Literally cool butterfly wings

Infrared images of butterflies: The bright areas are linked to the ability to cool radiation. (Image: Nanfang Yu and Cheng-Chia Tsai / Columbia Engineering)

Butterfly wings are by no means lifeless flight membranes, as one might think: there is active cell tissue in the filigree structures – and this has to be protected against overheating, as a study shows: Researchers have discovered clever insect behavior and refined structures that cool the wings. In addition to their biological significance, the corresponding surface features could also be used to develop materials for radiation cooling, the researchers say.

Apart from their educational areas, there is no living tissue in our hair and fingernails or in bird feathers. But this does not apply to the wings of the butterflies, as studies have already shown: Large parts of these tissues are traversed by blood vessels and nerves and contain active cells. In male wrinkles there is even an organ in the wing: it consists of glandular cells that release fragrances that help find a partner. A fine network of air tubes in the wing – the so-called tracheae – also provides oxygen for these areas.

Focus on life in the wing

But the structures and features of the butterfly wings have been studied far less well than one might think. That is why the scientists around Naomi Pierce from Harvard University in Cambridge have once again dedicated a study to them. Using modern optical examination methods, they first looked into the fine structure of the filigree structures and thus made it possible to gain new insights: Among other things, they were able to make mechanical sensors for coordinating the wing movements visible as well as thermal sensors for temperature detection. There is even a “wing heart”, the investigations made clear. This is a small area in the area of ​​the scent gland that contracts a few times per minute to drive the insect blood.

“Most of the research on butterfly wings has focused on colors and patterns that serve to communicate within the species or to camouflage and confuse predators,” says Pierce. “This work, on the other hand, makes it clear that we should consider butterfly wings as dynamic, living structures.” This is exactly what Pierce and her colleagues did as part of their further investigations. The central question was how the insects deal with the danger that the thin wings with their low heat capacity overheat in the sunshine.

Behaviors and sophisticated structures are used for cooling

Through behavioral studies on various types of butterflies, the scientists were able to document the reactions triggered by the temperature sensors that they had previously made visible in the wings. To do this, they aimed warm light at specific areas of the wings of their experimental animals. As it turned out, a critical temperature has been reached at 40 degrees Celsius: To avoid further heating, the butterflies skilfully turn their wing surfaces out of the light. By quickly changing the angle of incidence of the warming light source, butterflies can even be animated to a real dance, show the video recordings of the scientists.

As they report further, however, there are also structural elements of the cooling system of the beautiful insects. This became clear by analyzing the thermodynamic characteristics of the butterfly wings and by examining the nanostructures on their surface. “We found that the fine structure of different scale structures and the uneven thickness of the surfaces result in a different distribution of heat dissipation by radiation in different wing areas,” says co-author Nanfang Yu from Columbia University in New York. Through this system, the living parts in the area of ​​the wing veins and the fragrance organ are specifically kept cool, the investigations made clear.

As the scientists emphasize, the thermoregulating surface characteristics of the butterfly wings are not only interesting from a biological point of view. According to them, the nanostructures that enable the tiny wing scales to strongly dissipate heat to the environment could also be used to develop materials for radiation cooling. Appropriate coatings could cool objects without the use of energy – a strategy that is known to be becoming increasingly important. There could possibly be a literally cool coating based on the model of the butterfly wings.

Source: Columbia University School of Engineering and Applied Science, technical article: Nature Communications, doi: 10.1038 / s41467-020-14408-8

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