A new cockroach (Blattodea, Corydiidae) with pectinate antennae from mid-Cretaceous Burmese amber

Abstract A new species of fossil cockroach, Fragosublattapectinatagen. et sp. nov., is described from mid-Cretaceous Burmese amber. The new species is assigned to the family Corydiidae based on the following combination of characters: pronotum with tubercles, tegmina obovate with smallish anal region and spinules on the antero-ventral margin of the front femur (type C1). The new species is the second reported cockroach with ramified antennae. This finding broadens the diversity of Blattodea in mid-Cretaceous Burmese amber and provides further evidence of convergent evolution for antennal structures among different insect lineages.


Introduction
Blattodea is an order of insects consisting of cockroaches and termites (Inward et al. 2007;Zhao et al. 2019). Up to date, about 5000 extant cockroach species and 1500 fossil species have been documented (Liang et al. 2019;Li et al. 2020).
Diverse insects have been documented from the mid-Cretaceous Burmese (Myanmar) amber recently (Ross 2020(Ross , 2021). An ecosystem with a humid climate in the mid-Cretaceous enriched the diversity of cockroach species (Liang et al. 2019). Up to now, 11 families, 28 genera and 36 species of cockroaches in Burmese amber have been documented as shown in Table 1 (Ross 2021). However, only two extinct species of Corydiidae have been reported in Burmese amber so far. The specimens in Burmese amber give us an opportunity to better understand the morphological characters of ancient insects.
Herein, we describe a new genus and species, Fragosublatta pectinata gen. et sp. nov., assigned to Corydiidae. This new finding broadens the diversity of Blattodea in mid-Cretaceous Burmese amber, clarifies the varieties of their antennal morphology, and suggests a potential sexual dimorphism for these cockroaches.

Material and methods
The type specimen was collected from deposits in the Hukawng Valley of Kachin in northern Myanmar, approximately 100 km southwest of the village of Tanai. The age of Myanmar amber is documented as 98.79±0.62 Mya, in the mid-Cretaceous (Grimaldi and Ross 2017). Myanmar amber pieces have preserved abundant specimens of plants, insects and other invertebrates. The latest comprehensive list of insect taxa from Myanmar amber comprises 28 orders, 421 families, 975 genera and 1383 species (Ross 2020(Ross , 2021. The type specimen is housed in the Key Laboratory of Insect Evolution and Environmental Changes, College of Life Sciences and Academy for Multidisciplinary Studies, Capital Normal University, Beijing, China (CNUB; Dong Ren, Curator).
The new specimen was examined and photographed using a Leica M205C dissecting microscope with a Leica DFC450 digital camera system. The detailed and enlarged photos were taken by using a Nikon SMZ 25 microscope with a Nikon DS-Ri 2 digital camera system. Cool white transmitted light from microscope's LED illuminators passed through the specimen from the top, and cool white light, emitted from double optical fibers, irradiated the specimen from two sides simultaneously. Line drawings were prepared by using Adobe Illustrator CC and Adobe Photoshop CS5 graphics software.
Morphological terminology largely follows Roth (2003); venational terms follow Snodgrass (1935), with further interpretations by Smart (1951) and Li and Wang (2017) as a frame of reference.

Order Blattodea Brunner von Wattenwyl, 1882 Family Corydiidae Saussure & Zehntner, 1893
Genus Fragosublatta Chen, Shih & Ren, gen. nov. http://zoobank.org/97CB1AFA-A97C-4A12-AA36-CD3070D3F840 Diagnosis. (male only). Sc field narrow (about a third of the width of the R region) with Sc short and branched. CuA almost straight with comb-like branches. CuP sharply curved. The first and the second hind tarsomeres with no plantulae but with spines. Cercus monoliform. Etymology. Fragosublatta is a combination of fragosus (Latin for fractured), referring to the fractured pronotum, and the generic name of Blatta. Gender is feminine.
Remarks. The new species is assigned to the family Corydiidae based on these characters: pronotum with tubercles, tegmina obovate with smallish anal region and spinules on the antero-ventral margin of the front femur (type C1). The new genus is differentiated from other extinct genera mainly by the forewing and legs: CuA with comb-like branches and the first and the second hind tarsomeres apparently lacking plantulae but with spines. Besides, the subgenital plate of the new species is almost symmetrical, which is similar to Nodosigalea burmanica (Li & Huang, 2018), but the new species has comb-like CuA branches to justify the erection of a new genus. Locality and horizon. Hukawng Valley, Kachin State, northern Myanmar; lowermost Cenomanian, mid-Cretaceous.
Diagnosis. As for the genus due to monotype. Description. Medium-sized brown cockroach, body narrow and flattened, overall body length 8.21 mm/width 2.97 mm (Fig. 1A, B). Major parts of head not preserved. Eyes and labial palps invisible. Mandibles with two sharp teeth preserved (Fig. 3A). Only four maxillary palps preserved (total length 1.02 mm), with terminal palpomere oval in shape. Sensilla on palps dense and small, < 0.01 mm wide. Both antennae detached from head and missing some antennomeres ( Fig. 2A, B); antennae with 19 and 40 antennomeres respectively; length of antennae slightly shorter than forewing length; both antennae with comb-like extensions at end of each flagellomere. Basal flagellomeres simple, thick and short, medial 20 successive flagellomeres pectinate and apical 13 flagellomeres simple (Fig. 3). Longest comb-like extension of pectinate flagellomeres 0.19 mm. Antennomeres roundish to cylindrical with widest base of 0.13 mm. Pronotum (length 2.15 mm/width 1.84 mm, as preserved) with dense tubercles, nearly vaulted (Fig. 1C), partly sclerotized and melanized, anterior margin covered with obvious hairs. Scutellum distinct, long and wide (ca 0.75/ca1.18 mm).
Forewing obovate, overlapping each other and completely covering abdomen. Left forewing overlapping right forewing. Right forewing 7.7 mm long, anterior margin arched, apex rounded (Fig. 2C). Right forewing costa 2.13 mm long. Sc field narrow, slightly curved, dichotomized with two veins not meeting margin, occupying about one third of forewing length. R regularly branched. M with only two branches. CuA almost straight, posterior-most veins comb-like, up to nine veins preserved. CuP sharply curved. Most of clavus area sclerotized, anal area obviously smallish, with seven veins. Left forewing 7.37 mm long, damaged basally. R with six visible branches. M with only two branches preserved. CuA richly branched with distinct intercalary veins. CuP simple, probably with only two and relatively straight A veins. Hind wing membranous, transparent. R branched, with 6-7 visible veins, reaching wing margin.
Etymology. The name pectinata is derived from the Latin word of pectinatus referring to the pectinate antennae.
Remarks. The antennae are detached from the head of Fragosublatta pectinata gen. et sp. nov., but the basal antennomeres of both antennae are close to the head (Fig. 3A). As shown in Figs 1B and 2B, the length of the left antennae, as preserved, is slightly shorter than the forewing length, which is consistent with the length ratios of the antennae/forewing for many documented fossil cockroaches (Liang et al. 2019). Therefore, we have high confidence that these two antennae belong to Fragosublatta pectinata gen. et sp. nov. based on these observations. Besides, there are two syninclusions in this amber piece, including a Mycetophiloidea Diptera and a Hemiptera 'Homoptera' (suspected) close to the hind legs of the new species. Due to poor preservation, we cannot identify the detailed taxonomic classification for these two syninclusions.

Discussion
The new genus and species, Fragosublatta pectinata gen. et sp. nov., displays distinctive comb-like extensions of pectinate antennae. This antennal modification of comb-like extensions also occurs among Cretaceous fossils of other insect orders, such as Trichoptera, Mecoptera, Hymenoptera, Coleoptera and Neuroptera (Table 2, Fig. 5). Nevertheless, there are some differences in the number and the length of comb-like extensions of pectinate or bipectinate flagellomeres. Other than the fossil insect orders mentioned above, pectinate or bipectinate antennae are known in extant insect orders, for example, Diptera (Keroplatidae, Ditomyiidae), Lepidoptera (Lymantridae, Saturniidae, etc.) and Megaloptera (Corydalidae) (Ševčík 2000;Tegoni et al. 2004;Liu and Yang 2006;Symonds et al. 2011;Ševčík et al. 2015). This new finding of pectinate antennae for a cockroach in the mid-Cretaceous, in conjunction with the other 26 fossil insects in six orders (Table 2), provides further evidence to support structural convergent evolution for ramified antennae among different insect lineages. The most direct effect of the ramified antennal structure to enhance insect sensing is the overall expansion of the antenna surface area and the corresponding increase in the number of receptors (Gao et al. 2016). Since there are only two reported male cockroaches with pectinate or bipectinate antennae, potential sexual dimorphism for mid-Cretaceous cockroaches is suggested, pending future reports of more examples and conspecific females.
The fore tibia spurs of the new species have serrations on their inner surface, which is special among cockroaches (Fig. 4B). To our best knowledge, only Nodosigalea burmanica (Corydiidae) possesses similar serrations in Burmese amber (Li and Huang 2018). Besides, the tarsal plantulae in fore and mid legs are usually considered as adhesive devices allowing the cockroach to perch or forage on leaves, while the tarsal spines on hind legs are supposed to help the cockroach with rapid movement (Bell et al. 2007).
In addition, the venation and cercus of the new species are also interesting. In the right forewing, there are two incomplete CuA and A (Fig. 2C). This character has been reported in the Raphidiomimidae (Liang et al. 2009). It is likely that this phenomenon was due to the fusion of veins. The basal part of cercus for this new species is cylindrical while the terminal part is moniliform. The function or derivation of this structure of the cercus are unknown, pending future research with new fossil specimens.

Conclusions
This study documents and reports a new species of cockroach, Fragosublatta pectinata gen. et sp. nov., assigned to the Corydiidae. The pectinate antennae of this new species have been compared to 26 other ramified antennal structures in six orders of insects in the Cretaceous. This finding enriches the diversity of morphological characters of cockroaches and suggests that some extinct representatives of this family might have had sexual dimorphism in their antennae. Furthermore, diversified structures of ramified antennae in different orders of fossil insects during the Cretaceous provide further evidence supporting the convergent evolution of antennal structures among different insect lineages.