A new species of Platygaster ( Hymenoptera , Platygastroidea ) from India with an unusual antenna

A new species, Platygaster harpagoceras Popovici & Veenakumari, is described from India. The most unusual features of this species are the acuminate shape of the last antennomere and the 9-merous antenna in both sexes. The male and female are described and illustrated with brightfield and scanning electron microscopy. We provide a comparative analysis of the acuminate distal antennomere in the superfamily Chalcidoidea, including several genera of Pteromalidae (Callitula Spinola, Homoporus Thomson, Norbanus Walker, Rhaphitelus Walker).


Introduction
The "mega-genus" Platygaster Latreille is apparently one of the most speciose genera of Platygastroidea, but its taxonomy is presently in a state of confusion.With around 640 described species (various contributors 2018) and few taxonomically reliable external characters, Platygaster offers a unique challenge to any specialist dealing with the taxonomy of Platygastroidea.In some Platygaster species, previous authors have found a high degree of variability.For example, in P. depressiventris Thomson the size of the last tergite varies significantly, which Huggert (1974) considered "an ecological adaptation" for different "local populations" and in P. mainensis MacGown & Osgood were found three distinct "phena", which included one wingless specimen among more than 50 fully winged specimens (MacGown and Osgood 1971).Most "classical" species of Platygaster were described from Western Europe, so describing a new species using material from outside this geographical area may reveal additional problems as a consequence of geographical variation.Because of this, only very characteristic species can be described as new without a thorough revision and, preferably, when both sexes are available.
There are no comprehensive modern reviews of Platygaster for any region of the world.The species of "classical" authors were reviewed as follows: the species of Haliday and Walker preserved in the National Museum of Ireland and in the British Museum by Vlug (1984); the species of Zetterstedt and Thomson in Lund University, Sweden (Biological Museum -Entomological collections) by Buhl (1995), and the species of Förster by Buhl (1996).To study Platygaster, students have to use the monograph of Kieffer (1926) or some identification keys concerning local faunas, e.g.Buhl (1999) for species of Fennoscandia and Denmark, or Buhl (2006) for the species of Denmark.

Material and methods
The material described in this paper is deposited in the collection of the Hungarian Natural History Museum (Budapest) and in the National Bureau of Agricultural Insect Resources, Bengaluru.
Photographs were produced using a Leica DFC-500 camera on a Leica 205A stereomicroscope (facilities of the Integrated Centre of Environmental Science Studies in the North East Region -CERNESIM, from the Faculty of Biology, Iași, Romania), using the illumination protocol described in Fusu and Polaszek (2017) and Popovici et al. (2019).Single montage images were produced from image stacks with Zerene Stacker (Zerene Systems LLC, http://www.zerenesystems.com/)using the PMax algorithm.For SEM, the dried specimen was mounted on a double adhesive tape, coated with gold and imaged with a VEGA TESCAN SEM (WD=6.0146mm; HV=30.00 kV).

HNHM
Hungarian Natural History Museum (Budapest, Hungary) BMNH The Natural History Museum (London, UK) NBAIR National Bureau of Agricultural Insect Resources (Bengaluru, India)
Male (Fig. 2): similar to female, differing in the structure of antenna (Fig. 6), with A4 longer than A3, A5-A8 having almost the same shape and size, and metasoma consisting of 7 visible tergites with a rounded apex.

Discussion
The generic limits of Platygaster Latreille have not been clearly established and the genus is characterized primarily by the absence of characters that define other genera.There are no studies regarding the phylogeny of the genus Platygaster, and its monophyly is dubious, considering its high degree of morphological heterogeneity.Detailed study of the distribution of morphological characters throughout Platygaster is badly needed to infer monophyletic lineages that may be treated as separate genera and to construct a classification that is navigable at the species-level.
Spiculated antennae are an extreme case of acuminate antennae, where the apical part of the clava distinctly narrows into a terminal projection of various lengths.In Pteromalidae a 'terminal button' (Heraty et al. 2012), which could be regarded as the 4 th clavomere, is common in many genera.Whether the spicula represents the terminal button was investigated using SEM images.No suture has been observed in the antenna of the investigated genera, but in the male antenna of Norbanus it is evident that the spicula is distinct from the last antennomere.Hence, it is possible that the 4 th clavomere of the female antenna is fused to the previous one, at least in Pteromalidae.
Another possibility we investigated was that this terminal structure represents a large sensillum.Tselikh (2010) and Zeiri et al. (2015) state that the female antenna of Rhaphitelus has a 'baculiform sensillum' at its apex.However, our SEM images of the female clava of Rhaphitelus show the presence of setae on the terminal structure and no discontinuity with the rest of the clava, which suggests the first is part of the antennomere and not a distinct structure.This is also true for the other pteromalid genera, and for P. harpagoceras.
Of all previously mentioned pteromalid genera, the clava of P. harpagoceras is most similar to that of Rhaphitelus, where the male also displays a much reduced spicula compared with the female.Pteromalidae and Platygaster are only distantly related, so this character state is clearly a convergence.This seems to be the case within Pteromalidae as well, except for Norbanus and Homoporus, which are probably closely related based on other features.The function of this particular shape of the distal antennomere is not known.
The anteroventral projections of A6-A8 are another peculiarity of P. harpagoceras and are similar to the projections of A7 and A8 in some species of Allotropa Förster (Sceliotrachelinae).Similar projections on A6-A9 were illustrated by Buhl (2001) in Platygaster dilata Buhl, which has 10 antennomeres, but A10 is not acuminate.
P. harpagoceras is the second known species of Platygaster having 9-merous antennae.As Buhl (2009) states, it is not necessary to erect a new genus for these species, at least not at the present level of study, taking into account that among platygastrids there are some genera containing species with 9 or 10 antennomeres (e.g.Fidiobia Ashmead, Metanopedias Brues).We consider the reduced number of antennomeres and the peculiar morphology of the apical antennomere to be apomorphic characters.Erecting new genera for species with striking apomorphies have to be carefully decided as it could be "detrimental to the construction of a natural classification if it renders other taxa paraphyletic" (Talamas and Buffington 2014).
BMNH, and to Dr. Luciana Musetti and Dr. Norman Johnson for access to Hymenoptera Online (hol.osu.edu)-the most valuable resources for any student of Platygastroidea.We are grateful to Dr. Elijah Talamas for his suggestions regarding the etymology and name construction, and to Dr. István Mikó and to Mr. Zachary Lahey for providing useful comments on the manuscript.This work was supported in part by the SYNTHESYS project HU-TAF-6368 and by the UEFISCDI project PN-III-P4-ID-PCE-2016-0233.