How thin, delicate butterfly wings keep from overheating

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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.”

thermal imaging of butterfly wings
A thick layer of chitin over butterfly wing veins and scent patches, plus nanostructures in the patches, gives the tissues higher emissivity than the surrounding area (middle), meaning they release more heat and are consequently cooler (right).Nanfang Yu and Cheng-Chia Tsai
thermal imaging of butterfly wings
A thick layer of chitin over butterfly wing veins and scent patches, plus nanostructures in the patches, gives the tissues higher emissivity than the surrounding area (middle), meaning they release more heat and are consequently cooler (right).Nanfang Yu and Cheng-Chia Tsai

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.

Butterfly wings are equipped with living structures such as veins and scent patches that release more heat than surrounding areas, helping to cool the wings down when the insect basks in the sun.

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.



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