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Butterfly watch: four legs vs. six legs

The views expressed are those of the author and are not necessarily those of Scientific American.


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After last years rains and the late snows of winter, this summer has been a really good one for British butterflies. As August has now come to an end, and summer technically turns into autumn, I thought it was time for another butterfly post. In particular, I wanted to write about one of the stranger things about these beautiful little creatures: while they all have six legs, most butterflies in one family, the Nymphalidae, only really use four of them.

All the photos in this post were taken by my dad during a recent butterfly-watching walk we both went on. Please ask and link-back if you want to use them!

A Small Tortoiseshell, Aglais urticae. Image (c) James Gould

There are around 59-ish species of butterfly in the UK depending on how many you decide are now extinct and 26 of them are in the Nymphalidae family. Like all insects, butterflies have three pairs of distinct segmented legs attached to the thorax. The picture below shows a Large White butterfly (often called the cabbage white) which is not a Nymphalidae and below its body are three white jointed legs clinging onto the flower below. The other three legs are on the other side. (Click on the image to get a bigger version).

A Large White, Pieris brassicae, image (c) James Gould

The Nymphalidae are the largest and most impressive of the butterflies in the UK – containing the most recognisable species including the Red Admirals, Peacocks and Painted Ladies. For these butterflies only the last four of the six legs are used for standing on. The front two are small and vestigial, tucked up near the head, and no longer used as legs. The picture below shows a Meadow Brown which, if you look closely, is using only two legs to sit on the leaf below, compared with the Large White butterfly above. Once again, click on the picture to get a bigger and clearer image.

A Meadow Brown, Maniola jurtina, image (c) James Gould

So why don’t the Nymphalidae use their front two legs? There doesn’t seem to be a clear and obvious answer. It isn’t because they’re small butterflies and therefore don’t need the extra support - Nymphalidae are all fairly large and common across the world. In some species the tiny front legs have little brushes and hairs that the butterflies can use to sense smell and taste (which is why they have the nickname of “brush-footed butterflies”), so it might be that the Nymphalidae have decided to use two of their legs to improve signalling and communication, and spend the rest of the time where they’re not flying balancing on just four.

Butterfly numbers will start to drop as the autumn properly kicks in, so it’s back to the regularly scheduled bacteria for the next post.

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.





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