New observations on the prothoracic “wing” of treehoppers

Prud’homme et al. (2011) published a manuscript last May (2011) in Nature in which they demonstrated that the dorsal ornamentation of treehoppers (see examples here) was, in effect, the (re)expression in the prothorax of the genetic pathway responsible for wing development. No extant insects (of the >950,000 known species) have wings on this segment (prothorax), and the scientific community was flushed with excitement at this cool, new evolutionary story, especially over the apparent strength and diversity of their evidence (see some of our colleagues’ blog posts and news stories from last May: 1, 2, 3, 4, 5, 6).

The key finding, upon which they expanded their molecular and morphological observations, was the presence of a true articulated (moveable) dorsal appendage on the 1st thoracic segment (T1; prothorax). According to their paper, the “helmet”, i.e. the pronotal ornamentation, is an independent sclerite that articulates with the rest of the prothorax. They provided a more detailed morphological description of the putative “prothorax-pronotal wing” articulation as well as studied the genes that are regulate the development of the “helmet”.

Reading the morphological description provided by them and the annotated figures in their supplementary documents, we were unable to understand this articulation. Insect morphology, however, is what drives us – it’s our passion. As such we were compelled to explore the system further, to enlighten ourselves about the complex anatomy of the first (extant, anyway) pronotal wing ever described.

Using razor blades and some glycerine-stored treehopper specimens, we quickly determined that there is no pronotal wing-prothorax articulation to describe and understand. The previous authors were describing the articulation between the prothorax and the mesothorax, and interpreting it as the pronotal wing articulation treating. The whole prothorax was considered to be a pronotal wing.

After realizing that there were fundamental morphology flaws we were faced with a more difficult problem: How to present our findings to non-morphologists, to a broader audience, who might not be too interested in an exhaustive comparative morphology result, especially if the descriptions involve internal structures.

How do we communicate complex morphology to a broader audience? Rich, annotated, and compelling images certainly help! A μ-computed tomography image of a treehopper, showing the limits of relevant sclerites. Referencing rapidly growing (and increasingly robust) anatomy ontologies is another way to be more explicit about structures.

After five moths of observation, writing, reviewing, responding to reviews, rewriting, re-imaging, etc. our observations are finally published in PLoS ONE ( doi:10.1371/journal.pone.0030137) (we committed to an open access journal, given some of the complaints about Nature and other, similar journals being inaccessible to many interested readers).

In this paper we not only aim to offer a more comprehensive interpretation of treehopper thoracic morphology but also to take the next step in insect appendage/projection research. Although, there is no articulated appendage on the treehopper pronotum, treehoppers share the presence of a very interesting posterior flattened evagination, that itself shares multiple characteristics with the wing (although it is not articulated to the prothorax, but is the part of it):

  1. flattened, and hence the two layers are located relatively close to each other;
  2. the layers are separated in newly eclosed adults but are connected via columnar structures in older adults;
  3. hollow, trachea-containing, longitudinal structures extend along the length of PFE in mature adults;
  4. the lumen of the flattened evagination is continuous with the body cavity and hence contains fat body cell-like structures)

Our main finding is that this wing-like evagination is NOT unique to Membracidae, but rather is widely distributed across the phylogeny of Hemiptera. It was almost shocking to realize that the enlarged pronotum of a leaf-footed bug (Coreidae) is a flattened, wing-like, trachea-bearing evagination of the prothorax, and that the functional prothorax (area that serving as site of origin of muscles moving the head and the prolegs and connecting the prothorax with the mesothorax) is restricted only to a narrow anterior ring.

In the process of resubmitting our manuscript after its first round of changes, we learned that a colleague in Japan (Yoshizawa 2012; doi:10.1111/j.1365-3113.2011.00606.x) independently came to a very similar set of conclusions. It’ll be interesting to follow future research on this exciting observation!

Tags: , ,

Leave a Reply