Treehoppers have wings on their heads

Treehoppers are a group of insects distinguished by their impressive head ornamentation -- called a "helmet" -- which often stretches back along the length of their body. The helmets come in a wide variety of shapes and styles, mimicking everything from thorns to aggressive ants, and they help the treehoppers avoid predation through camouflage or intimidation. These structures don't just look cool; it turns out that they've got a great evolutionary story behind them!

For a long time, scientists thought that the helmet was simply an outgrowth of the exoskeleton, basically just an intricately shaped extension of the armor on top of the thorax. That changed in 2011, when Benjamin Prud'homme and his colleagues discovered that, at least in some species, the helmet is attached to the thorax by a joint. Exoskeletal outgrowths are rigid, so the presence of a joint means the helmet is an appendage, like a leg or a wing.

Since wings are the only dorsal appendages on the insect thorax, the team speculated that the helmet might be a modified wing. They found support for their contention in a variety of anatomical similarities, and then went on to show that a key transcription factor in wing development is expressed in the growing helmet. It's a nice piece of work and makes a convincing case that helmets, in all their morphological splendour, are wings that have been reshaped by evolution.

The strange thing is that no other insect species has wings in that position. The helmet is on the first segment of the thorax, while insects are only known to have wings on the second and third segment. Ancestral insects had wings on other segments, but over the past several hundred million years they've become confined to the only the second and third segments.

So how do treehoppers develop (modified) wings where no other insects have them? Wing development in the first segment is normally blocked by a particular gene, so it might be that this gene is missing or defective in treehoppers. The researchers tested this hypothesis, but they found that treehoppers have the gene and that it was able to block wing development when engineered into other insects. The team think the mutation which led to the reactivation of wing development must have been somewhere in the genetic pathway between the master regulator and the final steps, but they haven't figured out exactly which step.

I've written several times about the importance of constraints in evolution, and this story is a great example of the strength of constraints and their absence. As the paper points out, the absence of wings on the first thoracic segment in insects might be because of either developmental or selective constraints -- the limits of the body plan or the action of counterselection. The presence of wings in this position on treehoppers shows that the can develop in that location, so their absence in other insects must be due to selective pressures rather than a lack of developmental potential. That developmental potential is realized in treehoppers, and the resulting organs "illustrate how a structure [...] relieved from its original function (for instance by duplication or disuse) is 'left to the free play of the various laws of growth' and provides a new substrate for morphological diversification."