Treehoppers

This is right up my street - evolutionary origin of morphological novelty by modification of a core developmental switch:

Body plan innovation in treehoppers through the evolution of an extra wing-like appendage

This article is a perfect demonstration of the power of evo-devo biology: how a previously enigmatic structure can be re-interpreted with knowledge of developmental genetics to give a profound and elegant explanation. The morphological novelty at issue here is the 'helmet' of treehoppers. Previous thought to be modifications of dorsal sclerites on the first thoracic segement (like the horns of rhinoceros beetles - see http://goo.gl/n3Oxz for a fascinating  new look at beetle horn development using ESTs and microarrays), this paper demonstrates evidence that the helmet is in fact an appendage of the first thoracic segment (because of the presence of jointed articulations between the body and the helmet).

Whilst ancient fossil insects have been interpreted as having T1 appendages, in all modern insects only the second and third thoracic segments in insects bear paired appendages in the form of wings. The authors demonstrate in this paper that the  dorsal appendages of all 3 segments (wings and helmet) show similar expression of the gene nubbin, conclusively demonstrating the homology of the wings and the helmet.

The authors suggest that the novel structure (helmet) has arisen due to an escape from repression of Scr by dorsal appendage genes including nubbin; treehopper Scr is still capable of repressing nubbin in flies. If this is true, it is a fantastic demonstration of the importance of simple regulatory changes leading to profound morphological change. Another very fine paper by the Prud'homme/ Gompel lab.