Taxonomic history and review of the Förster genera of Platygastridae (Hymenoptera: Platygastroidea)

. Platygastridae is a ʻ dark taxon ʼ , with many genera and species in dire need of professional attention. The taxonomic impediment is especially severe in the Palearctic Platygastrinae due to the abundance of names with vague concepts. Historical descriptions and their associated type material must be examined and clari ﬁ ed before further revisionary work can occur. Arnold Förster described 18 genera of Platygastridae, most of which represent distinct and recognizable lineages. The present study reviews their taxonomic history, providing diagnostic remarks, English translations, and illustrations of important specimens from the Förster collection in the Natural History Museum Vienna. The collection also includes original exemplar specimens of European species whose types have been lost. Neotypes and lectotypes are designated from this material to improve nomenclatural stability in the group. Neotypes are designated for Amblyaspis forticornis (Nees, 1834), Isocybus grandis (Nees, 1834), Platygaster striolata Nees, 1834, and Trichacis tristis (Nees, 1834). Lectotypes are designated for Leptacis spinigera (Nees, 1834) comb. nov. and for Platygaster corvina Förster, 1861, with Platygaster henkvlugi Buhl, 1996 treated as a junior synonym. Platygaster mutica Nees, 1834 stat. rev., nomen dubium, is transferred from Synopeas .

The current work presents the fi rst photographic catalogue of Platygastridae from the NHMW, with notes on the taxonomic history and diagnoses of the Förster genera. The section of Hymenopterologische Studien dealing with Platygastridae, including extensive commentary on each genus, has been entirely translated into English (Supp. fi le 1). It is our hope that the illustrations and information presented here will be of use to future researchers who seek to untangle the complex taxonomy of Palearctic Platygastridae.

Material and methods Photography
Images were produced with a Macropod imaging system with a Canon EOS 6D Mark II camera, EF 70-200 mm lens, and 10 x and 20 x M Plan APO Mitutoyo objective lenses. Microphotography software included EOS 6D Mark II camera utility and Helicon Focus Pro 6.8.0 for image stacking. Adobe Photoshop was used for addition of scale bars and post processing. Images of type specimens from Naturhistoriska Riksmuseet Sverige and Biological Museum Lund University were provided by those institutions. Images of type specimens in the United States National Museum were provided by Talamas et al. (2017).

Informatics
Specimen data were transcribed from labels, with English translations and inferred collecting data in square brackets. Selected photographs of original labels were added to highlight the diverse taxonomic history of specimens. Associated data such as country of origin, collector, and collecting dates were gleaned from the original published descriptions. These data, along with specimen images, were uploaded to the Ohio State University's Museum of Biological Diversity database (mbd-db.osu.edu) and can be retrieved by entering the collecting unit identifi er (CUID) into the search form.
the type material, albeit with the name Acerotella. The genus can be distinguished from Inostemma by the semicircular mesoscutellum in dorsal view (Fig. 1D).

Remarks
Although this specimen represents the fi rst species to be placed in Acerota, it is not congeneric with specimens identifi ed by Förster. Ashmead (1887) named three species in the same paper, which were all later synonymized under the name Inostemma americanum.

Remarks
The genus Amblyaspis is defi ned by a conical and setose mesoscutellum in both sexes and the close diagonal appression of the apical two clavomeres in the female. The type species is Platygaster tritici Walker, 1835, the male lectotype of which is in the NMINH, Dublin. According to Vlug (1995), the specimen identifi ed as Pl. tritici in the Förster collection is a diff erent species.
The NHMW includes the female lectotype of Am. walkeri Förster, 1861 and an original exemplar of Am. forticornis (Nees, 1834), the type specimen of which has been lost. We here designate this specimen (NHMW-HYM#0005324) as the neotype of Am. forticornis.

Remarks
The type species of Amblyaspis was fi rst designated by Ashmead (1893) as "Amblyaspis aliena Förster". Although Amblyaspis aliena (Nees, 1834) is a valid species with which Förster was familiar, Ashmead's combination was considered a nomen nudum by Muesebeck & Walkley (1956), who designated "Platygaster tritici Curtis, 1831" instead. The latter name fi rst appeared in Curtis's British Entomology, a series of illustrations published periodically between 1824 and 1839. Curtis (1830) attributed the species AWAD J. et al., Platygastrid genera described by Förster to Haliday. However, there was no description and the combination is therefore also a nomen nudum. The species was not formally described until Walker (1835), who also attributed it to Haliday. Vlug (1995) designated "Platygaster tritici Walker, 1835" as the type species, with a male lectotype from the Haliday collection in Dublin. Unfortunately, the country of origin of this specimen cannot be determined, as Haliday collected in both England and Ireland, and both countries are mentioned in the original description. Förster, 1861 Amblyaspis walkeri Förster, 1861: 41.

Remarks
Förster defi ned Anopedias by the fl attened mesoscutellum in lateral view. The type of Anopedias is An. obscurus Thomson, 1859. The type specimen is in the MZLU, Lund. No specimens of taxonomic note were found in the NHMW. Thomson, 1859 Fig. 5 Anopedias obscurus Thomson, 1859: 80.

Remarks
In the addendum, Förster (1856) unnecessarily proposed the replacement name Polymecus to avoid confusion with the plant genus Ectadium E.Mey. Förster distinguished Ectadius from Synopeas by the elongate female metasoma in the former. It was later treated as a junior synonym of Synopeas by Fouts (1924). The type species of Ectadius is Platygaster craterus Walker, 1835, by monotypy, for which the primary type is deposited in NMINH. The specimens in Vienna are original exemplars, likely provided by Walker himself and used in the original description of the genus. They are identical to the lectotype and paralectotype except for being somewhat lighter in color. (Walker, 1835) Fig Triplatygaster Kieff er, 1913-Szelényi 1938.

Remarks
Förster describes the principal character of Hypocampsis to be the "widely turned edges of the metasoma". Presumably, this refers to the laterotergites, which are relatively broad. Additionally, the female antenna is fi liform and the clavomeres are not expanded in diameter. The original description does not include any species, but specimens were reared from "Cecidomyia strobi in Abies cones". Based on the current taxonomy of the midges and the tree, this is probably Kaltenbachiola strobi (Winnertz, 1853) on Picea abies (L.) H.Karst., which according to Kieff er (1926) is also the host of Platygaster contorticornis Ratzeburg, 1844, the type species of Triplatygaster Kieff er, 1913. Fouts (1920, 1924 synonymized Hypocampsis with Platygaster, based on Förster's description. Later, Szelényi (1938) treated Triplatygaster as a junior synonym of Hypocampsis.
The type specimen of Pl. contorticornis is almost certainly destroyed, presenting an obstacle to species identifi cation and casting uncertainty on the species group that MacGown (1979) based on Pl. contorticornis. The contorticornis group is associated with conifers and characterized by coarse microsculpture, complete notauli, a strongly fl attened metasoma, and a wedge-shaped male antennal plate organ. There are two specimens in Förster's collection identifi ed as Pl. contorticornis. However, their provenance is unknown and they do not resemble the prevailing concept of Pl. contorticornis.

Remarks
Förster described Isocybus as having a cubical head, a mesoscutellum with a tuft of setae, and a relatively large body size. He included the species Platygaster cotta Walker, 1835, Pl. erato Walker, 1835, and Pl. matuta Walker, 1835, the types of which are located in the NMINH and the NHMUK. He also included Pl. rufi cornis (Latreille, 1805), which infl uential authors of the time considered to be a senior synonym of Pl. grandis Nees, 1834 (Walker 1835;Curtis 1837). Platygaster grandis Nees, 1834 is the type species of Isocybus. The type material is unknown, probably lost. Four specimens in the Förster collection are marked as original exemplars of Pl. grandis, but we do not have reliable evidence that they could be considered part of Nees's syntype series. However, we do believe that they correspond to Nees's concept of the species and they fully match his description. Based on this, we here designate specimen NHMW-HYM #0005320 (top left) as the neotype of Pl. grandis.

Remarks
Förster described Leptacis on the basis of a long and pointed mesoscutellar spine, ocular ocellar length (OOL) less than lateral ocellar length (LOL), and four-segmented clava in the female. The type species of Leptacis is Ichneumon tipulae Kirby, 1798, the type specimen of which is in the NHMUK. The NHMW includes paralectotypes of L. scutellaris Thomson, 1859and L. torispinula Huggert, 1980. Huggert (1980 synonymized L. scutellaris with L. tipulae. Vlug (1985) synonymized L. torispinula with L. nydia (Walker, 1835). One specimen of Platygaster spinigera Nees, 1834 (NHMW-HYM#0005297), identifi ed by Förster, rightfully belongs to Leptacis and the label on this specimen is consistent with Nees's style (Graham 1988). However, another specimen identifi ed as P. spinigera in NHMW (NHMW-HYM#0005298) belongs to Synopeas. Based on the authenticity of the label, we consider the former specimen to be part of the syntype series and the latter specimen to be misidentifi ed. We here designate NHMW-HYM#0005297 as the lectotype of Platygaster spinigera Nees, 1834 and transfer it to Leptacis.

Material examined
Paralectotypes

Remarks
Metaclisis was treated by Masner & Huggert (1989). The genus is diagnosable by the fan of striae in the malar space and the forked knob of the submarginal vein in the fore wing. Förster did not recognize these characters, and instead defi ned it by the number of clavomeres (3, although the number can actually range from 2 to 4) and what he interpreted as a basal fore wing vein (actually a nebulous and pigmented RS&M vein). The type species is Inostemma areolata Haliday, 1835, the lectotype of which is deposited in NMINH. The original exemplar in NHMW is a paralectotype.

Remarks
Förster diff erentiated Monocrita from Metaclisis based on the antennal segments. What he interpreted as a one-segmented clava in Monocrita is in fact an elongate A10 (Masner & Huggert 1989

Remarks
The name Catillus, proposed by Förster 1856, was preoccupied by a genus of snail (Catillus Gray, 1847). Förster (1856) quickly proposed the replacement name Piestopleura in the addendum to Hymenopterologische Studien. This genus is defi ned by the strong lateral compression of the mesosoma, antero-posterior compression of the head, and dorsoventral compression of the metasoma. The type specimen of Platygaster catillus Walker, 1835 is in the NHMUK. The specimen in Vienna is an original exemplar, likely provided by Walker himself and used in the original description of the genus.

Remarks
Although Platygaster is not a Förster genus, his concept of the genus has prevailed and was the baseline against which he assigned generic status to lineages with recognizable characters. Förster regarded Platygaster as a large and confusing genus, an assessment that has been shared by many. It appears at the end of his key, being defi ned by its lack of distinguishing characteristics. No species names are cited, although he did mention the type species of Platygaster, Pl. rufi cornis, as an example of Isocybus.
Förster described nine species in Platygaster, six of which remain valid. The types of all nine species are in the NHMW. Vlug (1973) expressed doubt that the specimen labelled as the type of Pl. corvina represented the original because the label and pinning style were not consistent with Förster's work. It is true that the locality label is missing and that the specimen is pinned on a light-colored piece of pith instead of a dark one. However, it is possible that a label was lost and that the specimen was remounted after Förster's death. Furthermore, the verdigris on the pin itself suggests that it is of a similar age and chemical composition to the other pins in Förster's collection. The specimen generally matches Förster's description, except for minor diff erences in fl agellomere proportions, which are easily explained by viewing angle. We disagree with Vlug (1973) and here assert that the type specimen of Pl. corvina is accurately labelled. This renders Pl. henkvlugi Buhl, 1996

Remarks
Polygnotus was described as having a wide head with strong sculpture, a cushion-shaped mesoscutellum, and a mesoscutal lamella. It resembles Isocybus in size and sculpture, but lacks the tuft of setae on the mesoscutellum. Fouts (1920) considered it a junior synonym of Platygaster. The type species is Platygaster striolata Nees, 1834, the type specimen of which is unknown. The NHMW includes an original exemplar of Pl. striolata, which bears a label consistent with Nees's style, as well as the holotype of Po. signatus Förster, 1861. Based on the authenticity of the label, we here designate a neotype for Platygaster striolata Nees, 1834.
The NHMW also houses original exemplars of Platygaster orus Walker, 1835 (type specimen in NHMUK), which was considered by Förster to belong to Polygnotus but was never formally transferred.

Remarks
Sactogaster was clearly a favorite of Förster, as he writes about it quite eff usively in Hymenopterologische Studien (1856). It is also the only genus in the platygastrid section of this publication to include a key to species and new species names (Sa. curvicauda, Sa. longicauda, Sa. pisi, and Sa. subaequalis Förster, 1856). Förster defi ned Sactogaster by the shape of the female metasoma, which has T2 ventrally expanded. Sactogaster was synonymized with Synopeas by Kozlov (1978) and Buhl (1997) synonymized   (1926), which was mistakenly replicated by Vlug (1995).

Remarks
Förster's description of Synopeas is rather minimal, consisting mainly of the mesoscutellum bearing a short wart-or awl-like spine. More recently, Jackson (1969) defi ned the genus by the fusion of T1 and T2 and the presence of a ventral pronotal pit. Our interpretation of Synopeas includes a highly variable shape of the mesoscutellar spine, from almost non-existent to very prominent (Awad et al. 2021). The type species of Synopeas is Sy. inerme Thomson, 1859(Muesebeck & Walkley 1856. Ashmead (1893;1903) incorrectly listed Sy. prospectus Förster, 1861 andSy. melampus Förster, 1861 as the type species, respectively.
The type specimens of all three Förster species are in the NHMW. The collection also includes an original exemplar of Platygaster muticus Nees, 1834, which was transferred to Synopeas by Thomson (1859)  and these body parts were the basis for Graham (1988) considering it to be a species of Platygaster. The original exemplar in NHMW clearly belongs to Synopeas, and is thus of no use in providing additional information about the species. Platygaster muticus is essentially a nomen dubium because it cannot be recognized at the species level based on the remaining parts of the type and the original description, but it clearly does not belong in Synopeas, where it is currently placed, and we here transfer it to Platygaster.  Thomson, 1859 Fig. 20 Synopeas inermis Thomson, 1859: 74.  Förster, 1861 Synopeas rigidicornis Förster, 1861: 41.

Remarks
Trichacis is characterized by a tuft of setae on the mesoscutellum and a round or ovoid head (as opposed to the blocky head of Isocybus). Species of Trichacis typically have a smooth or fi nely sculptured frons with transverse striation or rugulae above the toruli. Upon its description, Förster included T. didas (Walker, 1835), T. pisis (Walker, 1835), and T. remulus (Walker, 1835). Trichacis pisis was designated as the type species by Ashmead (1983). The NHMW includes one female original exemplar of T. tristis, a species for which there is no known type material. We here designate this specimen (NHMW-HYM#0005319) as the neotype of Platygaster tristis Nees, 1834.

Remarks
The taxonomic history of Xestonotus is complex. Förster defi ned it by the nearly parallel notauli and smooth, blunt, relatively elongate mesoscutellum. Because he did not include any species, the type species was X. andriciphilus Ashmead, 1887 by fi rst subsequent inclusion. However, Ashmead's description and illustration were incongruous with Förster's concept. Fouts (1924) transferred it to Platygaster, despite the fact that the type had already been lost. Ghesquière (1948) synonymized Xestonotus with Leptacis based on the drawing of X. andriciphilus from Ashmead (1893), an opinion seconded by Masner (1964).
Adding to the confusion, the name Xestonotus was preoccupied by a beetle, Xestonotus Leconte, 1853. Gahan (1919 proposed the replacement name Xestonotidea, but also named a new type species, Xestonotidea foersteri Gahan, 1919. This type designation was determined to be invalid (Muesebeck & Walkley 1956) because a replacement name for a genus always retains the original type species. Later suggestions for replacement names were Axestonotus Kieff er, 1926 andEoxestonotus Debauche, 1947, but priority makes these invalid. Debauche made the same mistake as Gahan, attempting to designate a new type species along with the replacement name. Gahan and Debauche both intended to restore Förster's concept of the genus, which was so obviously misinterpreted by Ashmead. However, once the type species was fi xed by fi rst subsequent inclusion, the genus was permanently linked to that concept. The only way to restore the concept was to erect a new genus with its own type species, hence Euxestonotus Fouts, 1925. The type species is Platygaster error Fitch, 1865, which is housed in the USNM. The NHMW has no material of special taxonomic signifi cance to either Xestonotus or Euxestonotus.

Discussion
This study highlights three issues which are essential to the practice of good taxonomy: designation of type species and type specimens, conservatism in the generation of new names, and strict adherence to the rules of the ICZN. If Förster had designated types for his genera, if Ashmead and Kieff er had been more conservative in their approach to nomenclature, if Gahan and Debauche had obeyed the Code, much confusion would have been avoided. In Förster's defense, few guidelines were available for zoological taxonomists in the early to mid-19 th century (Melville 1995). On the other hand, Kieff er did designate type specimens, in a sense, but he did not always label them as such and rarely made note of their institutional location.
Clearing up these taxonomic problems is time-consuming and often involves multilingual detective work. Such obstacles may explain why the present study is the fi rst comprehensive update to Förster's pioneering work on the Platygastridae. The data and commentary here provide easy access to each generic concept and its nomenclatural history, as well as the species described by Förster and some of his contemporaries. These resources will be of use in the development of identifi cation tools and revisionary taxonomic works at the genus and species levels.
Neotypes of Platygaster forticornis Nees, 1834, Platygaster grandis Nees, 1834, Platygaster striolatus Nees, 1834, and Platygaster tristis Nees, 1834 are here designated in accordance with Article 75 of the International Code of Zoological Nomenclature. These are some of the earliest described species of Platygastrinae and establishing their identity is essential for taxonomic stability and the treatment of later names. The loss of Nees von Esenbeck's collection is well documented, precluding selection of neotypes from this material (Graham 1988;Horn et al. 1990;Vlug 1995). The historical provenance of the original exemplars, accompanied by label data, provides our best evidence for matching specimens with original concepts, and neotype specimens morphologically congruent with the descriptions in Nees (1834). All neotypes are property of the NHMW and are accessible for study.
Much work remains to generate a functional taxonomy for Palearctic Platygastridae. There is a need for a rigorous treatment of the world genera, and many species remain nearly unidentifi able, despite being described somewhat recently. For each genus, a thorough chronological exploration of type specimens will be required to characterize morphology, identify junior synonyms, and develop reliable diagnoses. If types are missing, neotypes must be appropriately designated. Future workers should not be discouraged by these challenges. Although the process can be slow, high-quality taxonomic research is worthwhile, as its products are critical to understanding the diversity of life on our planet.