On dorsal prothoracic appendages in treehoppers (Hemiptera: Membracidae) and the nature of morphological evidence

Abstract

A spectacular hypothesis was published recently, which suggested that the "helmet" (a dorsal thoracic sclerite that obscures most of the body) of treehoppers (Insecta: Hemiptera: Membracidae) is connected to the 1st thoracic segment (T1; prothorax) via a jointed articulation and therefore was a true appendage. Furthermore, the "helmet" was interpreted to share multiple characteristics with wings, which in extant pterygote insects are present only on the 2nd (T2) and 3rd (T3) thoracic segments. In this context, the "helmet" could be considered an evolutionary novelty. Although multiple lines of morphological evidence putatively supported the "helmet"-wing homology, the relationship of the "helmet" to other thoracic sclerites and muscles remained unclear. Our observations of exemplar thoraces of 10 hemipteran families reveal multiple misinterpretations relevant to the "helmet"-wing homology hypothesis as originally conceived: 1) the "helmet" actually represents T1 (excluding the fore legs); 2) the "T1 tergum" is actually the anterior dorsal area of T2; 3) the putative articulation between the "helmet" and T1 is actually the articulation between T1 and T2. We conclude that there is no dorsal, articulated appendage on the membracid T1. Although the posterior, flattened, cuticular evagination (PFE) of the membracid T1 does share structural and genetic attributes with wings, the PFE is actually widely distributed across Hemiptera. Hence, the presence of this structure in Membracidae is not an evolutionary novelty for this clade. We discuss this new interpretation of the membracid T1 and the challenges of interpreting and representing morphological data more broadly. We acknowledge that the lack of data standards for morphology is a contributing factor to misinterpreted results and offer an example for how one can reduce ambiguity in morphology by referencing anatomical concepts in published ontologies.

Figure 1. Brightfield images of Cyrtolobus vau (Membracidae) showing the body in different stages of subsequent separation of different body parts.

A: Habitus, lateral view. B: Fore leg + head + “helmet” complex, lateral view, “helmet” is annotated with overlay. C: Posterior body parts after removal of fore leg + head + ”helmet” complex, lateral view, “T1 tergum” is annotated by overlay. D: Anterior margin of T2 tergum and T2 pectus with the “flexible cuticle of helmet joint” and the “pteralium”. E: T1, lateral view, PFE is annotated by overlay. Abbreviations: “T1t” – “T1 tergum” ( = median area of T2 tergum); “fchj”  = “flexible cuticle of helmet joint” ( = intersegmental membrane between T1 and T2); “pt” = “pteralium” ( = prepectus); PFE = posterior flattened evagination of the pronotum.

Figure 2. Brightfield images and CLSM micrograph of Cyrtolobus vau (Membracidae) showing the articulation between T1 and T2.

A: T2, anterior view, “T1 tergum” is annotated by overlay. B: T1, median view, white line marks the site of origin of the T1–T2 intersegmental membrane that separates the “helmet” for the bi-layered PFE and an anterior, mono-layered area. C: Dorsal and ventral layers of PFE with columnar structures corresponding to external pits. D: T1 and head, posterior view, the posterior part of PFE is removed. Abbreviations: 18 = 1st mesopleuro-mesonotal muscle; 19 = 1st phragmo-2nd phragmal muscle; dl = dorsal layer of PFE; hm = helmet muscle; ism1,2 = intersegmental membrane between T1 and T2; PFE = posterior flattened evagination of the pronotum; pre = prepectus; sp2 = 1st thoracic spiracle; “T1t” = “T1 tergum” = median area of T2 tergum; T2pec = T2 pectus; tr = trachea; vl = ventral layer of PFE.

Figure 3. Brightfield images of Corythucha pallida (Tingidae) showing the similarity between the membracid and tingid body plan (compare with Figure 1A–E).

A: Habitus, lateral view. B: fore leg + head + T1 complex, lateral view, T1 is annotated with overlay. C: Posterior body parts after removal of fore leg + head + T1 complex, lateral view. D: T1, lateral view, PFE is annotated by overlay. Abbreviations: PFE = Posterior flattened evagination of the pronotum; pre = prepectus; T1 = 1st thoracic segment; T2t = T2 tergum.

Figure 4. Diagrammatic representation of the major body regions of a treehopper.

A: Interpreted by Prud'homme et al. . B: Interpreted in the present study. Tagmata are separated (dashed arrows) to more clearly show anatomical structures.

Figure 5. Micro-computed tomography of Stictocephala bisonia (Membracidae), showing the relationship between the “helmet” and other anatomical structures (volume renderings of μ-CT data).

A: Habitus, anterolateral view, left half of “helmet” removed. Head, T1 muscles, T2 tergum and “helmet” are annotated by overlays. B: Habitus, lateral view, “helmet” is annotated by overlay. Abbreviation: T2t = T2 tergum.

Figure 6. Micro-computed tomography of Stictocephala bisonia (Membracidae) showing the relationships between skeletal structures and muscles in T1 (surface rendering of 3D-reconstrution).

A: Fore leg + head + “helmet” complex, posterior view, PFE in large part removed. B: T1, anterior view, PFE is annotated with overlay. C: Fore leg + head + “helmet” complex with T2 tergum, posterior view, “helmet” is PFE is partly removed. D: Detail of T1, anterior view. E: Fore leg + head + “helmet” complex, posterior view, posterior “helmet” is partly removed. Abbreviations: Numbers refer to muscles listed in Table S3; cx1 = procoxa; h = head; hm = helmet muscle; PFE = posterior flattened evagination of the pronotum; T2t = T2 tergum.

Figure 7. Confocal laser scanning micrographs of hemipteran insects showing the articulation between the “helmet”, head and T2 tergum and the structure of PFE.

A, B: Ceresa sp. (Membracidae) A: Head-“helmet” and “helmet”-T2 tergum joints, sagittal section, anterior left, detail of Fig. 6B. B: Body, sagittal section, anterior to the left. C–E: Leptocoris trivittatus (Coreidae). C: Head-“helmet” and “helmet”-T1 joint, sagittal section, anterior to the left, detail of Fig. 6D; D: Body, sagittal section, anterior to the left. E: PFE, sagittal section; F: Atymna querci (Membracidae), 1st thoracic spiracle with pteralium ( = prepectus). Abbreviations: 16 = Occlusor muscle of 1st thoracic spiracle; 19 = 1st mesopleuro-mesonotal muscle; dl = dorsal layer; fb = fat body cell-like structures; h = head; ism1,2 = intersegmental membrane between T1 and T2; ismh,1 = intersegmental membrane between head and T1; PFE = posterior flattened evagination of T1; pre = prepectus; sp2 = 1st thoracic spiracle; T1 = 1st thoracic segment; T2p = T2 postnotum; T2t = T2 tergum; vl = ventral layer.

Figure 8. Confocal laser scanning micrographs and brightfield image of hemipteran insects showing the separated layers of PFE in pharate adult and the ecdysial line of 5th instar.

A: Notonectidae sp., (Notonectidae) body, sagittal section, anterior to the left. B: PFE, sagittal section, anterior to the left. C: Platycotis vittata (Membracidae), head and T1 of 5th instar during ecdysis (please note the presence of red mark on the PFE of emerging adult). Abbreviations: ism1,2 = intersegmental membrane between T1 and T2; PFE = posterior flattened evagination of the pronotum; vl = ventral layer of PFE; dl = dorsal layer of PFE.

Figure 9. Brightfield images of Leptoglossus fulvicornis (Coreidae) showing the relationship and articulation between T1 and T2.

A: Anterior half of body, dorsal view, anterior to the top. B: Anterior half of body, dorsal view, anterior to the top, fore leg + head + T1 complex detached from the posterior part of body. C: T1, posterior view. D: anterior margin of PFE, posterior view, detail of Fig. 7C. Abbreviations: cx1 = procoxa; PFE = posterior flattened evagination of the pronotum; T1 = 1st thoracic segment; T2t = T2 tergum; h = head; ism1,2 = intersegmental membrane between T1 and T2; ws = wing vein-like structure.