Why Do a Butterfly's Wings Match? 🦋

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Did you know a butterfly's two wings grow separately? What? And yet the spots, stripes and colors on one side usually match the other side remarkably well. How does that happen?

Hello friends and welcome back to Sleep Tight Science, a bedtime podcast that answers your questions about science. This episode's question this time comes from two listeners. Luna, who is six and lives in Chester, England, wanted to know all sorts of things about butterflies, including why their wings are so identical and pretty. And Elodie from Calgary, Alberta asked almost the exact same thing in their own way. How are butterflies symmetrical and why?

If you've ever looked closely at a butterfly, you've probably noticed it too. The pattern on one side is usually repeated on the other. The colors, stripes, spots and other markings can look almost like mirror images. Now, it can be difficult to walk up to a butterfly and ask it to sit still while you gently examine its wings. Sometimes, one might rest on your finger for a while, or, if you're really lucky, even land on your nose. But, if you have access to a library, I'm sure you can find a butterfly book and see exactly what we are talking about. But you can leave that trip to the library for tomorrow. And just in case you fall asleep before the end, here's the short answer to our question. The two sides of a butterfly develop separately, but they are using the same genetic instructions for making a butterfly. During development, genes help control where different colors and patterns appear on each wing. Because the same system is at work on the left and right sides, the patterns usually end up looking remarkably similar. They aren't always perfect copies, though. If you look closely enough, you can sometimes find small differences between the two sides. We are going to share how scientists discovered some of the genes involved in making butterfly wing patterns. And we'll see what can happen when the two sides don't quite match. Here are three words to listen for and slowly add to your growing science vocabulary. Gene, a section of DNA that carries information cells can use. Genes help living things grow and develop. Symmetry, when parts on opposite sides of something have a similar size, shape, or arrangement. A butterfly has bilateral symmetry, which means its left and right sides roughly mirror each other.

Pattern, a design or arrangement of things such as shapes, lines, spots, or colors.

Thank you to Poppy from Sydney, Australia, for introducing our show. Take a slow, deep breath in.

And let's get started.

Why don't we start with a few basics about the butterfly itself? Butterflies belong to a big group of insects called Lepidoptera, and so do moths. In fact, butterflies are one branch of the Lepidoptera family tree. Many butterflies are active during the day, while many moths are active at night, though there are plenty of exceptions to both. There are around 18,000 known species of butterfly living around the world, with an amazing variety of wing shapes, colors, spots, stripes, and other patterns. A butterfly has two pairs of wings, one pair in front and one behind, covered in thousands of tiny overlapping scales. Those scales create the colors and patterns you see. Some contain pigments. Others have microscopic structures that interact with light and can produce shimmering colors that seem to change as the butterfly moves.

Butterflies also drink in an unusual way. A butterfly has a long thin feeding tube called a proboscis, which stays curled up when it's not being used. When a butterfly visits a flower, it can unroll its proboscis and use it to drink nectar. Now, let's think back to what Luna and Elodie noticed. Look at a butterfly with its wings spread out and compare the left side with the right. You can use a book later or simply use your imagination for now. A spot on one side will usually have a matching spot on the other. A band of color might curve across one wing and then appear in almost the same place on the opposite wing. The match can be so close that the two sides look as though one has been copied from the other. But they haven't been copied. Each wing developed on its own side of the butterfly's body, and the pattern was already developing before the adult butterfly ever opened its wings. How do two separately developing sides end up looking so much alike?

To figure that out, researchers have been looking closely at the genes involved in making those patterns. Butterfly wing patterns are controlled by lots of genes working together. But researchers have discovered some that have particularly important jobs. One is called Wint A. You can think of it a little like a pencil. In several groups of butterflies, Wint A helps decide where parts of the pattern will appear on the wing, including bands and the boundaries between different colors. Another gene is called Optics. If Wint A is a bit like a pencil, Optics is closer to part of the coloring kit. In some butterflies, it helps determine which scales become red or orange rather than black. Researchers have also found that Optics can affect a microscopic structure of scales, producing some of those shimmering colors that change as the light moves.