Platygastridae (Hymenoptera) is a diverse family of parasitoid wasps for which few studies of internal morphology have been conducted. The monophyly of the group is undisputed based on recently published molecular data, but based on morphology, the family is diagnosable from other platygastroids only by a combination of character reductions. In the present study we explored the mesosoma of Platygastroidea and found two new synapomorphies for Platygastridae: an externally visible anterior mesofurcal pit, which corresponds to an invagination that connects to the anterior portion of the mesofurca, and internally, a posteriorly shifted origin of the first wing flexors. The absence of a mesofurcal bridge and the exclusively mesopectal origin of the fore wing flexors are treated as synapomorphies for Platygastridae+Janzenellidae. Phylogenetic implications and evolutionary hypotheses regarding these traits are discussed.

Fossils, furca, Janzenellidae, morphology, Platygastridae, Scelionidae

Platygastroidea (Hymenoptera, Proctotrupomorpha) are parasitoid wasps whose hosts represent ten insect orders and spiders (Austin et al 2005; Chen et al. 2021) and has been recently classified into eight families (Table 3; Chen et al. 2021). Platygastridae is highly diverse and extremely common with more than 2000 species worldwide (mbd-db.osu.edu). Based on molecular analyses, Platygastridae is sister to Janzenellidae, but this relationship is yet to be supported by any morphological characters. Platygastridae currently contains two monophyletic subfamilies, Platygastrinae and Sceliotrachelinae (Chen et al. 2021).

Platygastrinae parasitize gall midges (Diptera, Cecidomyiidae) (Austin et al. 2005) and are known to be effective against new invasive and serious quarantine pests in North America, such as the Swede midge (Ferland 2020) and the apple leaf midge (He and Wang 2015; Cossentine et al. 2021). They are considered propitious natural enemies of the wheat midge (Echegaray et al. 2016; Chavalle et al. 2018; Dufton et al. 2021) and have potential utility in suppressing populations of the soybean midge as well (Harris 2019; Kee 2019). In contrast to Platygastrinae, sceliotrachelines are egg, nymphal, and larval parasitoids of multiple orders including Coleoptera, Hemiptera, and Hymenoptera (Chen et al. 2021).

Despite their economic importance, the systematics of Platygastridae at the species level is in major need of revision, characterized by a high number of superficially described species with incomplete or missing differential diagnoses (Buhl and Jałoszyński 2019; Buhl 2021). At the generic level, identification of platygastrines is nearly impossible for non-specialists due to a lack of comprehensive identification keys and inadequate generic concepts.

The monophyly of Platygastridae is strongly supported by a recent molecular analysis (Chen et al. 2021). Despite this strong support of monophyly, only three morphological synapomorphies have been proposed for the family: absence of a mesepimeral sulcus (Talamas et al. 2019), absence of the mesofurcal bridge (Heraty et al. 1994), and the absence of a netrion (Mikó et al. 2007). However, only the mesepimeral sulcus has been examined in a wide range of platygastrid taxa and on many specimens. In this study, we examined the skeletomuscular system of the mesosoma in a range of extant and fossil platygastroids to better characterize these and other traits, and to explore to what degree they are phylogenetically informative.

In this section, we provide annotated descriptions of morphological traits on the pronotum and mesopectus that have significance in the higher classification of Platygastroidea with special emphasis on Platygastridae. Numbers following each trait description represent characters: states indicated in Table 3.

Description

The netrion is absent (1:0; Figs 1–4) from Platygastridae and Janzenellidae, and the first flexor of the fore wing arises exclusively from the mesopectus (2:0; Fig. 23B, C). The netrion is present (1:1) in Nixoniidae, Scelionidae, and Sparasionidae, and the first flexor of the fore wing arises partially from the pronotum (site of origin of the muscle corresponds to netrion, 2:1; Fig. 23A). The muscle was not observed in other platygastroid families that we examined. The ventral bridge of the pronotum is visible laterally, directly anterior to the ventral mesopectus (3:1; Fig. 4) in some platygastrid genera (Allostemma, Alfredella Masner & Huggert, Amitus, Oligomerella Masner & Huggert, Prototeleia Talamas, Popovici, Shih & Ren), but the first flexor of the fore wing does not partially originate from that area. The ventral bridge of the pronotum is visible laterally in some Proterosceliopsidae but not in other families of Platygastroidea.

Figures 1–4. 

Mesopectus, ventrolateral view 1 Amitus (USNMENT00989622_2) 2 Fidiobia (OSUC 698059) 3 Iphitrachelus (USNMENT01029164) 4 Sceliotrachelinae sp. (OSUC 698061).

The mesodiscrimen is absent from Platygastridae (4:0; Figs 1–4), present only posteriorly in Janzenellidae (5:0; Fig. 8), and present and extending anteriorly to the acetabular carina in other platygastroid families (5:1; Figs 9–12). The mesodiscrimenal lamella is not adjacent anteriorly to the acetabulum (6:1) in Janzenellidae and Platygastridae (except Synopeas), and in the latter, its anterior-most point is marked by the anterior mesofurcal invagination (Figs 6, 15, 17). The lamella extends anteriorly to the acetabulum (6:2; Fig. 5) in Synopeas (Platygastridae) and the other taxa examined.

Figures 5–6. 

Mesodiscrimenal lamella 5 Synopeas sp. (UNHC_1046722B) 6 Amitus sp. (UNHC_1046725B).

The anterior mesofurcal pit is present (7:1; Figs 1–4, 6–7, 14–17) in all Platygastridae except Synopeas and is absent (7:0; Figs 8–13) from other platygastroid families. The pit marks the anterior mesofurcal invagination that is continuous with the invagination of the mesofurca (Figs 15, 17). The anterior mesofurcal pit is located more posteriorly, near the middle of the ventral mesopectus (8:0; Fig. 3), in Sacespalus, Iphitrachelus, and Inostemma, and the pit is in the anterior third of the ventral mesopectus in other Platygastridae (8:1; Figs 1, 2, 4)

Figures 7–10. 

Mesopectus, ventral view 7 Orwellium enigmaticum (OSUC 226542) 8 Janzenella innupta (OSUC 264384) 9 Archaeoteleia gracilis (OSUC 163002) 10 Sceliomorpha (USNMENT00989930).

In Nixoniidae, a setose pit is present on the acetabulum that does not correspond to an invagination and is not connected to the mesofurcal invagination (9:1; Fig. 11). The pit is absent from the other taxa examined (9:0). In Gryon aetherium (Scelionidae), the left and right halves of the acetabular carina medially enclose a pit-like structure without any distinct invagination (10:1; Fig. 13). The medial invagination is absent from other platygastroid families (10:0).

Figures 11–13. 

11 Nixonia (SAM-HYM-P093322), mesopectus, ventral view 12 Proteroscelio (CNU-HYM-MA-2017545), internal mesosoma, lateral view 13 Gryon aetherium (DPI_FSCA 00008713), mesopectus, ventral view.

The mesofurcal bridge is absent (11:0; Figs 18–21, 23E, F) from Platygastridae and Janzenellidae and is present (11:1; Fig. 23D) in other platygastroid families.

Paired, shallow postacetabular pits are present (12:1; Figs 1, 3, 14) in many platygastrid genera (e.g. Amitus, Alfredella, Oligomerella, Fidiobia) and absent from other platygastroid families (12:0; Figs 2, 4).

In Platygastridae, the mesepimeral sulcus and the posterior mesepimeral area are absent (13:0; Figs 1–4), the mesepimeral ridge extends along the posterior margin of the mesopectus (14:0; Figs 19, 20), and the first flexor of the hind wing arises exclusively from the metapectus (15:0; Fig. 23C). In other platygastroid families, the mesepimeral sulcus and posterior mesepimeral area are present (13:1; Fig. 9) and the mesepimeral ridge is anterior and parallel to the posterior margin of the mesopectus (14:1). The first flexor of the hind wing arises partially from the mesopectus in Janzenellidae, Nixoniidae, Scelionidae, and Sparasionidae (15:1; Fig. 23A, B). The muscle was not observed in other families.

Figures 14–17. 

14 Trichacis sp. (FSCA 00000276), ventral view 15 Amitus (UNHC_1046725A) interior of head and mesosoma, lateral view 16 Isocybus (OSUC 334137), mesopectus, ventral view 17 Isocybus (unvouchered), interior of mesopectus, lateral view.

Figures 18–21. 

18 Synopeas sp. (UNHC_1046722A), interior of mesosoma, dorsal view 19 Allostema sp. (FSCA 00000289), internal mesopectus posterior view 20 Janzenella innupta (UNHC_0032469), interior of mesosoma, dorsal view (CLSM) 21 Janzenella innupta (UNHC_0032469), interior of mesosoma, dorsal view (compound microscopy).

The external mesopleuron and metapleuron of Platygastridae are simpler than most other platygastroid families (Masner 1979, 1980; Masner and Huggert 1989), however, this simplicity belies an array of internal characters that are demonstrably informative. Our observations on pectal morphology provide a clear example of this. The proposed synapomorphies have utility for family-level identification and phylogenetic inquiry, which is especially useful for fossil taxa that often exhibit forms quite different from the extant fauna. For example, the anterior mesofurcal pit is recognizable in a specimen from Burmese amber, which contains the oldest representatives of Platygastridae (Fig. 22). This specimen is recognizable as a platygastrid by other characters, but only the ventral aspect is presently visible. This confirms the association of the anterior profurcal pit with Cretaceous Platygastridae and illustrates how this character may be helpful for family-level placement of fossil specimens. Similarly, in a bisected specimen of Proteroscelio Brues, a mesodiscrimen that extends anteriorly to the acetabulum is clearly indicated by a row of foveae (Fig. 12), as in Nixonia and Archaeoteleia. The family-level placement of Proteroscelio is equivocal and may be guided by further examination of its mesosomal morphology.

Figure 22. 

Platygastridae in Burmese amber (CNU-HYM-MA-2016107), habitus, ventral view.

Janzenella was described by Masner and Johnson (2007) with an uncertain position in the superfamily. The genus was later retrieved as sister to Platygastridae in the phylogenetic analysis of Chen et al. (2021). We found that Janzenella is a morphological “missing link” because characters are shared exclusively with Platygastridae while some plesiomorphies associated with the rest of the superfamily are retained (Table 2). The recent study by Bremer et al. (2021) documented characteristics of the pro-, meso- and metafurca in a specimen of Janzenella from Baltic amber, confirming the stability of these characters in Janzenellidae and demonstrating how emerging technologies can be applied to fossil taxa. In the case of Platygastroidea, this line of inquiry is clearly fruitful, as some of the most informative family-level characters are internal skeletomusculature.

Figure 23. 

Illustration of the mesodiscrimenal lamella, invaginations on the ventral mesopectus, sites of origin of first wing flexors (A–C), and configuration of the mesofurca (D–F) A, D Scelionidae, Nixoniidae, Sparasionidae B, E Janzenellidae C, F Platygastridae.

Figure 24. 

Modified phylogeny from Chen et al. (2021) with characters mapped to the topology. Parenthetical numerals indicate the corresponding character: state from Table 3 A mesofurcal bridge present (13:3) B mesepimeral sulcus present (13:1) C first flexor of the fore wing with pronotal site of origin (2:1) D first flexor of the hind wing with mesopectal site of origin (15:1) E discrimenal lamella not extending anteriorly to acetabulum (6:1) F mesofurcal bridge absent (11:0) G first flexor of the fore wing with mesopectal site of origin (2:0) H mesepimeral sulcus absent (13:0) I first flexor of the hind wing with metapectal site of origin (15:0) J anterior mesofurcal pit present (7:1) K discrimenal lamella extending to acetabulum (6:0) L anterior mesofurcal pit absent (7:0).

We suspect that the anterior mesofurcal pit and its corresponding invagination, loss of the mesofurcal bridge, and ancestral wing flexor muscle patterns are interrelated, and that stabilization of the mesofurca shifted to mesofurcal invaginations from the mesofurcal bridge in concert with repositioning of wing flexor muscles. This makes sense considering that the mesofurca serves as the site of origin of numerous muscles involved in flight (e.g. mesofurco-mesolaterophragmal muscle) (Vilhelmsen et al. 2010). We also surmise that the transepisternal line is related to these characters. In Platygastridae, the transepisternal line externally marks the ventral margin of the site of origins of the fore wing flexors (pl2-3ax2a, b) and the retractor of the mesoscutellum (pl2-t2b; Mikó et al. 2007), similar to some Cynipoidea and Diapriidae (Vilhelmsen et al. 2010). Thus, the presence of the transepisternal line might indicate a shift of the first flexor to the mesopectus, given that the line is absent from platygastroids where the first flexor arises from the pronotum (Mikó et al. 2007). The fossil genus Proterosceliopsis Ortega-Blanco, McKellar & Engel (Proterosceliopsidae) offers some additional clues. It has a mesepimeral sulcus, suggesting that the first flexor of the hind wing attaches on the mesopectus. The laterally visible ventral bridge of the pronotum in Proterosceliopsis, termed the netrion in Talamas et al. (2019), is similar to the condition found in Platygastridae (Talamas et al. 2021), leading us to believe that the first flexor of the fore wing arises from the mesopectus in this genus. If this is correct, Proterosceliopsis would have the same configuration of first wing flexor muscles as Janzenella. Platygastridae and Proterosceliopsis are the only platygastroids with a transepisternal line, which further supports the notion that there are mesopectal origins of first flexors of the fore wing in the latter taxon.

Elijah Talamas was supported by the Florida Department of Agriculture and Consumer Services, Division of Plant Industry (FDACS-DPI). We thank Jonathan Bremer (FDACS-DPI) and many participants in the Smithsonian internship program for contributing images. Dong Ren and Chungkun Shih kindly provided a loan of the specimens in Burmese amber from Capital Normal University, Beijing.