Structure and development of the complex helmet of treehoppers (Insecta: Hemiptera: Membracidae)

Abstract

Some insects possess complex three-dimensional (3D) structures that develop under the old cuticle prior to the last imaginal molt. Adult treehoppers (Insecta: Hemiptera: Auchenorrhyncha: Membracidae) have one such complex 3D structure, known as a helmet, on their dorsal side. The adult helmet likely forms inside the nymphal pronotum during the final instar nymphal stage. Previous morphological studies have reported that the adult helmet is a large, bi-layered, plywood-like structure, whereas the nymphal pronotum is a monolayer, sheath-like structure. The adult helmet is much larger than nymphal helmet. Thus, the emergence of the adult helmet involves two structural transitions: a transition from a monolayer, sheath-like pronotum to a bi-layer, plywood-like helmet, and a transition in size from small to large. However, when, how, and in what order these transitions occur within the nymphal cuticle is largely unknown. To determine how adult helmet development occurs under the nymphal cuticle, in the present study we describe the morphology of the final adult helmet and investigate developmental trajectories of the helmet during the final instar nymphal stage. We used micro-CT, scanning electron microscope and paraffin sections for morphological observations, and used Antianthe expansa as a model species. We found that the structural transition (from monolayer, sheath-like structure to bi-layer, roof-like structure) occurs through the formation of a "miniature" of the adult helmet during the middle stage of development and that subsequently, extensive folding and furrows form, which account for the increase in size. We suggest that the making of a "miniature" is the key developmental step for the formation of various 3D structures of treehopper helmets.

Ontogenetic trajectories and early shape differentiation of treehopper pronota (Hemiptera: Membracidae)

“…The unusual pronotum, likewise, acquires its elaborate adult shape gradually during nymphal development (Kudla et al, 2022). Almost all the studies on membracid pronotal development have concentrated on the fifth (last) nymphal instar because it is the last stage that leads to the metamorphic transition to the adult (Adachi et al, 2020; Fisher et al, 2020; Stegmann, 1998). This focus stems from the dramatic change in size that occurs between these two stages when the pronotum takes on the adult morphology.…”

The roles of growth regulation and appendage patterning genes in the morphogenesis of treehopper pronota

“…The comparable size change of the pronotum and wings is not found in most other insects. The membracid pronotum undergoes such tremendous growth that it develops as a complexly folded structure under the cuticle of the 5th instar before expanding during adult ecdysis [ 7 ]. Interestingly, in the 5th instar of another hemipteran, Nilaparvata lugens (Cicadellidae), the wings and genitalia are transcriptionally most similar to one another [ 5 ].…”

Impact of environmental asymmetry on epithelial morphogenesis

“…The epithelial morphology is characterized by undulating shapes and is formed by the complex folding of the epithelial sheet during morphogenesis, e.g. the formation of brain gyri 1 , villi of the intestinal tract 2 , 3 , sea urchin archenterons 4 , leg and wing imaginal discs of Drosophila melanogaster 5 , 6 , horn primordia of beetles 7 , 8 , and helmet primordia of treehoppers 9 . Thus, epithelial folding plays a major role in morphogenesis.…”

“…The lumen of the intestinal tract is covered by villi composed of a single layer of epithelial cells, which provide an abundant surface area for nutrient absorption 2 , 3 . Furthermore, the imaginal disc of insect exoskeleton is the folding structure in which the completed shape of the exoskeleton is coded and is stored in the small body of the larva 5 – 9 . Epithelial folding is often restricted by the physical environment.…”