Abstract

Therevidae is a diverse family of lower Brachycera, yet its early evolutionary history remains poorly understood due to the absence of available Mesozoic fossils. Here, we describe †Paleothereva longicoxa gen. et sp. nov. from mid-Cretaceous Kachin amber, representing the oldest member of known derived therevids. Phylogenetic analysis and morphological comparisons integrating both extant and fossil species, consistently place this fossil within Taenogera genus-group of Agapophytinae. This finding indicates that Agapophytinae had already diverged by the mid-Cretaceous, suggesting an earlier origin of Therevidae than previously inferred from molecular data. The occurrence of Paleothereva on the Gondwanan-derived West Burma Block further supports the hypothesis that the ancestors of Agapophytinae—or possibly the broader Agapophytinae + Therevinae clade—originated in Gondwana. Ancestral character state reconstruction of female terminalia within therevid flies reveals that Paleothereva and extant relatives likely shared similar oviposition behaviors associating with sandy-soil environments, which may have driven the corresponding specialization and diversification of acanthophorite spines in female. These new findings provide rare, direct evidence illuminating the early evolution of derived therevid lineages.

Keywords

mid-Cretaceous, Diptera, Gondwana, Kachin amber, oviposition

1. Introduction

The Stiletto flies (Diptera: Therevidae) are a highly diverse family of lower Brachycera, comprising more than 1,170 described species across approximately 128 genera worldwide (Winterton et al. 2016; Hauser et al. 2017). The internal phylogeny of extant therevids has been extensively studied through both molecular and morphological analyses (Winterton 1999; Yang et al. 2000; Winterton et al. 2001; Holston et al. 2007; Lambkin et al. 2009; Winterton and Ware 2015; Winterton et al. 2016), leading to the recognition of four subfamilies: Phycinae, Xestomyzinae, Therevinae, and Agapophytinae. Among these, Phycinae and Therevinae are the most widely distributed, occupying nearly all major zoogeographical regions except Australasia and Antarctica. In contrast, Xestomyzinae and Agapophytinae are distinctly distributed restrictively, showing a disjunct distribution pattern. The complex biogeographical patterns of stiletto flies present a significant challenge to reconstructing the evolutionary history of the family, necessitating an integrated approach combining molecular evidence with well-preserved fossil records (Winterton et al. 2016).

Female terminalia show considerable morphological diversity across subfamilies, corresponding to distinct oviposition strategies—a key adaptation behind the family’s diversification and ecological success (Irwin 1976). The female terminalia and their associated oviposition behaviors are well documented in extant therevids and recognizable in Cenozoic fossils (Metz and Irwin 2000; Hauser and Irwin 2005a, 2005b; Hauser 2007), but they remain poorly known for the early Mesozoic representatives due to scarce fossils. To date, only two Mesozoic species are tentatively placed within Therevidae: one from the Early Cretaceous Crato Formation and another from mid-Cretaceous Kachin amber (Cockerell 1920; Carmo et al. 2021). †Cretothereva antiqua Carmo, Lamas & Ribeiro, 2021 was initially regarded as the oldest known member of extant Therevidae, but the absence of several key diagnostic characters suggests that it more likely represents an early stem lineage of the family—or even of the entire therevid clade (Carmo et al. 2021; Greenwalt et al. 2022; Ribeiro et al. 2023). The second species, †Psilocephala electrella Cockerell, 1920, preserves only two legs and partial wings, leaving its taxonomic placement uncertain; it may be closer to †Kumaromyia burmitica, a potential stem taxon of the therevid clade (Gaimari and Mostovski 2000; Grimaldi et al. 2011). Thus, well-preserved Mesozoic specimens are essential to narrow the knowledge gap and clarify the early evolution of this family.

†Paleothereva longicoxa gen. et sp. nov., described here from mid-Cretaceous amber, represents the earliest known derived therevid from the Mesozoic. Phylogenetic and morphological evidence place it close to the Taenogera genus-group within Agapophytinae. Its occurrence indicates that Agapophytinae had already diverged by the mid-Cretaceous, suggesting an earlier origin of Therevidae than the molecular dating estimates suggested. Furthermore, the new fossil from west Burma Block provides new insight for understanding the early evolution of Therevidae, and its exceptionally preserved female terminalia offers rare morphological evidence that significantly advances our understanding of the early evolution of oviposition behavior within Therevidae.

2. Materials and Methods

2.1. Materials and Depository

The specimen CNU–DIP–MA2016125 is deposited in the College of Life Sciences, Capital Normal University, Beijing, China (CNUB; Dong Ren, Curator). The amber material analyzed in this study was collected from the Hukawng Valley, Tanai Township, Myitkyina District, Kachin State, Myanmar, a locality renowned for its exceptionally diverse and abundant insect inclusions (Kania et al. 2015). All newly examined specimens were acquired by Mr. Fangyuan Xia in 2015 and donated for scientific study in 2016, ensuring full ethical compliance for Kachin amber research (Engel 2020). The deposit has been dated to approximately 99 Ma (earliest Cenomanian) based on U–Pb analyses of zircons from the volcaniclastic matrix enclosing the amber (Shi et al. 2012).

2.2. Optical Microscopy

The amber pieces containing the specimens were ground and polished to an appropriate size to facilitate photography and morphological examination. Specimens were examined and photographed using a Nikon SMZ18 stereomicroscope equipped with a Nikon DS-Ri2 digital camera. Measurements are provided in millimeters (mm). Terminology follows the one used by Hauser et al. (2017).

Abbreviations are as follows: A1, acanthophorite A1 spines; A2, acanthophorite A2 spines; spines al, alula; an lb, anal lobe; a plp, apical plpomere; cerc, cercus; CuA, cubitus anterior vein; CuP, cubitus posterior vein; d, discal cell; dc: dorsocentral macrosetae; flg: flagellum; h, humeral crossvein; lbl: labellum; lbr: labrum; M, medial vein; m, medial cell; np, notopleural macrosetae; pa, postalar macrosetae; ped, pedicel; plp, palpus; R, radius vein; r, radial cell; r-m, radial–medial crossvein; Rs, radial sector vein; sa, supraalar macrosetae; Sc, subcostal vein; sc, subcostal cell; scp, scape; ST1–ST8, sternite 1 to 8; T1–T10, tergite 1 to 10.

2.3. Phylogeny

2.3.1. Taxa sampling

A total 27 species in 27 genera of Therevidae are sampled as the ingroup, representing all known extant subfamilies and most known fossil lineages (two fossils, †Thereva carbonum von Heyden, 1856 and †Thereva marcelini Theobald, 1937, were excluded due to poorly morphological information collected). Sampling details are provided in Table 1. One species of Scenopinidae, i.e. Scenopinus fenestralis (Linnaeus, 1758) was chosen as the outgroup, based on the established phylogenetic hypothesis of a sister relationship between Scenopinidae and Therevidae (Winterton and Ware 2015; Winterton et al. 2016).

Table 1.

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Taxon sampling.

	Subfamily	Species	Reference
Outgroup	Scenopininae	Scenopinus fenestralis	Pohjoismäki and Haarto 2021
Ingroups	Phycusinae	Acathrito robusta	Hauser 2017
		Efflatouniella aegyptiaca	Mohammad and Badrawy 2011
		Phycus angustifrons	Lyneborg 2003
		Ruppellia keiseri	Lyneborg 1989
		Salwaea burgensis	Winterton et al. 2012
		†Dasystethos hoffeinsi	Hauser 2007
		†Glaesorthactia magnicornis
		†Kroeberiella pinguis
		†Palaeopherocera scudderi	Hauser and Irwin 2005b
	Xestomyzinae	Lyneborgia ammodyta	Irwin 1973
		Microgephyra chrysothorax	Hauser and Irwin 2005c
		Xestomyza lugubris	Hauser 2012
		†Arctogephyra agilis	Hauser and Irwin 2005b
		†Peratrimera mexicana	Hauser and Irwin 2005a
	Therevinae	Ammonaios confusus	Hauser and Irwin 2003
		Ammothereva nuda	Liu et al. 2012
		Chromolepida bella	Webb and Irwin 1995
		Dialineura elongata	Liu and Yang 2012
		Neotherevella arenaria	Winterton et al. 2023
		Tabuda planiceps	Webb and Irwin 1999
		Tabudamima melanophleba	Webb and Irwin 1999
		Anabarhynchus oblongicornus	Winterton 2004
		†Ambradolon grimaldii	Metz and Irwin 2000
	Agapophytinae	Agapophytus collessi	Winterton 2013
		Bonjeania flavofemoralis	Winterton et al. 2000
		Collessiama narelleae	Lambkin and Turco 2013
		†Paleothereva longicoxa	This study

2.3.2. Morphological data

A total of 31 morphological characters were scored for all extant and fossil taxa included in the phylogenetic analysis. All character states were based on adult morphology, primarily following previous studies (Winterton et al. 1999, 2001; Yeates 2002; Hauser 2005; Winterton and Ware 2015). Among these, six characters were scored for head morphology (1–6), 17 characters for thorax morphology (7–21), and 10 characters for abdomen morphology (22–31). Characters were unordered in all analyses while missing characters were scored as “?”. All characters were equally weighted and unordered/nonadditive in MP phylogenetic analysis. The descriptions of character states are provided in File S1 and morphological character state coding is presented in File S2.

2.3.3. Phylogenetic analysis

Parsimony analysis was conducted under TNT v.1.6 (Goloboff and Catalano 2016) using a heuristic tree search protocol that employed a TBR searching strategy with 1000 times replicates of random addition. Bootstrap values (BS) and Bremer support values (BR) were calculated to test the stableness of clade. Characteristics were mapped on the tree using WinClada v1.61 (Nixon 2002).

2.3.4. Ancestral Character State Reconstruction

To elucidate the evolutionary trajectory of female terminalia morphology within Therevidae, a fully bifurcated tree and female terminalia state data were utilized in this analysis (File S3). Three distinct states of female terminalia were proposed: (1) absence of acanthophorite spines on tergite 10 and macrosetae on sternite 8, (2) absence of acanthophorite spines on tergite 10 but presence of macrosetae on sternite 8, and (3) presence of acanthophorite spines on tergite 10 but absence of macrosetae on sternite 8. The tree was imported in the “ape” package v.5.7 using the “read.tree” function to produce a readable tree (Paradis and Schliep 2019). To ensure the exact match of taxa with their respective trait dataset, we applied the “match.phylo.data” function from the “picante” package v.1.8.2 (Kembel et al. 2010). For modeling discrete character evolution, the Equal Rates model was implemented through the “fitDiscrete” function from the “geiger” package v.2.0.11 (Harmon et al. 2008). We accounted for uncertainty in ancestral states for discrete characters using the ace function from the ape package. Stochastic character mapping was performed with the “make.simmap” function from the “phytools” package v.2.1, and interpretability was enhanced with legends added via the “add.simmap.legend” function (Revell 2024). All computational analyses were conducted within the R v.4.3.3 (R Core Team 2013) and the R script is provided in File S4.

3. Results

3.1. Phylogenetic results

The phylogenetic analysis yielded four most parsimonious trees (MPTs) with a tree length of 69, a consistency index (CI) of 0.52, and a retention index (RI) of 0.81. One MP tree with a fully bifurcating topology is used to illustrate the internal relationships of Therevidae, which is nearly identical to the consensus tree (Fig. 1). In the results, all four subfamilies were recovered as monophyletic, in agreement with prior studies (Winterton and Ware 2015; Winterton et al. 2016). Phycusinae was identified as the earliest-diverging lineage, and sister to the remaining subfamilies. Four extinct species—†Dasystethos hoffeinsi, †Glaesorthactia magnicornis, †Kroeberiella pinguis, and †Palaeopherocera scudderi are assembled in this subfamily—as suggested in earlier works (Hauser and Irwin 2005a; Hauser 2007; Winterton and Ware 2015). The monophyly of Phycusinae is supported by one synapomorphy: a sensory pit at the tip of the palpus (char. 6:1). Xestomyzinae represents the second earliest-diverging subfamily and is sister to the clade Agapophytinae + Therevinae. †Arctogephyra agilis and †Peratrimera mexicana were placed with Xestomyzinae, consistent with previous studies (Hauser and Irwin 2005b; Hauser 2007). The monophyly of Xestomyzinae is supported by a synapomorphy: the presence of macrosetae on sternite 8 in female (char. 29:1). The sister-group relationship of Agapophytinae + Therevinae was recovered as reported earlier (Winterton et al. 2016). This clade is supported by four synapomorphies: a circumambient costal vein (char. 14:3), the presence of a sclerotized ridge between T8 and T9+10 (char. 25:1), the presence of a gonocoxal ventral lobe (char. 26:1), and well-developed acanthophorite spines (char. 27:1).

Figure 1.

Results of phylogenetic analysis of Therevidae. A The preferred MP tree exhibits a fully bifurcated topology that is largely consistent with the strict consensus tree; B Strict consensus tree. Unambiguous morphological character state changes were shown on the tree with a black circle as the homologous state and a white circle as the homoplasious state. Bremer support and Bootstrap values are shown next to relevant nodes.

Monophyly of Therevinae is supported by three synapomorphies: scutellum with 2 pairs of macrosetae (char. 8:1) and multiple types of femoral vestiture (char. 9:1), and a non-forked ventral apodeme of the aedeagus (char. 22:1). †Ambradolon grimaldii was resolved as a member of Therevinae, agreeing with earlier findings (Metz and Irwin, 2000). Agapophytinae is supported as monophyletic by one synapomorphy, suggested in earlier studies (Winterton and Ware, 2015; Winterton et al. 2016): three spermathecae that join the spermathecal duct to form a common duct (char. 29:1). The new species †Paleothereva longicoxa gen. et sp. nov. was recovered within Agapophytinae and is closely related to the Taenogera genus-group in this subfamily. Its placement is supported by two synapomorphies: absence of setae along the medial surface of the scape (char. 5:2) and the presence of a single seta anteroventrally at the apex of the hind femur (char. 12:1).

4. Systematic paleontology

Order Diptera Linnaeus, 1758

Suborder Brachycera Zetterstedt, 1842

Family Therevidae Newman, 1834

Genus Paleothereva Feng, Ren and Wang, gen. nov.

http://zoobank.org:act:44B46661-D750-4897-BCD1-716FA0A62F71

Figures 2, 3, 4, 5

Type species.

Paleothereva longicoxa Feng, Ren and Wang, sp. nov.

Figure 2.

Habitus of †Paleothereva longicoxa gen. et sp. nov., holotype, female, CNU–DIP–MA2016125. A left lateral view; B right lateral view.

Figure 3.

Head detials of †Paleothereva longicoxa gen. et sp. nov., holotype female, CNU–DIP–MA2016125. A, B Head in lateral view; C Antenna; D Flagellum; E Mouthpart. Abbreviations: a plp: apical plpomere; flg, flagellum; lbl, labellum; lbr, labrum; ped, pedicel; scp, scape.

Figure 4.

Body details of †Paleothereva longicoxa gen. et sp. nov., holotype female, CNU–DIP–MA2016125. A Thorax in lateral view; B Thorax in dorsal view; C Hind coxa; D fore-femur; E mid-femur; F Hind femur; G pulvilli and empodium; H Right wing; I Drawing of right wing; J Basal portion of right wing; K Distal portion of left wing.

Figure 5.

Abdomen of †Paleothereva longicoxa gen. et sp. nov., holotype female, CNU–DIP–MA2016125. A Abdomen, left lateral view; B Abdomen, right lateral view; C Female terminalia in dorsal view; D Female terminalia in ventral view.

Diagnosis.

Scape as wide as pedicel in width; palpus two-segmented; single seta present antero-ventrally on apex of hind femur; R₁ bare; cell m₃ open; A1 spines elongate and acuminate apically.

Etymology.

The generic name is derived from the Greek prefix “paleo-” (ancient), combined with Thereva, the type genus of Therevidae.

Paleothereva longicoxa Feng, Ren and Wang, sp. nov. (Figs 2, 3, 4, 5)

http://zoobank.org:act:9D34407D-86BB-4DDB-BCDE-1A01E388AE8C

Diagnosis.

Same as for the genus.

Locality and horizon.

Northern Myanmar, Kachin (Hukawng Valley), lowermost Cenomanian, dated 98.79 ± 0.62 Ma (Shi et al. 2012).

Type material.

Holotype female, No. CNU–DIP–MA2016125.

Etymology.

The specific epithet longicoxa is derived from the Latin “longus” and “coxa”, referring to the distinctly elongate coxa of the species.

Description.

Body: Slender, about 8.12 mm in length (Fig. 2A, B). Wing about 4.61 mm in length. — Head. Semispherical, without macrosetae; eyes dichoptic, bare; occiput with numerous setae; frons wide, sparsely setose (Fig. 3A, B). Antennae shorter than head height; scape approximately three times length of pedicel, bearing dense stout setae, absent on the medial surface; pedicel as wide as scape; flagellum 3-segmented, covered with dense pubescence; basal flagellomere bulbous and tapered apically, with numerous sensory pits near base; apical flagellomere longer than flagellomere 2; stylus short (Fig. 3C, D). Mouthparts short, labrum present, with well-developed fleshy labellum; palpus two-segmented; apical palpomere, gradually tapering to apex (Fig. 3E). — Thorax. Slightly arched dorsally, with 4 pairs of np, 1 pair of sa, 2 pairs of pa, 1 pair of dc; scutellum well-developed, with a pair of macrosetae (Fig. 4A, B). Legs slender, and equal length; coxae elongated with sparse macrosetae, 2× broader than femora; hind coxa knob present; single seta present antero-ventrally on apex of hind femur; tibiae with macrosetae; tarsus 1 equal to tarsi 2−5 combined in length; pulvilli well-developed; empodium setiform (Figs 2A, 4C–G). — Wing. Nearly equal to abdomen in length; costal vein circumambient; crossvein h situated near wing base; Sc ending at mid-position of costal margin; R₁ stout and straight, bare dorsally; Rs equal to the distance of R₄₊₅ origin to crossvein r-m; R₂₊₃ slightly sinuous at distal portion, distant to R₁ at wing margin; R₄₊₅ forked at the level of tip of M₃ ; R₄ and R₅ diverged; R₄ sinuous; cell r₄ encompassing the wing apex; crossvein r-m locating at mid-position of cell d; cell d slender, 2× longer than M₃; basal portion of M₁ arched; cell m₃ widely opened; crossvein m-cu present; cell cua closed; anal lobe wider than cell cua. Stem of haltere longer the apical knob in length (Fig. 4H–K). — Abdomen. Slender, densely pubescent; tergites 2–6 well exposed, tergite 2 longest (~2× tergite 1); tergite 7 partly covered by tergite 6; tergite 8 partly overlapping tergite 9+10; tergites 6−8 with long pilosity; both acanthophorite A1 and A2 spines present, acanthophorite A1 spines elongate and acuminate apically; cercus broad, one-segmented (Fig. 5).

Remarks.

The †Paleothereva gen. nov. is assigned to Asiloidea based on a combination of characters, including reduced flagellomeres, absence of tibial spurs, and wing venations (Yeates 2002). Within Asiloidea, the presence of vein M₃ and a forked R₄₊₅ excludes its affinities with Bombyliidae and Mythicomyiidae, while the configuration of R₂₊₃ and the extension of M₁ beyond the wing apex preclude its assignment to Apioceridae. A two-segmented palpus, together with unfused M₁ and M₂ distinguishes it from Mydidae, and the bare face and non-predatory labellum separate it from Asilidae. The presence of a hind-coxal knob evidently support its inclusion in therevid clade. Among therevid families, †Paleothereva lacks the specialized tergal setae typical of Scenopinidae, and differs from Evocoidae by its short stylus and distinct divergent R₄ and R₅. It is further distinguished from Apsilocephalidae by the termination of R₅ beyond the wing apex. The elongated scape with stout setae, three-segmented flagellomeres, sinuous R₄, and presence of acanthophorite spines collectively provide strong evidence to assign †Paleothereva to Therevidae.

Within Therevidae, †Paleothereva can be excluded from Phycinae based on several diagnostic characters: in Phycinae, the costal vein terminates at or before CuA, whereas it is circumambient in †Paleothereva (Fig. 4H–K); all extant and fossil members of Phycinae exhibit a dorsally setulose vein R₁, but R₁ is entirely bare in †Paleothereva (Fig. 4J); acanthophorite spines are markedly reduced in Phycinae, while they are well developed in †Paleothereva (Fig. 5C, D). Comparison with Xestomyzinae, the subfamily is most characterized by the presence of modified digging macrosetae on female sternite 8, but it is clearly absent in †Paleothereva. Additional characters further distinguish the genus from Xestomyzinae: cell m₃ is consistently closed in all known species of Xestomyzinae, whereas it is open in †Paleothereva (Fig. 4H, I); the longitudinal wing fascia characteristic of Xestomyzinae is absent in the new genus (Fig. 4H, I).

†Paleothereva is distinguished from the extant Therevinae by combining a single pair of macrosetae on scutellum (Fig. 4A, B) and lacking lanceolate setae on hind femora (Fig. 4F) (Hauser et al. 2017). Within Agapophytinae, several key diagnostic characters support a close relationship between †Paleothereva and the Taenogera genus-group: the scape lacks setae along the medial surface (Fig. 3C), an apical seta is present on the hind femur (Fig. 4F), and cell m₃ is open (Fig. 4H, I, K) (Winterton et al. 1999, 2001). Nevertheless, †Paleothereva differs from members of the Taenogera genus group in having a scape as wide as the pedicel and a flagellum lacking setae (Fig. 3C, D). Therefore, the new genus is erected here as a putative relative of the Taenogera genus-group within Agapophytinae.

5. Discussion

5.1. New insights into the divergence of derived therevid flies

Based on the results of phylogenetic analysis and morphological comparisons, †Paleothereva gen. nov. is well associated to the Taenogera genus group of Agapophytinae, representing a derived therevid lineage (Fig. 1). This discovery unequivocally provides critical new evidence for calibrating the timing of crown therevid divergence and for reconstructing their early biogeographic evolution. Its occurrence indicates that Agapophytinae had already diverged from Therevinae by at least the mid-Cretaceous. Together with the widespread Cenozoic record of therevid fossils across the Northern Hemisphere (Fig. 6A), it supports the hypothesis that Agapophytinae and Therevinae underwent a rapid radiation beginning in the Cretaceous (Winterton et al. 2016). Moreover, the geological age of †Paleothereva gen. nov. closely aligns with the Early Cretaceous divergence of Therevidae inferred from molecular data (Winterton et al. 2016), suggesting an earlier origin of the family than previously recognized.

Figure 6.

Distribution of fossil therevids and evolutionary implications of female terminalia. A Therevid fossils mapped on the modern worldmap. B Paleogeographic reconstruction of continental configuration during the Late Albian (100 Ma), highlighting the position of the Burma Terrane. C the lift is results of the ancestral character state reconstruction for the female terminalia; the right are types of oviposition modes in Therevidae. Type I is in Phycusinae; Type II is in Xestomyzinae; Type III is in Agapophytinae + Therevinae.

The extant Agapophytinae, are now restricted to Australasia and South America (Winterton et al. 1999, 2001; Hauser et al. 2017). Nevertheless, its Cretaceous relative, †Paleothereva gen. nov. occurs in the West Burma Block—an isolated island during mid-Cretaceous (Fig. 6B). The disjuctive distribution pattern between †Paleothereva gen. nov. and extant Agapophytinae implies a complex evolutionary scenario of this lineage. The West Burma Block once formed part of eastern Gondwana during the Middle to Late Jurassic supported by paleontological, paleomagnetic, and stratigraphic evidence (Ezcurra and Agnolín 2012; Seton et al. 2012; Westerweel et al. 2019). It is likely that the ancestors of Agapophytinae, or of the broader clade comprising Agapophytinae + Therevinae, originated in Gondwana prior to the rifting of the West Burma Block. Given the disjunct distributions of these therevid flies, this family may have experienced a more complex evolutionary history, as has been suggested for many other insect lineages (Jiang et al., 2021; Ma et al., 2022; Feng et al., 2025). Considering the scarcity of Mesozoic fossil records for stiletto flies, their evolutionary history is far from being understood. While †Paleothereva gen. nov. offers direct evidence and valuable new data for testing earlier hypotheses of therevid evolution and Cretaceous paleobiogeography, additional wellpreserved Mesozoic fossils and further integration of multiple lines of evidence will be required to fully clarify the early evolutionary history of Therevidae.

5.2. Oviposition behaviors of †Paleothereva gen. nov. and its implications

†Paleothereva gen. nov., with exceptionally preserved female terminalia, provide direct morphological evidence for reconstructing its oviposition behavior (Fig. 5). Its well-developed acanthophorite spines on tergite 10 closely resemble with their extant relatives, indicating that the new genus likely employed a similar oviposition strategy. This finding demonstrates that the acanthophorite spine-assisted substrate-anchoring oviposition mechanism characteristic in Therevidae was already established by the mid-Cretaceous.

Female terminalia within Therevidae exhibit notable structural diversity, which can be classified into three main types. Results of ancestral character state reconstruction indicated that their occurrence across subfamilies, under the phylogenetic framework of Therevidae, is outlined in Fig. 6C. Type I (Fig. 6C) representing as a plesiomorphic state within therevid flies, found in the subfamily Phycinae, is associated with an oviposition behavior in which females excavate pits using their hind legs, deposit eggs at the bottom, and then bury them through abdominal movements—a process that requires no anchoring device (Irwin 1976). This oviposition strategy resembles that of vermileonid flies (Hemmingsen and Nielsen 1971) and is regarded as the plesiomorphic condition within lower Brachycera. Type II (Fig. 6C), present in Xestomyzinae, involves modified macrosetae on abdominal sternite 8 that function both as the primary digging apparatus and as anchors during oviposition (Hauser 2012). Type III (Fig. 6C) is remarkably characterized by well-developed acanthophorite spines and represents the most common female terminalia within Therevidae, occurring in †Paleothereva gen. nov. as well as in Agapophytinae and Therevinae. In these subfamilies, females perform successive abdominal contortions to insert and stabilize the abdomen in the substrate, with the acanthophorite spines providing crucial anchoring support during oviposition. The structural diversity of female terminalia in Therevidae likely play an important role in adapting to different environments.

Extant therevids possessing well-developed acanthophorite spines (Type III) are most diverse in arid or semiarid regions with sandy soils (Winterton et al. 2001; Gaimari and Webb 2009; Mortelmans and Bree 2022; Marchiori 2023). The presence of similar structures in †Paleothereva gen. nov. suggests it inhabited a comparable environment. Notably, substrate-piercing oviposition mechanisms—mediated by acanthophorite spines or analogous terminal structures—occur in multiple lineages of Asiloidea preserved in mid-Cretaceous Kachin amber (Grimaldi et al. 2011; Dikow and Grimaldi 2014; Grimaldi 2016; Zhang et al. 2018; Ye et al. 2019; Ngô-Muller et al. 2020). It is implied that the West Burma Block possibly had a wide range of arid or semiarid regions with sandy soils, an environment that would have promoted the diversification of lineages employing this convergent oviposition strategy.

6. Conclusions

Paleothereva longicoxa gen. et sp. nov. is the first derived therevid known from the Mesozoic, extending the fossil record of crown-group Therevidae into the Cretaceous. Phylogenetic analysis and morphological comparisons support its affinity with the Taenogera genus-group and ancestral character state reconstruction of female terminalia indicates its behavioral and ecological traits comparable to those of extant Agapophytinae and Therevinae. This discovery bridges the gap between molecular divergence estimates and fossil evidence, providing a mid-Cretaceous minimum age for a derived therevid lineage, and refined insight into early therevid diversification.

7. Declarations

Authors’contributions. QF performed data curation, formal analysis, methodology, software, and writing of the original draft; XJ performed data curation and formal analysis and contributed to writing the original draft; CZ contributed to formal analysis and resources; DR and YW contributed to conceptualization, funding acquisition, supervision, and writing—review and editing.

Conflict of Interest Statement. The authors declare that there is no conflict of interest regarding the publication of this manuscript or participation in this project.

8. Acknowledgements

We sincerely thank the subject editor and the reviewers—Dr. Shaun L. Winterton and one anonymous reviewer—for their valuable comments and suggestions, which substantially improved the manuscript. This work was supported by the National Natural Science Foundation of China (grant 32370481 and 42472001); GDAS Special Project of Science and Technology Development (2022GDASZH–2022010106); Pearl River Talent Plan of Guangdong Province (2021QN02N101).

9. References

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Cockerell TDA (1920) A therevid fly in Burmese amber. The Entomologist 53: 169–170.

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Supplementary material

Supplementary material 1

Files S1–S4

Feng Q, Jia X, Zhou C, Ren D, Wang Y (2026)

Data type: .zip

Explanation notes: File S1. List of characters for phylogenetic analysis [.docx file]. — File S2. Morphological matrix for phylogenetic analysis [.xlsx file]. — File S3. State of female_terminalia_for character state reconstrcution [.xlsx file]. — File S4. Rcode for ancestral character state reconstruction [.txt file].

This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited.

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