New species of Hybristodryinus Engel (Hymenoptera, Dryinidae) from mid-Cretaceous amber of northern Myanmar, with notes on their possible hosts

Two new species of Hybristodryinus Engel, 2005, are described from mid-Cretaceous amber of northern Myanmar: H. castaneus sp. nov. (based on one male) and H. zaifui sp. nov. (based on one female). Keys to the females and males of Hybristodryinus species are modified to include the two new taxa. A syninclusion, a nymph of Cixitettiginae (Perforissidae), present in the same amber piece containing H. zaifui, is studied. This syninclusion, together with the presence of Antennal Dorsal Organs (ADOs) in Hybristodryinus, suggests that Perforissidae are possible hosts of Hybristodryinus.

The extinct genus Hybristodryinus is known only from Burmese amber . With 14 described species, it is the most diverse genus of pincer wasps from that type of amber (Perkovsky et al. 2020a;Tribull et al. 2020).
In this study, we examined a small collection of dryinids from Burmese amber and recognized two additional new species, described below. In addition, we studied a syninclusion, represented by a nymph of a possible host of Hybristodryinus.

Material and methods
The descriptions follow the terminology used by Olmi et al. (2019) and . The measurements reported are relative, except for the body length (head to abdominal tip, without the antennae). In the descriptions, POL is the distance between the inner edges of the lateral ocelli; OL is the distance between the inner edges of a lateral ocellus and the median ocellus; OOL is the distance from the outer edge of a lateral ocellus to the compound eye.
The term "metapectal-propodeal complex" is here used in the sense of Kawada et al. (2015). It corresponds to the term "propodeum" sensu Olmi (1984).
In all monographs on Dryinidae (Olmi 1984;Xu et al. 2013;Olmi and Virla 2014;Olmi and Xu 2015;Olmi et al. 2019), different names of the cells and veins of the fore wing were used. They are here used in the sense of Azevedo et al. (2018), and. The correspondence between old and new names is the following (the first name is the old name): median cell = radial cell (R); submedian cell = first cubital cell (1Cu); marginal cell = second radial 1 cell (2R1); stigmal vein = second radial cross & radial sector (2r-rs&Rs); metacarp = poststigmal abscissa of radial 1 (PostabR1). In the text, cells and veins are named by their respective abbreviations, including costal cell (C).
The term "ADOs" (= Antennal Dorsal Organs) is here used in the sense of Riolo et al. (2016). It corresponds to the term "rhinaria" sensu Olmi (1984) and Xu et al. (2013). According to Riolo et al. (2016), ADOs are sensory structures that might mediate the antennal responses to vibratory stimuli. As far as we know, they are present only in the antennae of dryinid females attacking Fulgoromorpha (Olmi 1984). Antennae without ADOs are present mainly in species that are parasitoids of Cicadomorpha.
Because of the nature of the fossils and distortions sometimes caused by artifacts, the word "apparently" is used when describing characters for which there is slight uncertainty about the true condition or where a false impression is obtained at first sight.
The types of all fossil species of Hybristodryinus were examined. The type material of the new taxa studied in this paper is deposited in the collections of the Key Lab of Insect Evolution and Environmental Changes, the College of Life Sciences, Capital Normal University, Beijing, China (CNUB) and the Department of Agriculture and Forest Sciences (DAFNE), University of Tuscia, Viterbo, Italy (DAF).

Generic placement
The new species described in this paper have been placed in the genus Hybristodryinus, because they fit the generic diagnosis reported below. Diagnosis of the genus. Female: Macropterous; occipital carina complete; mandible quadridentate, with teeth becoming regularly progressively larger from dorsal to ventral tooth; palpal formula 6/3; antenna without tufts of long hairs; antennal ADOs present; disc of metapectal-propodeal complex with posterior corners strongly projected posteriorly; fore wing with three cells enclosed by pigmented veins (C, R, 1Cu); chela with rudimentary claw; protarsomere 5 less than twice as broad as enlarged claw; enlarged claw shorter than protibia; tibial spurs 1/1/1 or 1/1/2. Male: Macropterous; antenna with scape much broader than pedicel; palpal formula 6/3; occipital carina complete; mandible with four irregular teeth; epicnemium visible, because lateral regions of prothorax not continuous with mesopleura; mesopleuron protruding laterally; fore wing with three cells enclosed by pigmented veins (C, R, 1Cu), 2r-rs&Rs vein and pterostigma; fore wing with PostabR1 slightly shorter than pterostigma; tibial spurs 1/1/2.
Male. Unknown. Antenna slenderer, with antennomere 9 about six times as long as broad (Fig. 1E)

Discussion
Following the descriptions of the above two new taxa, the number of known Hybristodryinus species has increased from 14 to 16 Tribull et al. 2020;present paper), indicating that this extinct genus of Dryininae is the most diverse from Burmese amber, based on the known records. However, the main problem of this genus is the significant sexual dimorphism between the male and the female, so that it is impossible to associate the opposite sexes based on morphologic characters. This extreme sexual dimorphism is common to almost all pincer wasps (except the subfamily Aphelopinae, whose females are often similar to males, so that the association of the opposite sexes is less difficult). Males and females of extant species can be associated by rearing or DNA analysis, which apparently are not applicable to species in amber. Although males and females of this genus have been assigned to different new species, some of them are possibly the opposite sexes of one species. Therefore, currently there are two separate keys for Hybristodryinus species, one for females and one for males.
In the same subfamily Dryininae, Dryinus (13 species, see Martynova et al. 2020) is another relatively diverse genus in Burmese amber. However, Dryinus is an extant genus and has been reported not only from Burmese amber, but also from Priabonian Baltic, Scandinavian and Rovno amber (nine species, see Martynova et al. 2020;Perkovsky et al. 2020b), amber from upper Cenomanian Taimyr (Nizhnyaya Agapa River, Siberia, one species), Campanian Medicine Hat (Canada, one species) and Middle Miocene (Mexico, one species; Dominican Republic, nine species) ). In conclusion, there are 34 species of Dryinus reported from amber, much more than those of Hybristodryinus.
In Hybristodryinus, the antenna has the ADOs, which are sensorial structures present in dryinid females parasitizing Fulgoromorpha . Perforissidae belong to Fulgoromorpha and therefore they could be hosts of Hybristodryinus. As mentioned previously, in mid-Cretaceous Burmese amber the most common dryinids were species of Dryinus and Hybristodryinus, both with antennal ADOs Martynova et al. 2020). Hybristodryinus is an extinct genus, whereas Dryinus is not extinct. The morphology of these two genera is similar (differences il-lustrated in Martynova et al. 2020 and and cannot explain why one genus is extinct and the other is not. It could be hypothesized that Hybristodryinus is extinct as their hosts became extinct . From this point of view, as Perforissidae are extinct planthoppers, they are perfect as one of the hosts of Hybristodryinus. Finding of perforissids with a thylacium of dryinids could strengthen the above hypothesis. The above conclusion contrasts with the hypothesis proposed by  and Martynova et al. (2020), who asserted that Perforissidae are less likely hosts of Hybristodryinus, because they are well known from Baeomorpha realm (see Perkovsky and Vasilenko 2019 and references therein), where Hybristodryinus is unknown, and are scarce from Burmese amber. However, already two genera and three species of Cixitettiginae (Perforissidae), were described from Burmese amber (Luo et al. 2020), indicating that Perforissidae from Burmese amber were more diverse than expected.