How and why butterflies keep their wings cool

Butterflies regulate their wing temperatures through structural and behavioural adaptations, new research shows. And they need to. 

Far from being colourful but lifeless membranes, the wings contain a network of cells that require a constrained range of temperatures for optimal performance, according to a team of US engineers, mathematicians and biologists led by Columbia University. 

Writing in the journal Nature Communications, they describe how delicate wings with a small thermal capacity can both overheat rapidly in the sun and cool down too much while flying in a cold environment. 

“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, from Columbia Engineering.

By removing the wing scales to enable them to peer into the interior of the wings, then staining the neurons found within the wing, Yu and colleagues found that butterfly wings are a network of mechanical and temperature sensors. 

They also discovered a “wing heart” that beats a few dozen times per minute to facilitate the directional flow of insect blood through a “scent pad” or an androconial organ located on the wings of some species. 

“Most of the research on butterfly wings has focused on colours used in signalling between individuals,” says Columbia biologist Naomi E Pierce. “This work shows that we should reconceptualise 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 non-invasive technique based on infrared hyperspectral imaging – with each pixel of an image representing one infrared spectrum – that enabled them to make accurate measurements of the temperature distributions over butterfly wings. 

They then mimicked the butterflies’ natural environment in the lab, allowing them to quantify the contributions of several 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. 

They found that in all simulated environmental conditions, despite diverse visible colours 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. 

Behavioural studies of living butterflies from six of the seven recognised butterfly families, showed that they use their wings to sense the direction and intensity of sunlight – the main source of warmth or overheating – and to respond with specialised behaviours to prevent overheating or overcooling of their wings. 

“Each wing of a butterfly is equipped with a few dozen mechanical sensors that provide real-time feedback to enable complex flying patterns,” Yu says. 

“This is an inspiration for designing the wings of flying machines: perhaps wing design should not be solely based on considerations of flight dynamics, and wings designed as an integrated sensory-mechanical system could enable flying machines to perform better in complex aerodynamic conditions.”

Yu and Pierce are now conducting a large-scale systematic optical study of the collections in Harvard University’s Museum of Comparative Zoology, which include thousands of individual specimens of hundreds of butterfly species across the entire phylogenetic tree.