1. Introduction

The Fulgoromorpha or planthoppers, which extend back to the Early Permian, comprise a large mainly tropi-copolitan group of very diverse taxa classified into three superfamilies (Szwedo et al. 2004). The superfamily Fulgoroidea presently contains some 34 extant and fossil families (Bourgoin 2022). Basic characters of the Fulgoroidea are tegulae at the base of the forewings, antennae ventral to the eyes, antennal pedicel enlarged and often swollen, bearing “placodea sensilla”, and the frons occupying most of the facial part of the head. The 21 extant families and 13 extinct Fulgoroidea families are distinguished mainly by the shape of the head, forewing venation and arrangement of teeth and spines on the metatibia and metatarsi (Dietrich 2009; Fletcher & Carver 1991; Szwedo 2018; Song et al. 2019; Bourgoin 2022).

Planthoppers damage plants by ovipositing in their tissues and feeding on the phloem, sometimes vectoring a variety of plant pathogens. Cixiidae and Delphacidae are the families with the most injurious species. Cixiids are vectors of phytoplasma, mycoplasma and Mollicutes, whereas delphacids are mainly virus vectors. These diseases cause serious damage on a great number of economically important crops (Bourgoin 2022). Planthoppers were considered as one of the significant obstacles to the success of the Green Revolution in tropical Southeast Asia with losses from them estimated in hundreds of millions of dollars. The rice crop in Asia, which feeds 60% of the world population, is particularly damaged by the delphacid Nilaparvata lugens (the rice brown planthopper), which is considered as the most important pest of rice in Asia. Some 150 species of planthoppers from various families are currently directly or indirectly recorded as pests of 99 economic plants (O'Brien 2002).

The present study describes a new genus and species of planthopper in Burmese amber. The fossil is the first Cretaceous representative of the Fulgoridiidae, a family only known from the Jurassic. It depicts some unique characters, thus adding to the already extensive morphological variation that occurs in these small, fragile insects. A mature, post-parasitic dryinid larva (Hymenoptera: Dryinidae) is adjacent to the fossil planthopper. The biology and occurrence of adult members of dryinids in Burmese amber are briefly discussed.

2. Material and methods

The fossil planthopper originated from the Noije Bum 2001 Summit Site mine located southwest of Maingkhwan in Kachin State (26°20′N, 96°36′E) in Myanmar. Based on paleontological evidence this site was dated to the Upper Albian of the Early-Mid Cretaceous (Cruickshank & Ko 2003), placing the age at 97 to 110 mya. A more recent study using U-Pb zircon dating determined the age to be 98.79 ± 0.62 Ma (Shi et al. 2012). Nuclear magnetic resonance (NMR) spectra and the presence of araucaroid wood fibers in amber samples from the Noije Bum 2001 Summit Site indicate an araucarian tree source for the amber (Poinar et al. 2007).

Observations and photographs were made with a Nikon SMZ-10 R stereoscopic microscope and Nikon Optiphot compound microscope with magnifications up to 800 X. Helicon Focus Pro X64 was used to stack photos for better depth of field.

Tegminal venation and morphological terminologies follow the works of Fletcher & Carver (1991), Fletcher (1999) and Bourgoin et al. (2015).

This specimen was first sampled in 2015, well prior to the onset of challenges regarding amber from Myanmar and therefore is in compliance with all ethical standards for the study of Burmese amber fossils (see also Poinar & Ellenberger 2020).

LSID for publication: urn:lsid:zoobank.org:pub:135AC1AC-0C39-4616-A5C1-602CF7FF(7)D06

3. Systematic palaeontology

Fig. 1.

Holotype of Stonymetopus megus gen. et sp. nov. in Burmese amber. A – Dorsal view. Scale bar = 2.3 mm. B – Ventral view. Scale bar = 2.3 mm. C. Lateral view. Scale bar = 2.5 mm.

Fig. 2.

Wing venation of Stonymetopus megus gen. et sp. nov. in Burmese amber. A – Dorsal-lateral view of specimen. Scale bar = 1.5 mm. B – Wing vein nomenclature. Scale bar = 1.7 mm.

Fig. 3.

Dorsal view of head of Stonymetopus megus gen. et sp. nov. in Burmese amber. Scale bar = 0.5 mm. B. Major head parts labeled. CO = compound eye; SbO = suborbital plate of compound eye; MsD = mesonotal disk; Pn = pronotum; V = vertex. Scale bar = 0.5 mm.

4. Discussion

The diagnostic characters of Stonymetopus megus separate the genus from all other described Burmese amber fossils (Table 1). Several morphological configurations, such as those on the metatibiotarsi or the mesonotum, are unique for planthoppers. The forewing venation places the genus in a group of taxa considered to have evolved either from “primitive” lineages related to fulgoridiids, or from a “pre-cixioid group” (Shcherbakov 2007a). This would include many of the recently described mid-Cretaceous new fossil planthopper families such as the Perforissidae Shcherbakov, 2007, Jubisentidae Zhang, Ren & Yao, 2019, Mimarachnidae Shcherbakov, 2007, Dorytocidae Emeljanov & Shcherbakov, 2018, Neazoniidae Szwedo, 2007, Katlasidae Luo, Jiang & Szwedo, 2020 as well as the recently described Inoderbidae Shcherbakov & Emeljanov, 2021. However, Stonymetopus cannot be included within any of these because of its many other characters that depict a unique general habitus within this group of mid-Cretaceous planthoppers.

While previous studies have described new genera from Burmese amber in monotypic families, we decided against this action with Stonymetopus. While such a decision emphasizes the great disparity and diversity of the mid-Cretaceous planthopper fauna, on the other hand, it also conceals the evolutionary dimensions of these taxa. Elevating these monotaxic units to family rank avoids considering the question of their evolutionary relationship. Accordingly, and in agreement with its venational pattern being similar to the Fulgoridiidae genus Cixiites, we place, at least provisionally, Stenometopus in Fulgoridiidae with which it shares a multibranched CuA. Its exact placement within this family, artificially restricted to Jurassic genera relative to other taxa, will be determined in future comparative and phylogenetic analyses.

Fig. 4.

Holotype of Stonymetopus megus gen. et sp. nov. in Burmese amber. A – Ventral view of head. Upper arrow shows fronto-clypeal suture. Lower arrow shows rostrum. Scale bar = 1.0 mm. Insert shows slanted flutes on front. Scale bar = 1.2 mm. B – Lateral view of head. Arrow shows obelliptic eye. Scale bar = 0.3 mm.

The discovery of a dryinid larva (Hymenoptera: Dryinidae) (length 2.1 mm: width 0.8 mm) adjacent to Stonymetopus megus is quite interesting (Fig. 7). Dryinids are specialized wasps that have evolved a unique method of parasitizing nymphs and adults of leafhoppers (Hemiptera: Cicadellidae), planthoppers (Hemiptera: Fulgoroidea) and treehoppers (Hemiptera: Membracidae). Adult female wasps deposit their eggs in these hemipterans and are also predaceous, so dryinids can be both parasites and predators at the same time, which makes them excellent biological control organisms (Clausen 1962).

Female dryinids oviposit in various locations on their hosts, but two of the most preferred are beneath the wing lobes and between the first two abdominal segments (Clausen 1962). One of these sites was probably where oviposition and larval development occurred on Stonymetopus megus.

Fig. 5.

Holotype of Stonymetopus megus gen. et sp. nov. in Burmese amber. A – Frontal view of head. Arrow shows medial carina. Scale bar = 0.4 mm. B – Antenna. S = Scape; P = pedicel; SP= sensilla placodea; B = basal swelling of flagellum; F = flagellum. Scale bar = 0.8 mm.

As the dryinid larvae develop inside the host, with each successive molt, they rupture the host's body wall and start to emerge from the host, but are always contained in the last larval skin, which protrudes as a sac, or thylacia. When mature, the active white, 5^th stage dryinid larva leaves the host and searches for a place to pupate. This is probably what the dryinid larva adjacent to Stonymetopus megus was preparing to do when its host landed in resin. The shock of entering the resin probably caused the mature parasite to emerge, similar to the behavior of mature mermithid nematodes that leave their planthopper host when the latter falls in resin (Poinar 2001).

A number of adult dryinid wasps, also known as pincer wasps, have been described from Burmese amber, however, it is not possible to determine if the larva adjacent to Stonymetopus megus belongs to any of these described wasp taxa (Olmi et al. 2014, 2020, 2021).

Dryinid parasites are known to alter their host morphology, usually causing a condition known as “parasitic castration” where the reproductive organs are reduced or destroyed (Clausen 1962). While there are no known records of planthopper hosts modified in size or pigmentation due to dryinid parasitism, whether other features are altered, such as wing venation patterns, is unknown.

Fig. 6.

Holotype of Stonymetopus megus gen. et sp. nov. in Burmese amber. A – Lateral view of apex of fore and hind legs. T = metatibia with two long lateral spines and five smaller apical spines; M1 = metatarsus 1 with two apical spines; M2 = metatarsus 2 with truncate spine; M3 = metatarsus 3 with claw and arolium; PT = protarsus. Scale bar = 0.2 mm. B – Apex of hind leg. M3 = slender metatarsus 3 with claw and arolium; T = metatibia with two long lateral spines and five smaller apical spines; E = truncate spine on metatarsus 2. Scale bar = 0.7 mm.

Fig. 7.

A – Holotype of Stonymetopus megus gen. et sp. nov. in Burmese amber with adjacent dryinid larva (D). O = Oviposi-tor; G = ventral margins of paired gonoplacs of abdominal segment lX. Scale bar = 0.6 mm. B – Anterior portion of dryinid larva. Arrow shows head capsule. Scale bar = 0.2 mm.

Table 1.

Other planthopper families described from Mesozoic deposits with diagnostic features that separate them from Stonymetopus megus gen. et sp. nov.

5. Conclusions

Here we present yet another fossil planthopper with its own set of unique characters, showing the tremendous diversity of the Fulgoroidea in the mid-Cretaceous, which remains by far underestimated. This high diversity of Cretaceous planthoppers has been repeatedly credited to the rise and diversification of angiosperms (Berendse & Sheffer 2009; Poinar 2018; Bateman 2020; Luo C. et al. 2020, 2021; Luo Y. et al. 2021), which were rapidly branching out into new habitats in competition with the established gymnosperms. While this is probably true for the extant planthopper families, old lineages within the Fulgoridiidae might also have been too specialized to switch from gymnosperm to angiosperm host plants, and became extinct. A clearer understanding of the basal planthopper phylogeny will allow scientists to better estimate the role of angiosperms as key factors in planthopper evolution. Perhaps we can also determine why such a wide diversity of planthoppers in the Mid-Cretaceous is not reflected in more recent Cenozoic fossils as well as with extant planthopper families.