Butterfly and moth wings are composed of an intricate network of living cells that must “weather the storm,” meaning that they must adjust to ever-fluctuating temperatures they encounter without overheating. But elucidating how butterfly wings accomplish such an arduous feat is not easy. In the past, scientists attempting to characterize how butterfly wings respond to temperature changes wrestled with challenges in applying standard temperature measurement techniques to these thin, delicate structures that typically have low heat capacities. Even attempts to measure wing temperature using infrared cameras are frequently thwarted by the devices’ inability to differentiate thermal radiation from multiple sources: that generated by the wing itself, that originating from the local environment, and that transmitted and reflected by the wing.

Now, new information sheds light on how butterflies’ wings adjust to temperature variations without overheating—thanks to research published in a recent issue of Nature Communications.

“This work is the among the first to investigate the thermodynamic and thermoregulatory properties of the wing itself,” says Nanfang Yu, associate professor in the Department of Applied Physics and Applied Mathematics at Columbia University in New York and corresponding author of the study. “We discovered that butterflies have developed a number of physical and behavioral adaptations to regulate their wing temperatures.”

The majority of previous research on thermoregulation in Lepidoptera (the order of insects to which butterflies and moths belong) focused on the relation between flight and thoracic temperatures since the muscles of the thorax need to be sufficiently warm for an insect to fly; research exploring the thermoregulatory properties of wings themselves has been scarce—largely owing to the difficulties in capturing such data from these delicate structures.

Before investigating the temperature-adaptive properties of the wings, the researchers had to investigate the living components of the wings.

“We had to do more work on living aspects of the wing than we expected because a reviewer was not convinced that these components were alive throughout the entire lifespan of the adult insect, which is why we started to look at hemolymph flow—the circulation of the insect’s ‘blood’ or fluid—in the wing veins and scent pads,” says Naomi Pierce, professor of biology and curator of Lepidoptera in the Museum of Comparative Zoology at Harvard University and also a contributor to the study. “But this led to some additional interesting observations.”

The researchers confirmed that the wings are “alive” in the sense that they contain sensory and circulatory tissues made up of living cells, including hemolymph cells and sensory cells that are distributed primarily within and along the wing veins in both sexes, and the scent glands in males, and that these continue to function during the entire adult lifespan. Although they make up the bulk of the wing in terms of surface area, the fused, thin membranes found between the wing veins are largely inanimate, non-living tissues.

The researchers then focused on assessing behavioral adaptations as well as thermodynamic aspects of the wings that act to protect these living tissues. “We found that, despite the wide variation in visible coloration and pattern over a butterfly wing, it typically has substantially lower solar absorption in the near-infrared wavelengths than in the visible,” Yu says.

Hyperspectral imaging in the mid-infrared wavelengths (l = 2.5-20 µm) revealed a second adaptation: the living parts of the butterfly wings have elevated thermal emissivities—a feature that allows for radiative cooling through which heat dissipates via thermal radiation. More specifically, the wing veins, scent pads, and scent patches all have emissivities that approach unity, and thus possess a radiative-cooling capability approaching that of perfect thermal emitters. The high emissivity of wing veins is due to their thickened cuticles, and that of scent pads and patches is due to special nanostructured wing scales that cover these organs.

The research team created laboratory environments that simulated various weather conditions of the butterfly’s natural habitat, and took thermal images of wing specimens situated in these simulated environments. By using a hyperspectral imaging technique, the researchers calculated mid-infrared emissivity, transmissivity, and reflectivity distributions, from which they were then able to make a distinction between thermal radiation generated by the wing itself, as well as environmentally generated thermal radiation reflected from and transmitted through the wing, and use these to derive accurate wing temperature distributions.

These wing temperature distributions revealed that the living parts of the wing (i.e., the scent patches, pads, and wing veins) consistently maintained a temperature 10-20°C lower than the inanimate wing tissues (i.e., regions with fused membranes). Additionally, a cooler radiative background (i.e., a cloudless, blue sky with low humidity) reduced the overall wing temperature by allowing a larger net transfer of radiative heat from the wings to the cooler background.

Butterflies also use behavioral traits to counter overcooling of their wings. By using a thermal camera to observe butterflies in their natural habitat, the researchers found that strong thermal convection caused wing temperatures to drop to ambient temperature following flight. These temperatures are often too cold. To combat this issue, butterflies frequently bask in the sunlight right after ceasing flight in order to warm up their wings.

Yu says, “This work on the thermodynamics and thermoregulation of butterfly wings motivates us to better understand the effects of abiotic or physical factors on the structure and pattern of butterfly wings.” He continues, “We believe that a point of view combining biological and physical factors will help better decipher the butterfly wing, which is a truly complex ‘book’ to interpret.”

Read the article in Nature Communications.