A butterfly's wings can have many jobs besides keeping the insect aloft.
They may be called on to attract mates, to warn potential predators to stay away,
to mimic other animals or even to provide camouflage.
All of these roles, though, depend on their colouration—which is unchanging.
This plays into the idea that butterfly wings are dead tissue, like a bird's feathers or a mammal's hair.
In fact, that is not true. For example, in some species males' wings harbour special cells that release pheromones which attract females.
Nanfang Yu, a physicist at Columbia University, in New York, has been looking into the matter.
One of his interests is the optical properties of biological materials.
That has led him to study butterfly wings in more detail.
And, in collaboration with Naomi Pierce, a butterfly specialist at Harvard University,
he has now shown, in a paper published this week in Nature Communications,
that butterfly wings are, indeed, very much alive.
Initially, Dr Yu and Dr Pierce wanted to know how the insects keep their body temperatures up without their wings overheating.
Unlike birds and mammals, butterflies do not generate enough internal heat to run their metabolisms at full pelt.
Instead, they rely on outside heat sources—usually the sun—to bring their bodies up to speed.
But their wings, being thin protein membranes, have a limited thermal capacity.
Those wings can therefore overheat quickly if the insects bask too long in sunlight,
or, conversely, can cool down too rapidly if they are flying through cold air.
In their experiments, the two researchers used a laser to heat up spots on the wings of dozens of butterfly species,
including the painted lady and the hickory hairstreak.
When the temperature of the area under the laser reached 40°C or so,
the insects responded within seconds by doing things that stopped their wings heating up further.
These actions included a butterfly turning around to minimise its profile to the laser, flapping its wings or simply walking away.