Unique pronotum morphology of treehoppers: Common structures among species revealed

The prominent horns (pronotum) of treehoppers are known to be morphologically very diverse. Now, a research group led by Assistant Professor Hiroki Gotoh of the Department of Bioscience, Graduate School of Science and Technology at Shizuoka University, has identified structures (homologous parts) commonly found in the complex horns among species of treehoppers (Hemiptera: Membracidae). They also presented hypotheses on how different horn regions were modified to constitute the respective horn morphologies. The findings are expected to help improve the accuracy of description and comparison of horn morphology of treehoppers and were published in the European Journal of Entomology.

Treehoppers belonging to the family Membracidae of the order Hemiptera are small insects characterized by their unusual and diverse pronotal structures (horns). They have long attracted the attention of collectors, enthusiasts, and scholars. However, the morphological complexity and variety of the horns of treehoppers have been the primary cause of ambiguity in the interspecific correspondence of the various structures of the horns. This problem hinders comparisons of horn morphology among different species of treehoppers. Moreover, when describing new species, confusion can be caused by using a structure name different from that used for the same structure in other species.

The research group selected 13 species from all members of the family Membracidae. They initiated this study to determine the horn structures (homologous parts) commonly found among the selected species by comparing relative positional relationships and structures, starting with the subfamily Smiliinae. First, they observed six species of this subfamily, of which morphology has been described in detail in previous studies. As a result, the following four structures were commonly observed in the horns of the species observed in this study: (1) the humeral angle, (2) the median carina, (3) the starting point of the anterior point, and (4) the posterior apex of the pronotum.

Observations to see if these horn structures could also be found among seven species of the subfamilies Membracinae, Centrotinae, Stegaspidinae, and Endoiastinae, which are phylogenetically distant from the subfamily Smiliinae, revealed that all the species had structures (1)-(4). Thus, structures (1)-(4) appear to be shared commonly among species of treehoppers, i.e., homologous parts.

Using the identified homologous parts as landmarks, they divided the horn into six regions. They proposed a ground plan of the treehoppers' horns to explain how the respective regions were morphologically modified to constitute the horn morphologies characteristic of the individual species.

The present study analyzed species from only five of the nine subfamilies belonging to the family Membracidae. In reference to papers describing the subfamily Darninae, homologous parts can be likewise confirmed. However, the horn structures remain to be analyzed for the remaining three subfamilies. More species should be observed to cover the entire Membracidae family.

Some of the species observed in this study had indeterminate homologous parts, requiring further histological observations. In addition to the relative positional relationship used in this study, there are several other criteria for defining morphological homology, such as genetic and developmental homology-based criteria. Observations and comparisons of horn morphology from the perspective of these other homologies are expected to consolidate the homology-based approach presented here.

The horn morphology of treehoppers is an evolutionary hotspot and a very interesting subject in terms of morphogenesis and morphological diversification. The homology-based approach and the ground plan, which are outcomes of the study, are expected to help increase the accuracy of description and comparison of horn morphology in treehoppers, which is the basis of any research, and to serve as the key to understanding the evolutionary history of horn morphology.

Gotoh said, "Treehoppers have such rare and unique shapes that a photo book has been published. This study revealed that their morphological variations can be understood in terms of modifications to the basic type. You may not be familiar with this insect, but I can assure you that you will be surprised when you see it."

Journal Information
Publication: European Journal of Entomology
Title: A general theory of the complex pronotum morphology of treehoppers in Smiliinae and its relatives (Hemiptera: Membracidae) and its applicability to other subfamilies
DOI: 10.14411/eje.2025.005