When butterflies display their spectacular wings on sunny days they are not just showing off their good looks. Spreading out in the sun allows their wings to heat up after they have been cooled by circulating air during flight. However, butterflies cannot fly if their wings get too warm, so they have to be able to monitor the temperature of their wings and stop them from overheating, either by closing their wings, tilting their bodies to absorb less sunlight or walking to a less sunny spot. To understand how the animals decide when to stop basking, a team of researchers at Columbia and Harvard Universities, USA, developed a camera fine-tuned to see the heat spectrum produced by the sun on thin material like wings. They found that the colourful displays that we see on butterfly wings are part of a story, which only becomes evident when you can see temperature as well.
In order to compare reactions to heat on different wing shapes, the team looked at more than 50 butterfly species. They heated the wing at different points with a sun-mimicking laser and found that all of the butterflies stopped basking as the temperature rose, regardless of their wing shape or the location of the heat spot. This finding suggests that heat-sensitive structures that detect temperature are distributed evenly across the surface of the wing. The camera also showed that the undersides were cooler than the upper sides, which may explain why butterflies fold up their wings when they are overheating.
In addition, the camera revealed that temperature can vary enormously across a single wing, by as much as 15°C. Under the camera, it was clear that the veins and sensory structures remain cooler than the membrane in between. The team predicted that this difference in temperature is due to the structure of the scales covering areas of the wing that are important for sensing the environment. To test this theory, they gently removed scales and imaged the wings again with their temperature-sensitive camera. Without scales, most of the previously cool parts of the wing were the same warm temperature as the wing membrane, while other cool parts of the wing remained cool due to the thinness of the underlying membrane.
The structure of the wings, along with actively avoiding the sun, allows butterflies to catch our eyes while staying cool enough to fly.
