SHORT NOTE
A pepsine forewing fossil (Hymenoptera, Pompilidae) from the Late Miocene Yamaguchi Formation, Yamagata Prefecture, Japan
2026 Volume 30 Pages 205-208
Details
2026 Volume 30 Pages 205-208
Graphical Abstract
Spider wasps (Hymenoptera, Pompilidae) are brood parasitic, parasitoid, or hunting wasps that prepare spiders for their offspring, usually a single spider for their single egg (Iwata, 1976; Shimizu et al., 2012). This group is one of the largest families among the aculeate wasps and bees and comprises approximately 5,000 extant species (Pitts et al., 2006; Aguiar et al., 2013) and 130 genera (Aguiar et al., 2013) belonging to one of the five subfamilies Ctenocerinae Arnold, 1934; Ceropalinae Radoszkowski, 1888; Notocyphinae Fox, 1894; Pompilinae Latreille, 1804; or Pepsinae Lepeletier, 1845 (Waichert et al., 2015). They are distributed in all zoogeographic regions but have greatly diversified in the tropical regions of the world (Pitts et al., 2006).
In Japan, 142 species and 37 genera belonging to three subfamilies (Ceropalinae, Pepsinae and Pompilinae) are recorded (Shimizu, 2020). Among these subfamilies, Pepsinae and Pompilinae are particularly diverse, representing most species of the Japanese Pompilidae. However, the pompilid fossil fauna of Japan is poorly understood due to lack of fossil examples. So far, only one sample identified as Pompilidae gen et sp. indet. has been reported from the Middle to Late Pliocene or Early Pleistocene Togo Formation in Kagoshima Prefecture (Fujiyama and Iwao, 1975). In this study, we describe a new fossil specimen of Pepsinae (Pompilidae) from the Late Miocene lacustrine deposit distributed in Tendo, Yamagata.
The Yamaguchi Formation is a lacustrine deposit mainly composed of tuffaceous fine-grained sediments and pumiceous tuff (Nagasawa and Yamanoi, 1990). The late Miocene age of the Yamaguchi Formation was constrained by fission track dating of andesitic lapilli tuff (6.8 ± 0.9 Ma) from the underlying Uwanodai Formation (Amano, 1980). The specimen described herein was obtained from a quarry in Barazaki, Tendo, Yamagata, Japan (Supplementary figure 1), which corresponds to the middle to upper part of the Yamaguchi Formation. The fossil-bearing horizon is lithologically distinct, consisting of alternating unstratified sandy tuff and thinly laminated tuffaceous siltstone. The stratigraphic horizon from which the new fossil was derived is unclear because it was collected as a float fragment. Nagasawa and Oba (1998) listed 22 genera belonging to 20 families of plant fossils and Hayashi and Nagasawa (2000) reported a termite fossil (Hodotermopsis sp. aff. H. iwatensis). These fossils and studied material are housed at the Yamagata Prefectural Museum.
The fossil was observed under a Leica M205 C microscope (Leica Corporation, Wetzlar, Germany). Images were captured using a Leica MC170HD microscope (Leica Application Suite Version 4.1.3., Leica Corporation). The images were edited and their contrast and tonality were adjusted using Adobe Photoshop TM Version CS6 (Adobe Systems Incorporated, San Jose, CA, USA). Measurements of wing parts were taken at their maximally valued portions unless otherwise noted.
The nomenclature of the veins and cells follows Day (1988), with abbreviations for the wing veins (italicized in the main text and figures) and cells as follows: A, anal vein; C, costa; CuA, cubitus anterior; M, median; m-cu, crossvein between veins M and Cu; R, radial; Rs, radial sector; Sc, subcosta; rs-m, crossvein between veins Rs and M; and SMC, submarginal cell. The numbers before and after the abbreviations for veins and cells indicate their respective sequence numbers.
Order Hymenoptera Linnaeus, 1758
Family Pompilidae Latreille, 1804
Subfamily Pepsinae Lepeletier, 1845
Pepsinae gen. et sp. indet.
Specimen.—1Af007118 (Figure 1A), collected by T. Yamanoi in 1988 and reposited at Yamagata Prefectural Museum, Yamagata, Yamagata, Japan. Nearly complete forewing without antero-outer part. Counterpart missing. Sex unknown.
Occurrence.—Yamaguchi Formation (Late Miocene) in Barazaki, Tendo, Yamagata, Japan.
Description.—Forewing 4.8 mm long, entirely infuscate. Pterostigma and marginal cell not preserved. Four submarginal cells present. SMC1 0.8 mm long (as preserved), 0.3 mm wide, much longer than SMC2 and 3. SMC2 rhomboid, 0.5 mm long, 0.3 mm wide. SMC3 0.5 mm long, 0.3 mm wide, almost as long as SMC2 on vein M, narrowed on vein Rs. SMC4 largest among submarginal cells, 0.9 mm long on vein M. Second abscissa of vein Rs (upper part of basal vein) straight. Crossvein 2rs-m gently curved outward, oblique to vein M, meeting discal cell 2 at one half of its upper side. Crossvein 3rs-m curved outward, not parallel to crossvein 2rs-m, meeting discal cell 3 at one-third of its upper side. Basal cell large, triangular, 1.3 mm long, 0.6 mm wide. Second abscissa of vein M (lower part of basal vein) strongly curved inward below. Vein M attaining outer wing margin. Discal cell 1 1.2 mm long, 0.4 mm wide. Crossvein 1m-cu almost straight, meeting SMC2 at half-length of its lower side. Discal cell 2 1.0 mm long, 0.6 mm wide. Crossvein 2m-cu strongly curved outward at its middle, meeting SMC3 at one-third of its lower side, also meeting subdiscal cell 2 at half-length of its upper side. Discal cell 3 1.1 mm long, 0.6 mm wide. Subbasal cell elongated, 1.7 mm long, 0.2 mm wide. Vein M+CuA parallel to vein A. Crossvein cu-a almost interstitial to 2nd abscissa of vein M, slightly oblique to vein A. Subdiscal cell 1 0.8 mm long on vein A, 0.4 mm wide. Vein CuA branching into CuA1 and CuA2 at two-third of outer side of subdiscal cell 1. Vein CuA1 attaining outer wing margin, not deflected downward at its base. Clavus elongated, ca. 2.8 mm long, 0.2 mm wide, its lower margin being gently undulated.
Remarks.—1Af007118 can be referred to Pompilidae based on the following features: (1) 10 closed cells (Day, 1988); (2) four submarginal cells; (3) three discoidal cells; and (4) discoidal cell 1 not particularly long as in Vespidae (Borror and White, 1970). Additionally, features (5) vein CuA1 not distinctly deflected posteriorly at its base and (6) vein M reaching outer wing margin indicate that the species is a member of Ceropalinae or Pepsinae. The fossil forewing, however, differs from those of Ceropalinae (Ceropales Latreille, 1796 and Irenangelus Schulz, 1906) in having the following combination of characteristics: (7) crossvein 2rs-m curved outward and oblique to vein M; (8) crossvein 3rs-m not very oblique to vein M; (9) SMC3 not much larger than SMC2; and (10) the second abscissa of vein M strongly curved below. Hence, the fossil is derived from a Pepsinae wasp.
Pepsine taxa having feature (6) are relatively uncommon and include Sphictostethus Kohl, 1884 and some Calopompilus Ashmead, 1900 (Americas); Trichocurugus Haupt, 1937 (New Zealand); Apinaspis Banks, 1938 (Southeast Asia); Masisia Arnold, 1934 (Africa); Caliadurgus Pate, 1946, Priocnemis Schiødte, 1837, and Dipogon Fox, 1897 (worldwide); Minagenia Banks, 1934 (pantropical); and Eopompilus Gussakovskij, 1933 (eastern Asia). Among these genera, Dipogon shares the combination of features (1)–(10) with 1Af007118, as well as feature (11): crossvein cu-a originating slightly posterior to the fork of vein M+CuA. In the genus Dipogon, some species of the subgenus Nipponodipogon Ishikawa, 1965, [D. (N.) mandibularis Ishikawa, 1965; D. (N.) nagasei Ishikawa, 1965; D. (N.) orientalis Loktionov et al., 2017; D. (N.) rossicus Lelej, 1986; D. (N.) shimizui Loktionov et al., 2017; D. (N.) sudai (Shimizu, 2015)] have crossvein cu-a almost interstitial to the second abscissa of vein M like 1Af007118 [see Shimizu et al., 2015; the taxonomic rank of Nipponodipogon follows Shimizu (2020)]. Seven known species of this subgenus are smaller than most members of the other subgenera of Dipogon (forewing length is 3.6–6.6 in females and 3.1–5.3 in males: Shimizu et al., 2015), corresponding with the length of the fossil forewing (ca. 4.9 mm long; Figure 1). Thus, 1Af007118 is most likely referable to Nipponodipogon.
Although all known species of Nipponodipogon have bifasciate fore wings (Ishikawa, 1965; Shimizu et al., 2015; Loktionov et al., 2017), 1Af007118 appears to lack such fasciae. In contrast, Rodriguez et al. (2017) and Xu et al. (2025) recorded fossil pompilids in which the fore wing bands were clearly preserved. From these facts, there is a possibility that 1Af007118 originally lacked wing fasciae. Thus, we refrain from identifying the fossil at the (sub)genus level and instead refer it to the subfamily Pepsinae. This represents the first fossil record of a pepsine wasp from Japan.
We are grateful to the Keio Gijuku Fukuzawa Memorial Fund for the Advancement of Education and Research for financially supporting the study. We also express our gratitude to H. Aiba (Institute for Educational Practice Studies) and R. Kukihara and T. Suguro (Keio Yochisha Elementary School) for supporting our study. Lastly, thanks are extended to two anonymous reviewers for their constructive comments.
Supplementary figure 1. Locality of the present fossil (1Af007118). A, map of northeastern Honshu, Japan; B, location of Tendo City, Yamagata Prefecture; C, enlarged map of fossil locality in Barazaki, Tendo City.
YT initiated the study and mainly wrote the manuscript and made figures. AS revised the manuscript and taxonomic aspects, and HS summarized the geology of the fossil locality. All authors contributed to the writing of the paper.
