Nanostructures and Living Cells in Butterfly Wings Could Inspire Radiative-Cooling Materials & Advanced Flying Machines
Infrared photographs of butterflies, where brightness correlates with the capability of radiative cooling.
Credit: Nanfang Yu and Cheng-Chia Tsai/Columbia Engineering
Researchers discover that butterflies have specialized behaviors and wing scales to protect the living parts of their wings; nanostructures found in the wing scales could inspire the design of radiative-cooling materials to help manage excessive heat conditions; sensory network in the wings could inspire the design of advanced flying machines.
A new study from Columbia Engineering and Harvard identified the critical physiological importance of suitable temperatures for butterfly wings to function properly, and discovered that the insects exquisitely regulate their wing temperatures through both structural and behavioral adaptations.
Contrary to common belief that butterfly wings consist primarily of lifeless membranes, the new study demonstrated that they contain a network of living cells whose function requires a constrained range of temperatures for optimal performance. Given their small thermal capacity, wings can overheat rapidly in the sun when butterflies cease flight, and they can cool down too much during flight in a cold environment. The study, published online today (January 28, 2020) by Nature Communications, is the first to explore the implications of temperature in shaping the wing structure and behavior of butterflies.
“Butterfly wings are essentially vector light-detecting panels by which butterflies can accurately determine the intensity and direction of sunlight, and do this swiftly without using their eyes,” says Nanfang Yu, associate professor of applied physics at Columbia Engineering and co-PI of the study.
The team, which was co-led by Naomi E. Pierce, Hessel Professor of Biology in the department of organismic and evolutionary biology, and Curator of Lepidoptera at the Museum of Comparative Zoology, Harvard, used their expertise in biology and optics to make a number of significant discoveries. By carefully removing the wing scales to enable them to peer into the interior of the wings, and by staining the neurons found within the wing, they found that butterfly wings are loaded with a network of mechanical and temperature sensors. The living tissues in the wings are actively supplied by circulatory and tracheal systems throughout the adult lifetime — in the case of painted lady butterflies, for more than three weeks.
They also discovered a “wing heart” that beats a few dozen times per minute to facilitate the directional flow of insect blood or hemolymph through a “scent pad” or an androconial organ located on the wings of some species of butterflies. “Most of the research on butterfly wings has focused on colors used in signaling between individuals,” says Pierce., “This work shows that we should reconceptualize the butterfly wing as a dynamic, living structure rather than as a relatively inert membrane. Patterns observed on the wing may also be shaped in important ways by the need to modulate temperatures of living parts of the wing.”
Yu’s lab designed a noninvasive technique based on infrared hyperspectral imaging, with each pixel of an image representing one infrared spectrum, that enabled them to make — for the first time — accurate measurements of the temperature distributions over butterfly wings. “This has been difficult to do until now,” Pierce notes, “because of the thinness and delicacy of butterfly wings.” “This imaging technique enables us to examine physical adaptations that decouple the wing’s visible appearance from its thermodynamic properties,” Yu adds. “We discovered that diverse scale nanostructures and non-uniform cuticle thicknesses create a heterogeneous distribution of radiative cooling — heat dissipation through thermal radiation — that selectively reduces the temperature of living structures such as wing veins and scent pads.”
The effect of this regional and selective enhancement of thermal radiation was amply demonstrated in the team’s thermodynamic experiments on butterfly wings. Experimental conditions that mimic the butterflies’ natural environment were created in Yu’s lab, and allowed the researchers to quantify the relative contributions of several environmental factors to the wing temperature. These included the intensity of sunlight, the temperature of the terrestrial environment, and the “coldness” of the sky, which can serve as an efficient heat sink of thermal radiation from heated wings. The team found that in all simulated environmental conditions, despite diverse visible colors and patterns, the areas of butterfly wings that contain live cells (wing veins and scent pads) are always cooler than the “lifeless” regions of the wing due to enhanced radiative cooling. “The nanostructures found in the wing scales could inspire the design of radiative-cooling materials to cope with excessive heat conditions,” says Cheng-Chia Tsai, a Ph.D. student in Yu’s group who was lead author of the study.