How thin, delicate butterfly wings keep from overheating
Living parts such as veins have protective structures that keep them cooler than dead scales
Delicate butterfly wings are pretty cool — literally, thanks to special structures that protect them from overheating in the sun.
New thermal images of butterflies show that living parts of the wing — including veins transporting insect blood, or hemolymph, and scent patches or pads that males use to release pheromones — release more heat than surrounding dead scales, keeping the living areas cooler.
Small changes in body temperature can affect a butterfly’s ability to fly, as muscles in the thorax must be warm so that the insect can flap its wings fast enough for takeoff. But because the wings are so thin, they heat up faster than the thorax and can rapidly overheat.
People might think that scale-covered butterfly wings are “like a fingernail, or a feather of a bird, or human hair — they are lifeless,” says Nanfang Yu, an applied physicist at Columbia University (SN: 5/23/08). But wings are also equipped with living tissues crucial for survival and flight, and high temperatures will make the insect “really feel uncomfortable.”
Butterfly wings’ thin, semitransparent nature has made it difficult for thermal infrared cameras to distinguish heat from the wing versus from background sources. So Yu and colleagues employed an infrared hyperspectral imaging technique to measure wing temperature and heat emissivity at single-scale resolution for more than 50 butterfly species.
Tube-shaped nanostructures and a thicker layer of chitin, a component of an insect’s exoskeleton, radiate excess heat from living wing tissue, the researchers report January 28 in Nature Communications. Wing veins are covered with that thicker chitin layer, and scent pads have those nanostructures, plus the extra chitin. Thicker or hollow materials are better at radiating heat than thin, solid materials, Yu says.
Those structures protect a wing only up to a point, prompting a butterfly to move away from intense light if it gets too warm. When the researchers beamed a laser on the wing’s scales, the temperature went up “but butterflies can’t feel it and they don’t care,” Yu says. But when the light warmed a butterfly’s veins too much, the insect would flap its wings or move away.
The team also discovered some butterflies have a structure that looks like a beating “heart” in their wings. It pumps hemolymph through the scent pads of male hickory hairstreak (Satyrium caryaevorus) and white M hairstreak (Parrhasius m-album) butterflies, and beats a few dozen times per minute.
A wing must be light for the insect to fly well so it’s surprising to find such a structure in the middle of it, Yu says. That it exists “can only mean that this wing heart is very important for function and health of the scent pad,” he says.