterflies. J Exp Biol 15 May 2021; 224 (10): jeb237917. doi: https://doi.org/10.1242/jeb.237917


Provided by Marine Biological Laboratory

The Surprises of Colour Evolution (Biology)

Nature is full of colour. For flowers, displaying colour is primarily a means to attract pollinators. Insects use their colour vision not only to locate the right flowers to feed on but also to find mates. The evolutionary interaction between insects and plants has created complex dependencies that can have surprising outcomes. Casper van der Kooi, a biologist at the University of Groningen, uses an interdisciplinary approach to analyse the interaction between pollinators and flowers. In January, he was the first author of two review articles on this topic.

Bees and other insects visit flowers to feed on nectar and pollen. In exchange for these goodies, they assist the reproduction of these plants by pollinating their flowers. That is the simple and slightly romantic view of pollination. The reality, however, is full of deception, chemical warfare and biomechanical trickery. ‘The combination of chemistry and physics with evolutionary biology has broadened our view of pollination,’ says Van der Kooi.

Anatomy

He is the first author of a review article on the evolution of colour vision in insects, which was published in the January 2021 volume of Annual Review of Entomology, and of a second review on the ‘arms race’ between plants and pollinators, which appeared on 25 January in Current Biology.

Butterfly visiting a flower | Photo Sara Leonhardt, Technical University of Munich

‘For many insect families, we know very little about how they see colours,’ says Van der Kooi. Bees have been studied in great detail but much less is known about colour vision in flies, even though many of their families, such as hoverflies, are very important pollinators. ‘They are difficult to study and to keep in the lab and the anatomy of their eyes is more complicated,’ explains Van der Kooi. ‘Furthermore, some long-standing ideas on fly vision have recently been overturned.’

Pigments

Van der Kooi and his co-authors tabulated which wavelengths can be seen by different insect species. ‘Basically, insect colour vision occurs at wavelengths between 300 and 700 nanometres. Most photoreceptors in insect eyes detect ultraviolet, blue and green light but there is great diversity.’ Insects evolved colour vision before the first flowers appeared. ‘The pigments in flowers appear to be fine-tuned to be visible to pollinators. But of course, insects have subsequently co-evolved.’

Apart from colour, plants use scent to attract insects to the food that they provide. As production of nectar and pollen is costly, plants need to protect themselves from robbers, which eat the food but do not pollinate the flowers. This is the topic of the second review paper. ‘This paper shows a huge diversity in the relationship between plants and pollinators, from real mutualism to outright abuse.’ Some plants do not provide any food at all. ‘Others have pollen or nectar that is toxic to most bee species. Only specific species can actually digest this food.’

Biomechanics

Pollinators also have their own agenda. ‘One particular plant is pollinated by moths in early spring. The moth also lays eggs on the plant and later in the year, the caterpillars will eat parts of it. Around that time, the main pollinators for this plant are flies.’ This is one example of the complex relationship between plants and pollinators. ‘There can be seasonal differences but the relationship can also be different in different locations – there is variation in time and space and through different biological interactions,’ says Van der Kooi.

The review focuses on different aspects of the complex relationship using views from chemical biology (e.g. the nutrient content of nectar or pollen), biomechanics (e.g. the barriers that flowers use to ward off unwanted insects or to make sure that pollen are dispersed by them) and sensory biology (e.g. the ways in which insects detect and recognize flowers).

Bumblebee | Photo M. Kraaij, University of Groningen

Vibration

Some plants, for example many species in the potato family, have evolved the method of ‘buzz-pollination’, where the pollen are stored in tubes and insects need to vibrate on the flowers to release them. ‘Honeybees, flies and butterflies cannot get to them but other bees such as bumblebees can shake the pollen free using their strong flight muscles.’ The stiffness of the tubes, the stickiness of the pollen and the vibration frequency of the buzzing bees all play a part in this process. ‘You really need tools from physics to understand their relationship.’ The interdisciplinary study of insect-plant interactions is what Van der Kooi loves. He started his career using optics techniques. ‘That is in part because I really like physics. But every new approach will show us new aspects of this complex relationship. ‘

A recent development in the field is the realization that plants are different in different geographic locations. ‘A cornflower in the Netherlands is not necessarily the same as a cornflower in Italy. For example, the chemical composition of the pollen or the nectar may be different, which affects the interaction with insects.’

Insect havens

This has serious ramifications for attempts to boost insect numbers by creating insect havens, explains Van der Kooi: ‘Sometimes, the seed mixtures for flowering strips are not sourced locally but from other countries. In that case, there may be a mismatch with the local insects, which may even harm insect numbers.’ Insect havens are therefore best created using local seeds.

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Video: Casper van der Kooi on his research interests | Video University of Groningen

Both review articles stress how complicated the relationship between plants and pollinators can be. So why do plants bother? Why are they not all using wind dispersal of their pollen? ‘Those are good questions,’ says Van der Kooi. ‘The efficiency of wind pollination is low but that is also true for animal pollination. Yet, roughly 90 percent of plant species use the latter method, so it is a huge success.’ But even this is complicated: ‘Grasses use wind pollination, and in some ways, they are successful groups too. As nearly everything in biology, the answer so often is “it depends…”.’

Featured image: Casper van der Kooi | Photo University of Groningen


References:
1) Casper J. van der Kooi, Doekele G. Stavenga, Kentaro Arikawa, Gregor Belušič, and Almut Kelber: Evolution of Insect Color Vision: From Spectral Sensitivity to Visual Ecology. Annual Review of Entomology January 2021 (first online: 23 September 2020)
2) C.J. van der Kooi, M. Vallejo-Marin, S.D. Leonhardt: Mutualisms and (a)symmetry in plant-pollinator interactions. Current Biology 25 January 2021


Provided by University of Groningen

Butterfly Wing Clap Explains Mystery of Flight (Biology)

The fluttery flight of butterflies has so far been somewhat of a mystery to researchers, given their unusually large and broad wings relative to their body size. Now researchers at Lund University in Sweden have studied the aerodynamics of butterflies in a wind tunnel. The results suggest that butterflies use a highly effective clap technique, therefore making use of their unique wings. This helps them rapidly take off when escaping predators.

The study explains the benefits of both the wing shape and the flexibility of their wings.

The Lund researchers studied the wingbeats of freely flying butterflies during take-off in a wind tunnel. During the upward stroke, the wings cup, creating an air-filled pocket between them. When the wings then collide, the air is forced out, resulting in a backward jet that propels the butterflies forward. The downward wingbeat has another function: the butterflies stay in the air and do not fall to the ground.

The wings colliding was described by researchers almost 50 years ago, but it is only in this study that the theory has been tested on real butterflies in free flight. Until now, the common perception has been that butterfly wings are aerodynamically inefficient, however, the researchers suggest that the opposite is actually true.

“That the wings are cupped when butterflies clap them together, makes the wing stroke much more effective. It is an elegant mechanism that is far more advanced than we imagined, and it is fascinating. The butterflies benefit from the technique when they have to take off quickly to escape from predators”, says biology researcher Per Henningsson, who studied the butterflies’ aerodynamics together with colleague Christoffer Johansson.

“The shape and flexibility of butterfly wings could inspire improved performance and flight technology in small drones”, he continues.

In addition to studying the butterflies in a wind tunnel, the researchers designed mechanical wings that mimic real ones. The shape and flexibility of the mechanical wings as they are cupped and folded confirm the efficiency.

“Our measurements show that the impulse created by the flexible wings is 22 percent higher and the efficiency 28 percent better compared to if the wings had been rigid”, concludes Christoffer Johansson.

Publication in Journal of the Royal Society Interface: Butterflies fly using efficient propulsive clap mechanism owing to flexible wings

Featured image: Silver-washed fritillary butterfly (Photo: Per Henningson)

Provided by Lund University

CCNY Biologist Finds Asian Butterfly Mimics Different Species as Defense Against Predators (Biology)

Many animal and insect species use Batesian mimicry – mimicking a poisonous species – as a defense against predators. The common palmfly, Elymnias hypermnestra (a species of satyrine butterfly), which is found throughout wide areas of tropical and subtropical Asia, adds a twist to this evolutionary strategy: the females evolved two distinct forms, either orange or dark brown, imitating two separate poisonous model species, Danaus or Euploea. The males are uniformly brown. A population group is either entirely brown (both males and females) or mixed (brown males and orange females).

Photo of butterfly in the wild, Elymnias hypermnestra beatrice. Photo credit: Gan Cheong Weei.

City College of New York entomologist David Lohman and his collaborators studied the genome of 45 samples representing 18 subspecies across Asia to determine their evolutionary history and to establish what genes were responsible for the color variation in females. They found that neither the orange nor brown females had a common recent ancestor.

“The conventional wisdom is that once something evolves and you lose it, it’s hard to re-evolve it,” said Lohman. “That suggests something is acting like a switch, switching the gene on or off.”

The researchers found two DNA nucleotides on the Elymnias hypermnestra genome that regulate WntA, a gene associated with color patterning in butterfly species.

The WntA gene can be switched on to recreate the phenotypic shift, even where it hasn’t appeared for several generations. Reaching back into genetic history allows a species to create a variant without having to re-evolve the intermediate biochemical pathways.

“Evolution of a phenotype can be more plastic than we thought,” said Shen-Horn Yen, one of Lohman’s collaborators from the Department of Biological Sciences, National Sun Yat-Sen University, Taiwan.

The study appears in the journal Proceedings of the Royal Society B.

To Lohman, studying Elymnias hypermnestra encapsulates the study of biodiversity in its entirety. There’s a universe of variety in color, form and size and genetic variability all found in a single genus of butterfly.

Reference: Dee M. Ruttenberg, Nicholas W. VanKuren, Sumitha Nallu, Shen-Horn Yen, Djunijanti Peggie, David J. Lohman and Marcus R. Kronforst, “The evolution and genetics of sexually dimorphic ‘dual’ mimicry in the butterfly Elymnias hypermnestra”, Proceedings of the Royal Society B, 2021. https://royalsocietypublishing.org/doi/10.1098/rspb.2020.2192 https://doi.org/10.1098/rspb.2020.2192

Provided by CCNY

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Butterfly Color Diversity Due To Female Preferences (Biology)

Butterflies have long captured our attention due to their amazing color diversity. But why are they so colorful? A new publication led by researchers from Sweden and Germany suggests that female influence butterfly color diversity by mating with colorful males.

Dorsal wing color by sex of European butterflies. © Kalle Tunström.

In many species, especially birds and butterflies, males are typically more colorful than females, a phenomenon known as dichromatism. In many dichromatic species, the more conspicuous sex is more vulnerable to predation. Certainly, the male peacock is a much easier target than the more camouflaged hen. Explaining why one member of a species would place itself in more danger was a challenge to Charles Darwin’s early views on evolution by natural selection, as Darwin envisioned natural selection acting to reduce such risks.

Examples of dichromatism in fact were one of the issues that lead him to develop his theory of sexual selection, where elaborate male traits could evolve through female preference for conspicuous males, even in the face of the increased dangers such males would encounter.

Today, many naturalists and biologists alike generally ascribe the exaggerated coloration of males as being due to sexual selection. However, when we see a species in which males are more colorful than females, sexual selection is not necessarily the only answer. An alternative route to dichromatism might begin with males and females both being very colorful, followed by natural selection acting upon females to make them less conspicuous, perhaps due to the cost of being easier prey. Stated another way, perhaps females become less colorful so they are better camouflaged and therefore preyed upon less. The argument that natural selection could give rise to dichromatism was posited by Darwin’s contemporary, Alfred Russel Wallace. Darwin and Wallace in fact argued for decades about the origins of dichromatism in birds and butterflies.

The reason for this long debate between Darwin and Wallace arises because, without knowing how males and females looked in evolutionary past, either sexual selection or natural selection could give rise to dichromatism. Since they had no way of formally assessing what species used to look like, their argument had few routes for resolution.

This is where researchers from Sweden (Stockholm University and Lund University) and Germany (University of Marburg) have recently made progress, by developing statistical means for inferring the ancestral color states of males and females over evolutionary time.

To do this, they first reconstructed the evolutionary relationships among European butterflies and put this into a time calibrated framework. Then they scanned scientific drawings of all these male and female butterfly species, and used that color information in its evolutionary context to estimate the direction of butterfly color evolution for each sex, and in relation to the amounts of dichromatism per species. “Tracking evolutionary colour vectors through time made it possible to quantify both the male and female contribution to dichromatism”, says Dr. Dirk Zeuss from the University of Marburg, who is coauthor of the new study.

“We find that the rates of color evolution in males are faster than in females”, says Dr. Wouter van der Bijl, the lead author of the study. While this finding itself suggested that males might be the target of sexual selection, further analysis was needed to rule out alternative explanations. For example, male color could be evolving rapidly when species are already dichromatic, but not when males and females start to first diverge from each other in color. By modelling both the changes in dichromatism and the changes in male and female color over evolutionary time, the researchers could calculate that changes in male color are twice as important to the evolution of dichromatism than changes in female color.

This finding suggests that Darwin was right, as it is consistent with female preference and thus sexual selection for colorful males being the driving force in color evolution. Thus, the researchers provided some resolution to the 150-year-old argument between Darwin and Wallace about the origins of dichromatism in butterflies, finding that Darwin’s, but not Wallace’s, model of dichromatism evolution explains the patterns better.

References: van der Bijl, W., Zeuss, D., Chazot, N., Tunström, K., Wahlberg, N., Wiklund, C., Fitzpatrick, J.L. and Wheat, C.W. (2020), Butterfly dichromatism primarily evolved via Darwin’s, not Wallace’s, model. Evolution Letters. doi:10.1002/evl3.199 link: https://onlinelibrary.wiley.com/doi/full/10.1002/evl3.199

Provided by Stockholm University