A new European species of Mesocrina (Hymenoptera, Braconidae, Alysiinae, Alysiini) with notes on the biology and systematics of the genus

Mesocrina chandleri Godfray & van Achterberg, sp. nov. , is described in the small Holarctic genus Meso-crina Foerster, 1863, the second European species. The holotype was collected in England and further specimens are recorded from Finland, France and the Netherlands. A key is provided to the Palaearctic Mesocrina species. DNA sequence from the CO1 barcode locus was obtained and the new species is 10% divergent from M. indagatrix (the other European species) and 5% divergent from an undetermined North American species. We provide evidence that Mesocrina spp. parasitise cyclorrhaphan Diptera in fungi (and that previous host records from phytophagous insects are incorrect) and that the taxon is not part of the Dapsilarthra genus-group as often previously assumed.


Introduction
The genus Mesocrina Foerster, 1863 was erected for M. indagatrix Foerster, 1863, which for a long time was the sole member of the genus as understood today.In the last quarter of the 20 th century four further species were described from the Eastern Palaearctic and India (van Achterberg 1983;Belokobylskij 1998) and two from North America (Wharton 1980).Females of the genus are distinctive as they have a laterally compressed metasoma with dorsal carina on the third and distal tergites, but both sexes can be identified to genus relatively easily in standard keys to the Alysiini (Fischer 1971;Tobias 1986;Wharton 1997;Belokobylskij and Tobias 1998;Zhu et al. 2017).
Mesocrina indagatrix, hitherto the only species known from Europe (Königsmann 1959;van Achterberg 1983van Achterberg , 2014)), is a widespread but uncommon species with relatively few specimens in major collections.An examination of the four Mesocrina in the collection of the National Museum of Scotland found that in addition to three M. indagatrix, there is a single female specimen that differs from M. indagatrix in several aspects, notably it is larger in size and has a longer ovipositor.DNA sequence from the CO1 barcode locus was obtained from both M. indagatrix and the aberrant specimen and were clearly distinct (10% divergence).The BOLD database contained a specimen from Finland with identical sequence to the new taxon which was borrowed and found to be a male.Four further specimens were then discovered in recent collections from the Netherlands and France.The wasps differ from the species known from the East Palaearctic and North America and here are described as belonging to a new species.

Methods
Details of the holotype and five paratypes are given below in the type designation.The holotype is deposited in the National Museum of Scotland, Edinburgh, the male paratype in the Zoological Collections of the Finnish Museum of Natural History, University of Helsinki, Finland, and the remaining paratypes in the entomological collection of the Naturalis Biodiversity Center, Leiden.
Photographs were taken through a Leica M125C microscope with focus stacking using the Leica Application Suite X (LAS X) image analysis software with final processing in Photoshop.Ratio measurements were also made using the LAS X system.Morphology terminology follows Wharton et al. (1997).
CO1 sequencing was carried out by the Biodiversity Institute of Ontario at the University of Guelph with initial Sanger sequencing supplemented in the case of the M. indagatrix specimen by "next generation" short read sequencing (Prosser et al. 2016).Our sequence data are publicly available at http://v4.boldsystems.org/which also provides full details of primers.Sequence analysis was carried out on the BOLD platform and using the programme MEGA11 (Molecular Evolutionary Genetics Analysis version 11, Tamura et al. (2021)).Name.The new species is named for Peter J. Chandler, the eminent Diptera entomologist who collected the first specimen in 1998.
Head.Antenna with 36 segments, 1.1 times length of fore wing and 1.5 times body, densely clothed with anteriorly directed setae projecting at an angle of 30°; length of third segment 0.7 times fourth segment, lengths of third, fourth and penultimate segments 3.9, 2.7 and 2.4 times their widths, respectively; maxillary and labial palps with 6 and 4 segments, respectively; length of maxillary palp ~ 2.6 times height of eye; OOL 2.3 times greater than POL, POL 1.4 times posterior ocellus diameter; in dorsal view head 2.0 times wider than maximum length, margin of temples posterior to eye slightly convex; vertex shiny with scattered anteriorly-directed setae especially near occiput and margin of eye, a weak furrow runs from between posterior ocelli to occiput; frons largely smooth and glabrous with small areas of rugosity and a small number of setae; dorsal length of eyes 0.6 times maximum length of head, glabrous; face 1.8 times wider than high medially, 0.6 times as wide as head, largely smooth with some slight rugosity medially, with moderately dense setae apart from a narrow medial glabrous band, the setae dorsally directed except at sides where they point laterally and lateroventrally; clypeus glabrous with sparse punctures, ventral margin very slightly convex, epistomal suture distinct; malar space 0.15 times basal width of mandible; mandibles 1.6 times longer than maximum width which is 1.3 times basal width, finely rugulose and setose; three teeth with the central pointed and 0.3 times mandible length, the others obtuse and slightly reflexed relative to the axis of the mandible, no sharp incisions between teeth; head 1.3 times as wide as mesoscutum.
Mesosoma.Length of mesosoma 1.25 times its height; pronope absent; side of pronotum glabrous and largely smooth with some crenulation antero-medially and ventrally; mesopleuron smooth with scattered setae, mesopleural sulcus finely crenulate, precoxal sulcus completely absent; mesosternum with moderately dense posteriorly directed setae and a narrow punctate sulcus; metapleuron shallowly rugulose with sparse setae; mesoscutum with a triangular area of dense setae and punctation on its anterior surface extending somewhat dorso-medially, but dorsal surface largely shiny with sparse setae, notauli punctate anteriorly but absent from dorsal surface, a medioposterior groove runs from 2/3 to posterior edge, margin adjoining pronotum with longitudinal striae; scutellar sulcus 3.0 times as wide as long, with a strong medial carinae and two pairs of weaker lateral carinae; scutellum smooth with a few setae; axillar depression well developed and weakly crenulate; metanotum rugose with an indistinct antero-medial carina that bifurcates posteriorly, not protruding in lateral view; propodeum rugulose anteriorly, medially and laterally, with a relatively smooth area lateromedially, posterior region adjacent to insertion of first metasomal tergite incised and raised with horizontal striae, lateral posterior margin with small protuberance, surface with scattered sparse setae.
Legs.Hind coxa smooth; hind femur ~ 5 times longer than maximally wide, densely clothed with short setae dorsally projecting at an angle of 30°, ventrally approximately half the width of the femur and projecting at 60°; hind tibia slender, densely clothed with setae (projecting at 30°), 1.15 times longer than tarsi; apical tibial spur inconspicuous, less than 0.2 times length of basitarsus; tarsi with similar setae to tibia, tarsal segment length ratios 1 (basitarsus): 0.53: 0.42: 0.31: 0.38; tarsal claws and arolium well developed and 0.75 times length of fifth tarsal segment; structure of fore and mid legs similar though femur more slender and legs shorter, ratio of hind: mid: fore femur 1: 0.90: 0.81.
Metasoma.Length of first tergite 1.7 times its apical width, the latter 1.7 times its narrowest width near its base, a pair of dorsal carinae arise basally from the lat-eral carinae and reach the dorsum at about ¼ and run close together in parallel to about ½ where they lose their distinctiveness, posterior dorsal surface with longitudinal sculpturing and sparsely scattered setae, a distinct dorsope present; metasoma beyond first tergite strongly lateral compressed with a strong dorsal medial carina extending from the third tergite to the end; second tergite smooth; scattered setae on second and posterior tergites; ovipositor straight and projecting, its exposed setose part 0.7 times the length of the hind tibia; ovipositor sheath with posterior directed setae projecting at an angle of 60-80°, their length up to twice the width of sheath; hypopygium slightly postero-ventrally produced, terminating at level of the cerci.
Colour.Head, mesosoma and metasoma dark brown except for yellow brown parts: scape, pedicel, base of the third antennal segment, mandibles (apart from tooth tips) and ventral part of gena, latero-ventral prothorax, tegulum, medio-ventral region of the laterally-compressed metasoma; precoxal area of mesopleuron slightly lighter than the remainder of the surface; palps and legs yellow, the mid and hind tarsi slightly darker, areola contrastingly dark brown; wing venation and pterostigma dark brown, wing membrane hyaline.
Variation amongst females.The French and Netherland female specimens generally match the holotype.The number of antennal segments were 34 and 37 (two specimens without complete antenna); extent of setation on dorsal surface of mesoscutum varies from comparatively well developed as shown in holotype (Fig. 1) to largely restricted to a few remote setae as figured in the male paratype (Fig. 2).In addition, the medio-longitudinal carina of propodeum is weakly developed in some specimens.
This specimen is somewhat damaged with both antennae truncated and some legs missing.Similar to the female but differing in the following features.
Mesosoma.Anterior surface of mesonotum setose but setae extending less onto the dorsal surface than in the female holotype; propodeum smoother.
Metasoma.The metasoma is not laterally compressed and does not have longitudinal carinae on the posterior tergites; thus, having a "normal" Alysiini appearance.
Colour.Ventral margin of clypeus yellow; metasoma more uniformly brown.Molecular analysis.Sequence data from the mitochondrial CO1 gene (the standard barcode locus) were obtained from the M. chandleri holotype (607 base pairs) and a British specimen of M. indagatrix (550 b.p.).A male Finnish specimen with identical gene sequence to the holotype was found in the BOLD database.No further Old World sequences were present in BOLD, but it did contain seven closely-related North American sequences, some identified as Mesocrina and others that appeared from images in BOLD to belong to the genus.A maximum likelihood phylogenetic tree (CO1 gene, Tamura-Nei model; default MEGA settings) of the 10 sequences was created (Fig. 3), and they segregated into six putative species or BINs (Barcode Index Numbers; BIN codes as of February 2024).
Mesocrina chandleri (BIN: BOLD:ADX0117) shows a 6.5% divergence from its nearest relative, a Californian BIN (BOLD: AFO2946) represented by a single specimen.The two together with a Canadian BIN (3 specimens, BOLD:ACD3275) form a clade separated by 6.1% from a clade containing M. indagatrix (BOLD:AED2809) and two Canadian BINs (BOLD:AAU8494 & ACL6319).M. indagatrix shows a 11.6% separation from M. chandleri and 6.0% from the nearest Canadian BIN.The genetic data thus strongly supports the distinctiveness of the new species.

Discussion
The Palaearctic species of Mesocrina Foerster can be distinguished by the following key which is based on Belokobylskij's (1998)    Females of both European species of Mesocrina and an American species (Wharton 1980) have been captured on gilled fungi, and it has been hypothesised that the laterally flattened female metasoma allows it to be inserted between closely packed fungal gills (Wharton 1980).We are unaware of any substantiated host rearing records but as all Alysiinae attack Diptera we presume Mesocrina spp.parasitise fly larvae feeding in fungal fruiting bodies.Königsmann (1959) quotes two non-fungi-associated host records for M. indagatrix which he referred to as Pseudomesocrina venatrix Marshall, later synonymised by van Achterberg (1983).The first a male from a Pegomya sp.(Anthomyiidae) leaf mining Rumex sp. in the UK (Morley 1933).The first author and his students have reared 1880 parasitoid specimens from Pegomya spp.feeding on Rumex in the UK of which the only Alysiini were Adelurola florimela Haliday which made up 35% of the rearings.As males lack the distinctive compressed metasoma, and as Morley says the male only differs from the female in antennal segment number (which would be a curious observation to make about a highly sexually dimorphic Mesocrina), we suspect Morley's record is a misidentification, possibly of A. florimela.The second is Nanna (=Amaurosoma) armillata and/or Nanna flavipes (Scathophagidae) in developing flower heads of grasses, Phleum spp.(Poaceae), in Sweden (Borg 1959).We suspect this is a misidentification of Synelix semirugosa Haliday (Braconidae, Alysiinae) which attacks these hosts (King et al. 1935;Telenga 1935) and also has a laterally compressed metasoma.Our working hypothesis thus remains that Mesocrina spp.are parasitoids of Diptera in fungi.
The UK, Netherland and French specimens of the new species were all captured in October while the more northerly record from Finland is from September.The three specimens of M. indagatrix in the NMS collection were also collected in the autumn.The abundance of fungal fruiting bodies peaks at this time of year, and so the phenology and putative biology are consistent.Mesocrina chandleri is a relatively large and distinctive species (for an Alysiini) and given how widespread it is we are surprised it has not been noted before, especially as some of the recent records come from sites that have been regularly surveyed for Braconidae.We speculate that the species may be undergoing a current expansion of range and/or abundance.
There are currently eight described species of Mesocrina, six from the Palaearctic and two from the Nearctic (Wharton 1980;van Achterberg 1983;Belokobylskij 1998).However, DNA CO1 sequences of the two European species cluster closely with four North American taxa that we believe are Mesocrina (that may or may not include the two described by Wharton) suggesting greater diversity in the Nearctic than currently recognised.
The genus Mesocrina was described by Foerster (1863Foerster ( [1862]]) though van Achterberg (1983) treated it as a subgenus of Dapsilarthra.Later Papp (1991Papp ( [1990]]) raised it to genus level which was followed by other authors (van Achterberg 2014).Other Dapsilarthra genus-group braconids attack phytophagous cyclorrhaphan Diptera and they are thought to be the sister group of the Dacnusini which have the same biology (Griffiths 1964).Van Achterberg (1983) noted that the association of Mesocrina spp. with fungi was atypical for the genus group, and Wharton (1980) suggested an affinity with Alysia which is supported by our (relatively limited) DNA sequence data.Stronger evidence comes from a recent phylogenomic study of the Braconidae that included single species of Alysia, Mesocrina and Dapsilarthra: the first two genera are closely associated, while Dapsilarthra is distantly related and is indeed placed near the root of the Dacnusini (Jasso-Martinez et al. 2022).

Figure 1 .Figure 2 .
Figure 1.Montage of photographs of the female holotype (Sample ID NR890) of M. chandleri a lateral view b dorsal metasoma c whole insect d lateroventral head e face and mandible f dorsal head and mesosoma.The length of the body (excluding antennae and ovipositor) is 3.8 mm.

Figure 3 .
Figure 3. Maximum-likelihood tree (CO1 gene, Tamura-Nei model) with branch lengths of the 10 Mesocrina sequences in the BOLD database.The height of the terminal triangular wedges represents sample number and their horizontal width the genetic variation within the species (where no variation the wedge is a vertical line).Letters represent putative and undetermined Canadian (A, B, D) or Californian (C) species of Mesocrina.

Figure 4 .
Figure 4. Montage of drawings (by C.v.A. using a camera lucida) of male and female Mesocrina indagatrix.Scale bar: 1.0 mm.