The treehopper helmet not only gives the treehoppers an other-worldly appearance but also is an example of an appendage that arose as a new feature. This is rare since the more common observation is the loss of appendages as opposed to the introduction of new features/ appendages, which is thought to add further constraint. The treehopper insects (Membracidae), a small group of hemipteran insects to cicadas, have evolved a highly diverse range of helmets, from those mimicking tree branches, aggressive ants, and other natural elements to helmets with thorns or ant animal droppings.
It is shown that the helmet is an appendage with a flexible joint, instead of an extension of the exoskeleton. The helmet actually seems to be attached bilaterally to the thorax by these flexible joints, much like regular wings. Similarities are drawn between the anatomy of the treehopper helmet and wings. A handful of transcription factors, including nubbin, mark wing developmental fate and differentiate between wing and other appendage precursors. Nubbin was found in the developing helmet and its expression parallels that of the wings. Two other genes, Distal-less and homothorax, are also expressed in the developing helmet and determine the helmet proximo-distal axis from hinge region to posterior tip. Recently, developmental studies have shown that the introduction of the helmet has been brought about through inhibitory mechanisms that prevent the formation of wings.
|Please welcome the Lady Gaga of the treehoppers.|
A gene involved in inhibiting wing formation, Sex combs reduced (Scr) was thought to have something to do with the formation of the helmet. Prud’homme et al. found that the evolution of the helmet was not due to a change in Scr expression or function but rather that nubbin may have become unresponsive to Scr repression. All in all, we see evidence that helmet development may rely on developmental mechanisms involved in wing formation.
Moczek, Armin. "The Origins of Novelty." News & Views. Macmillan Publishers Limited, 5 May 2011. Web. 18 Mar. 2012. <http://sites.bio.indiana.edu/~moczeklab/pdffiles/2011%20Moczek%20Nature.pdf>
Prud’homme et. al. "Body Plan Innovation in Treehoppers through the Evolution of an Extra Wing-like Appendage." Nature 473.7345 (2011): 83-86. Nature.com. Nature Publishing Group, 4 May 2011. Web. 18 Mar. 2012. <http://www.nature.com/nature/journal/v473/n7345/full/nature09977.html#/main>