Review of the rare genus Vanhornia Crawford, 1909 (Hymenoptera, Proctotrupoidea, Vanhorniidae) with description of a new species from the Russian Far East

A brief review of the proctotrupoid genus Vanhornia Crawford is given. A new species, Vanhornia yurii sp. nov. from Primorskiy Territory, Russia, is described and illustrated. Photographic illustrations of Vanhornia eucnemidarum Crawford, 1909 (specimens from USA), V. leileri Hedqvist, 1976 (specimens from the Russian Far East) and V. quizhouensis (He & Chu, 1990) (holotype) are given. An identification key to all known species of Vanhornia is provided.


Introduction
The superfamily Proctotrupoidea is a diverse group of poorly studied parasitic Hymenoptera of ancient origin. The concept of Proctotrupoidea has changed considerably over the last few decades. After the exclusion of several taxa, Proctotrupoidea s. str.
in North America (Champlain 1922;Brues 1927;Deyrup 1985). In Sweden, V. leileri was also reared from an eucnemid host, Hylis cariniceps (Reitter) (Hedqvist 1976). Despite the fact that species of Vanhornia do not seem to have any economic importance as biological control agents of their rare hosts, investigations of this group of parasitic Hymenoptera are of great interest for Hymenoptera morphology and phylogeny (Castro et al. 2006;Hogan et al. 2019).
Study of materials from different regions of Russia revealed three additional records of Vanhornia. In this paper one of the specimens from the southern Russian Far East is described as a new species, and the other records provide new important information on the distribution of the trans-Palaearctic V. leileri.
The main aim of this study is to describe a new Vanhornia species from Eastern Palaearctic, to review all the known species of the genus of the world fauna, to prepare a new illustrated key for determination of Vanhornia species, and to discuss some of its morphological characters and distribution.

Material and method
The morphological terminology used in the present study follows Hymenoptera Anatomy Ontology (Yoder et al. 2010) and Mikó et al. (2007); surface sculpture terminology follows Harris (1979); terminology of wing venation follows Naumann and Masner (1985) and van Achterberg (1993). Traditional morphometric measurements were made from properly mounted specimens or from digital photographs (e.g. Popovici et al. 2013).
Photographs were obtained using a Leica M165 stereomicroscope equipped with a Leica DFC450 camera (Paleontological Institute RAS, Moscow) and with a Canon EOS 70D digital camera mounted on an Olympus SZX10 microscope (Zoological Institute RAS, St. Petersburg). Image stacking was performed using Helicon Focus 5.0. Final plates were prepared in Adobe Photoshop CS6.
The following abbreviations are used: POL, shortest distance between the lateral ocelli; LOL, shortest distance between the lateral ocellus and the median ocellus; OOL, shortest distance between the lateral ocellus and compound eye; OD, maximum ocellar diameter.  Crawford, 1909.

Taxonomic synopsis of Vanhornia species
Vanhornia eucnemidarum Crawford, 1909 Vanhornia leileri Hedqvist, 1976 Vanhornia quizhouensis (He & Chu, 1990 Description. Female. Body length 3.9 mm; fore wing length 2.6 mm; antenna length 1.5 mm; ovipositor length 2.7 mm. Head: In dorsal view 1.35 times as wide as its medium length, 1.3 times as wide as maximum width of mesoscutum; in lateral view 1.1 times as high as long; vertex distinctly roundly convex. Head behind eyes (dorsal view) weakly convex in anterior two-thirds, then weakly roundly narrowed. Vertex posteriorly without median longitudinal sulcus. Temple bulging, almost equal to transverse diameter of eye (dorsal view). Eyes suboval, slightly narrowed ventrally, bare. Ocelli weakly enlarged, arranged in obtuse triangle with base (POL) 1.5 times as large as its sides (LOL). POL almost 4.0 times OD, 1.6 times OOL. Occipital carina interrupted dorsally over wide distance, present laterally. Occiput dorsally with effaced vertical median sulcus. Frons distinctly widely convex medially, with small shallow but distinctly rounded submedian depression. Antennal scrobes present as shallow, smooth, subvertical oblique lateral furrows, without marginal carinae delimiting them from the central area of frons. Interantennal process present as obtuse low convex vertical cushion. Clypeal suture distinct and deep. Clypeus narrow and very wide, with two medio-lateral acuminate corners, distinctly concave medially and weakly concave laterally, about 5.0 times as wide as median height. Malar space very short. Temple along its lower margin (between occipital carina and eye) with transverse crenulate narrow furrow. Mandible with four distinct and thick teeth and small additional tubercle on antero-lateral surface; apically two anterior teeth rather acuminate and two posterior teeth rounded and obtuse; anterior tooth distinctly separate from more closely situated three posterior teeth. Clypeus and base of mandibles covered by rather dense long whitish setae. Vertex and temple entirely in distinct but small, numerous and rather dense setiferous punctation; frons finely and sparsely punctate.
Antennae: Thickened, setiform, gradually weakly narrowed apically, 13-segmented, with very dense and short setosity. Scapus widened distally, 1.7 times as long as maximum apical width, about 2.5 times as long as subglobal pedicel. First flagellomere longest and widest, evenly widened towards apex, 2.6 times as long as its maximum width, 1.3 times as long as second flagellomere. Antennomeres A5-A9 elongate, uniform in length. Penultimate segment 2.0 times longer than wide, 0.75 times as long as apical segment; the latter weakly acuminate apically.
Mesosoma: Almost as high as wide, 2.1 times as long as maximum width (dorsal view), 1.9 times as long as maximum height (lateral view). Pronotal collar elongate, rugose-reticulate posteriorly and medially (dorsal view). Netrion almost spindleshaped, smooth, distinctly delimited by complete and crenulate with rugosity netrion sulcus. Mesoscutum distinctly transverse, 1.5 times wider than medial length, weakly convex, almost entirely distinctly and rather densely (but fine and sparse posteriorly, including between notauli) punctate with short and pale setae. Notauli arcuate, deep, complete throughout, distinctly and rather sparsely crenulate, not widened posteriorly. Parapsidal lines present and narrow. Scutoscutellar (prescutellar) sulcus deep, curved, with seven rather narrow foveae separated by distinct high striae. Scutellum rather large, distinctly narrowed posteriorly, almost as wide anteriorly as its median length, posterior fifth of scutellum separated by deep and distinctly crenulate transverse curved mesoscutellar sulcus. Metanotum (dorsal view) medially with subsquare and coarsely rugose convex area, laterally coarsely and sparsely crenulate with rugosity. Mesopleuron in wide and mainly concave median area (femoral depression) smooth and bare, antero-ventrally with wide crenulation at short distance; mesopleural carina (anterior ridge of mesopleuron) sharp, rugulose-crenulate in antero-dorsal half, extending posteroventrad as low and effaced ridge to postpectal (ventral mesopleural) carina; speculum smooth; mesepimeral sulcus comprised of large subcircular foveae above and smaller transversely elongate foveae below; lower convex longitudinal area of mesopleuron anteriorly distinctly rugulose-punctate, finely and sparsely punctate posteriorly; mesodiscrimen wide, percurrent, foveolate, in sparse and coarse crenulation. Propodeum mostly coarsely reticulate-areolate; basally with two curved short medial keels and two long, oblique, posteriorly converging lateral keels, delimiting three rather small mainly smooth but rugose posteriorly basal areas; with complete high coarse transverse curved keel in posterior 0.4 of propodeum. Fore wing: About 3.0 times as long as maximum width, entirely in dense and dark setosity. Radial cell distinctly shortened, surpassed by postmarginal vein, 3.5 times as long as maximum width. Costal section of radial cell 1.4 times length of pterostigma, about 1.5 times distance from apex of radial cell to apex of wing. Apical (terminal) abscissa of radial vein (3-Rs) weakly curved basally and almost straight in distal half, 3.0 times as long as preceding abscissa of this vein (2-Rs). Medial cell small and narrow, strongly narrowed distally. Vein cu-a straight and interstitial to vein 1-M.
Metasoma: Narrow (dorsal view). Syntergite 2-5 (dorsal view) without any traces of fusion of tergites, 2.4 times as long as its maximum width; basally with high and coarse weakly curved transverse keel, medially and laterally with coarse longitudinal keels in basal quarter of syntergite, with several and rather sparse longitudinal striation on basal 0.2; remaining part of syntergite entirely in rather dense but fine setiferous punctation. Synsternite 2-5 with a percurrent deep, relatively narrow and almost smooth groove on midline, almost entirely in dense and fine setiferous punctation, upper smooth on narrow stripe, posteriorly in narrow area in dense, straight or weakly curved vertical striation. Cowled tergite 6 abruptly deflexed, mostly in sparse and fine setiferous punctation, almost smooth in anterior vertical third; in posterior view, rather narrow, subtriangular shape, strongly narrowed to lower margin and distinctly convex upper, 1.4 times as high as maximum width, with distinct median obtuse and smooth vertical bar. Ovipositor mostly exposed, slender and flexible, its visible part 0.7 times as long as body.
Colour: Body mainly black, metasoma with brown parts. Antenna black; mandibles mostly brown with black teeth; palpi light brown to brown. Legs brown to dark reddish brown, fore and middle tibiae and tarsi and hind tarsus yellow to partly brownish yellow. Tegula dark brown. Fore wing faintly infuscate; sclerotised veins brown; pterostigma black. Ovipositor yellow.
Male. Unknown. Comparative diagnosis. This new species is similar to V. quizhouensis (He & Chu, 1990) from China (Guizhou) (Fig. 3) and Thailand, but differs from it in having the vertex without median longitudinal sulcus (vs. such sulcus present), mesoscutum in fine, small and dense setiferous punctation (vs. punctation distinct, enlarged and rather sparse), scutoscutellar (prescutellar) sulcus with seven narrow foveae (vs. only five foveae), vein cu-a in fore wing straight and interstitial to vein 1-M (vs. curved posteriorly and antefurcal); metasoma narrow, syntergite 2-5 in dorsal view 2.4 times as long as its maximum width (vs. 1.9 times).
The new species is also similar to the North American V. eucnemidarum Crawford, 1909 (Fig. 4), but differs from it in the scutoscutellar (prescutellar) sulcus with seven narrow foveae (vs. only five foveae), metasoma narrow, syntergite 2-5 in dorsal view 2.4 times as long as its maximum width (vs. 1.86-2.07 times), femoral depression on  mesopleuron mostly smooth and with crenulation anteroventrally (vs. with areolation medially and without lower crenulation), vertex without median longitudinal sulcus (vs. sulcus extends back from anterior ocellus proceeding into occipital vertical median sulcus), as well as distribution, in the East Palaearctic (vs. North America).
Etymology. This species is named in honour of its collector, Dr Yuriy N. Sundukov, a Russian coleopterist and hymenopterist.

Discussion
Vanhornia is a genus of rare and specialised parasitoids of remarkable appearance. Vanhornia is different from all other Proctotrupoidea in having exodont mandibles, which are generally rare in Hymenoptera. Exodont mandibles are characteristic of braconids in the large subfamily Alysiinae, which use them to cut their way out of the hosts' puparium (Tobias 1968;Wharton et al. 2006), and such mandibles are only occasionally found in some other braconid taxa (Wharton 1977;Mason 1991;Sharkey and Wharton 1994). Mymarommatoidea, an enigmatic group of small Proctotrupomorpha, also have mostly exodont mandibles (except in some extinct species), with their supposed function of rupturing the chorion of a putative host (Gibson et al. 2007). Likewise, the late instar larvae of several egg parasitoids are armed with powerful exodont mandibles to pierce the host egg chorion for allowing ventilation (Boivin 2010). Rather intriguing is the similarity of the mouthparts of Vanhornia with those of some eucnemid larvae (its hosts), in particular those of Melasini, to which Isorhipis ruficornis belongs, also equipped with large, freely movable, exodont mandibles (Deyrup 1985;Muona and Teräväinen 2020). Based on this similarity, Deyrup (1985) speculated, though recognized it as unconvincing, of the origin of Vanhornia from much more flatter ancestors, who completed their development in the larval galleries, and not in the pupal cell of their hosts. In any case, like Deyrup (1985), we have to state that the adaptive value of the exodont mandibles of Vanhornia is still unknown, but perhaps it can use exodont mandibles to chew through wood, similar to their host larvae.
Although all species of Vanhornia have exodont mandibles, their shape is substantially different between certain species. Hedqvist (1976) described V. leileri as having 5 mandibular teeth. However he pointed out in the diagnosis that mandibles of this species have fewer teeth than V. eucnemidarum. He illustrated the mandible of V. leileri with four teeth, and the latter species with five, so an error clearly occurred (Hedqvist 1976: fig. 2B, D). In all specimens of V. leileri that we have studied, the mandibles are elongate, 1.5-1.6 times as long as their width at the base, and have five distinct teeth, the anterior tooth being small, but rather distinctly separated from the large second tooth (Fig. 5B). The mandible of the specimens of V. eucnemidarum studied by us is very wide and short, about 0.7 times as long as its basal width, with four distinct and thick teeth and a small additional tubercle on antero-lateral surface (Fig. 4E). It resembles that of V. yurii sp. nov., but the new species has a somewhat deeper notch between the two posterior teeth (Fig. 1B), thus making them more distinctly separated. In V. eucnemidarum and V. yurii sp. nov., the anterior mandibular tubercle may be hidden behind the medio-lateral corner of the clypeus depending on the position of the mandible. According to the original description, the mandible of V. quizhouensis is very wide, with four triangular teeth (He and Chu 1990).
Another morphological character making Vanhornia unique among the Proctotrupoidea is the presence of a netrion, a morphological trait that was not mentioned in the previous original descriptions of species of this genus (Crawford 1909;Hedqvist 1976;He and Chu 1990). The netrion is a posteroventral portion of the pronotum delimited anteriorly by netrion sulcus and corresponds to the site of origin of the first flexor of the fore wing, and is well developed in all Vanhornia species. Other than in Vanhorniidae, a pronotal origin of the fore wing flexor is known only from Platygastroidea, and in all other Hymenoptera this muscle originates entirely from the mesopleuron (Mikó et al. 2007;Talamas et al. 2019).
The metasoma and ovipositor of Vanhornia are also very peculiar features. In Vanhornia, the heterogeneity of metasomal segments is especially pronounced among all Proctotrupoidea and is accompanied by the fusion of several sclerites into syntergites and synsternites, respectively. In V. eucnemidarum and V. leileri, the traces of fusion of metasomal tergites are barely discernible as areas with sparser pubescence (Figs 4I, 6B), whereas the syntergite is evenly punctate without any traces of fusion in V. yurii sp. nov. (Fig. 2C). The fusion of several metasomal sclerites is complemented by the reduction of metasomal spiracles, which is also a unique state for Proctotrupoidea s. str. (Naumann and Masner 1985). Only in V. eucnemidarum three minute pits are spaced at regular intervals along each side of the syntergite, which may be considered as vestiges of spiracles (Mason 1983). The ovipositor is extremely long, extruded, but very flexible, with weakly sclerotised third valvulae, transformed into the working element penetrating the substrate along with the first two pairs of valvulae. Investigation of the functional morphology of the ovipositor revealed that its external position in Vanhorniidae (as well as in Proctotrupidae) is a derived state (Rasnitsyn 1980). This type of ovipositor is adapted to attacking a concealed host and is no doubt unsuitable for drilling through hard wood. Vanhornia females have been observed to insert it into the cracks of logs or sticks using it like a probe to reach a potential host (Townes and Townes 1981;Deyrup 1985).
The presence of a set of peculiar morphological features implies a profound biological specialisation. However, biological information on Vanhornia is rather scarce. Vanhornia eucnemidarum is known as a parasitoid of Isorhipis ruficornis in North America (Champlain 1922;Brues 1927;Deyrup 1985) and V. leileri was reared from Hylis cariniceps in Sweden (Hedqvist 1976), with host species belonging to the subfamily Melasinae (Eucnemidae). Most of the false click beetles are associated with dead and rotting wood and are generally uncommon. If the assumption of the strict trophic relations of the parasitic wasps of the genus Vanhornia with eucnemids is true, it also explains the rare occurrence of these parasitoids. In Primorskiy Territory, Russia, 26 species of false click beetles of 18 genera have been recorded, and 22 species of them are found directly in the Lazovsky Nature Reserve (the type locality of V. yurii sp. nov.), all of them reported as rare (Gusakov 2009).
Species of the genus Vanhornia are found in the Nearctic (V. eucnemidarum) and Palaearctic (V. leileri and V. yurii sp. nov.), and one species (V. quizhouensis) is found in the Oriental region (South China and Thailand). Of these, V. eucnemidarum is the most common. The other species are especially rare, including V. leileri, which has been known to date only from North, West and Central Europe and from the Russian Far East. Therefore, the new records of V. leileri in East Siberia (Krasnoyarsk Territory) and in the Kuril Islands on the one hand expand its range, and on the other hand partly close the huge distribution gap for this species (Fig. 8).