Volume 473 Issue 7345, 5 May 2011

When is a wing not a wing? When it’s a helmet. Insect wings vary enormously in size and shape, but all have one thing in common — they grow out of the second and third of the three segments of the thorax. Or do they? Prud'homme and colleagues have been looking at the treehoppers, close relatives of cicadas that have a bizarre and varied structure called the 'helmet' growing from the wingless first thoracic segment. The helmet is classically described as a cuticular expansion of the first thoracic segment. Closer examination shows that, in evolutionary terms, it corresponds to a third pair of wings. A striking feature of body plans is their relative stability over long evolutionary times. The discovery of this previously unknown variation of the blueprint for insects illustrates how a structure relieved of its original role is free to evolve new functions and morphologies. On the cover, the treehopper Hemikyptha marginata. Cover image: Nicolas Gompel.

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