Biofluorescence in Butterflies: How Wings Glow Under UV Light
Biofluorescence in butterflies is an old and rich story. In 1924, British naturalist E. A. Cochkayne began checking the butterfly collection of the British natural history museum, making a number of interesting discoveries along the way. The main one being the abundance of biofluorescence in butterflies.
After those initial pioneering explorations there was a bit gap in the exploration of butterfly biofluorescence. It wasn’t until 2005 a team of biomimicry scientists at the University of Exeter that the next layer in the story of butterfly biofluorescence was discovered. The team were investigating the fluorescence of the green-banded swallowtail, Princeps nireus.
Butterflies have a complex array of filters in their eyes that in many species give them refined colour visioni. Each of their compound eyes is made up of many tiny units called ommatidia, that each act like a small lens to capture a part of the image. Different ommatidia can contain different mixes of light-sensitive cells and pigments, which act like filters. This setup lets butterflies to fine-tune their vision so each unit is sensitive to a very specific wavelength of colour. Given them a unique view of the world. And if that wasn’t enough to spark curiosity, they also have one of the most complex and controlled fluorescent emissions in nature. One that is so efficient it rivals the design of human LED’s.
Butterfly LED’s
The surface of the green-banded swallowtail butterfly’s wing when viewed under high magnification is not smooth. It is composed of microscopic nanostructures. In the upper layer, this surface takes the form of an orderly, grid-like network of tiny hollow tubes. Nestled within this structure are fluorescent pigments.The pigments absorb blue light and re-emit it as a vibrant green glow.
Fluorescence in nature is normally an inefficient use of energy, it usually wastes quite a lot of energy, but butterflies have evolved two clever ways to make their glow more efficient. First, they use photonic crystals, a microscopic maze that controls how light moves. The crystals selectively block or allow certain wavelengths to pass. They are shaped in such a way to prevent the fluorescence from escaping sideways, funnelling the light into a more intense and directed beam that shoots straight outwards. This allows the butterfly to direct its glow, rather than letting it diffuse randomly.
1.5 micrometres below the crystals, we can find a second structure, a multi-layered mirror known as a distributed Bragg reflector. This mirror is tuned to bounce back the exact colour of the butterfly’s fluorescence. Any fluorescence that would normally be lost by shining downward is reflected back up. These two features make a butterfly’s fluorescent emissions a lot more efficient. Their design is very similar to the technology humans developed for LED lights. Both structures work together to ensure the fluorescence is very precise in colour.
Glowing Butterfly Communication?
Of the 10,069 species of butterflies and moths surveyed with UV lights in museums, 3,122 were found to be fluorescent viii. Some species of butterfly also have completely different fluorescent patterns in males and females. Put this all together, and you have an animal that can see its own fluorescence, has a clear use for it, and uniquely has a special system that enhances it. Butterflies stand out as a great candidate for fluorescent communication, but so far, no one has carried out any behavioural studies to see what role, if any, fluorescence plays in their lives.