Three new stoneflies (Insecta: Plecoptera) from Eocene Baltic amber

107 European Journal of Taxonomy 1026: 107–122 https://doi.org/10.5852/ejt.2025.1026.3107 europeanjournaloftaxonomy.eu ISSN 2118-9773 2025 · Chen Z.-T. This work is licensed under a Creative Commons Attribution License (CC BY 4.0) Received: 28 February • Accepted: 2 September 2025 • Published: 13 November 2025 Topic editor: Marie-Béatrice Forel • Desk editor: Eva-Maria Levermann Research article urn:lsid:zoobank.org:pub:0FD44709-7300-4C48-A22A-110672041F11 Three new stoneflies (Insecta: Plecoptera) from Eocene Baltic amber Zhi-Teng CHEN  School of Grain Science and Technology, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu 212004, China. Email: [email protected] Abstract. ThreenewstoneflyspeciesaredescribedandillustratedfromEoceneBalticamberfoundin Lithuania: †Palaeopsole spinosa sp. nov. (family Leuctridae Klapálek, 1905), †Podmosta biloba sp. nov. (family Nemouridae Newman, 1853), and †Isoperla lituana sp. nov. (family Perlodidae Klapálek, 1909). The generic diagnosis of †Palaeopsole Caruso & Wichard, 2011 is also emended. These new fossils enhance the understanding of the extinct Plecoptera fauna in Baltic amber. Keywords. Leuctridae, Nemouridae, Perlodidae, new species. ChenZ.-T.2025.Threenewstoneflies(Insecta:Plecoptera)fromEoceneBalticamber.European Journal of Taxonomy 1026: 107–122. https://doi.org/10.5852/ejt.2025.1026.3107 Introduction Taxonomic studies of the Plecoptera Burmeister, 1839 (stoneflies)preservedinEoceneBalticamber began early in the history of paleoentomology but have often been limited by incomplete descriptions andthelossofholotypespecimens.Pictet&Hagen(1856)firstdescribed13speciesfromsixextant genera, including Taeniopteryx Pictet, 1841 (Taeniopterygidae Klapálek, 1905), Leuctra Stephens, 1836 (Leuctridae Klapálek, 1905), Nemoura Latreille, 1796 (Nemouridae Newman, 1853), Perla Geoffroy, 1762 (Perlidae Burmeister, 1839), Isoperla Banks, 1906 (Perlodidae Klapálek, 1909), and Perlodes Banks, 1903 (Perlodidae). However, the holotype specimens of 10 of these species have been lost, and these species are considered incertae sedis (Caruso & Wichard 2010; Jouault et al. 2021). Similarly, Ricker (1936) described †Megaleuctra neavei Ricker, 1936, whose holotype is also missing (Caruso & Wichard 2010). Furthermore, the original descriptions of these early species were rudimentary, lacking critical details such as genitalia morphology, which is essential in modern taxonomic work. Fossil Plecoptera from Baltic amber were reviewed by Caruso & Wichard (2010), who described three new species: †Zealeuctra cornuta Caruso & Wichard, 2010 (Leuctridae), †Lednia zilli Caruso & Wichard, 2010 (Nemouridae), and †Podmosta attenuata Caruso & Wichard, 2010 (Nemouridae). Subsequently, Caruso & Wichard (2011) introduced †Palaeopsole weiterschani Caruso & Wichard, 2011 (Leuctridae) and discussed the paleogeographic distribution of Leuctridae and Nemouridae. European Journal of Taxonomy 1026: 107–122 (2025) 108 More recently, Chen (2018a, 2018b, 2018c, 2018d) expanded the fossil record by describing a female of Podmosta Ricker, 1952 with a distinctive “rabbit-shaped” sclerite on sternum 8, and by establishing new leuctrid genera (†Baltileuctra Chen, 2018 and †EuroleuctraChen,2018)andthefirstBrachypterainae fossil (†BalticopteryxChen,2018,Taeniopterygidae).Additionalcontributionsincludethefirstfossil record of Brachyptera Newport, 1848 (Chen 2022a), †Balticonemoura Chen, 2022 (Nemouridae; Chen 2022b), and further studies on †Baltileuctra and its parasitic mites (Chen & Liu 2022). However,thediversityofstoneflyspeciesinBalticamberisstillunderestimated,andfurtherdiscoveries are anticipated. In this study, three species from the Eocene Baltic amber are described, namely †Palaeopsole spinosa sp. nov. (Leuctridae), †Podmosta biloba sp. nov. (Nemouridae), and †Isoperla lituana sp. nov. (Perlodidae), contributing to a better understanding of the diversity of Plecoptera and evolutionary patterns in this region. Material and methods The amber specimens analyzed in this study were legally acquired from MB Amber Inclusions, Vilnius, Lithuania. Baltic amber is generally considered to date from the Eocene (Bartonian to Priabonian, ca 34– 38 million years ago) (Kosmowska-Ceranowicz et al. 1997; Jouault et al. 2021). The amber pieces were polished using sandpaper and grinding paste to enhance visibility for observation. All measurements and observations were conducted using a SDPTOP SZM45 stereo microscope. Photographs were taken using a Canon EOS 5DSR digital camera with a Canon MP-E 65 mm 5 ×macrolens,andwidefieldfluorescence imageswerecapturedwithanOlympusCX31lightmicroscopeequippedwithafluorescenceimaging system.ImageprocessingandfigureassemblywereperformedusingAdobePhotoshopCS6.Thetype specimens are deposited in the Insect Collection of Jiangsu University of Science and Technology, Jiangsu Province, China (ICJUST). The examination of extant species of Podmosta Ricker, 1952 were conducted in the C.P. Gillette Museum of Arthropod Diversity, Colorado State University, Fort Collins, Colorado, USA (CSUC). Terminology of wing venation follows that of Béthoux (2005). Abbreviations for morphological terms AA1 = firstanterioranalis AA2 = second anterior analis an = antenna ce = cercus CuA = anterior cubitus CuP = posterior cubitus ep = epiproct LFW = left forewing LHW = left hind wing M = media mp = maxillary palp pp = paraproct RA = anterior radius ra-rp = crossvein between RA and RP RFW = right forewing RHW = right hind wing RP = posterior radius ScP = posterior subcosta st = sternum t = tergum ve = vesicle CHEN Z.-T., New species of Eocene stoneflies (Plecoptera) 109 Results Taxonomy Class Insecta Linnaeus, 1758 Order Plecoptera Burmeister, 1839 Family Leuctridae Klapálek, 1905 Subfamily Leuctrinae Klapálek, 1905 Genus †Palaeopsole Caruso & Wichard, 2011 Type species †Palaeopsole weiterschani Caruso & Wichard, 2011. Emended diagnosis Tergum 9 with one posteromedial spine; sternum 9 extended caudally forming rounded subgenital plate which undergirds sternum 10, basally with vesicle; lateral projections of tergum 10 ending caudad in a more or less concave margin; subanal lobes variable in shape and length; cerci short and unsegmented, apexvariablymodified;epiproctupturnedandhook-shaped,basalcushionvariablymodified. †Palaeopsole spinosa sp. nov. urn:lsid:zoobank.org:act:18C09F91-CCDA-44F6-9B37-94B2143CE78D Figs 1–3 Diagnosis Epiproct slender (wider in P. weiterschani); paraprocts sharp (rounded in P. weiterschani); cerci without humps (with humps in P. weiterschani); lateral projections of tergum 10 obtuse (pointed in P. weiterschani); ra-rp reaching RP after fork point (at fork point in P. weiterschani). Etymology Thespecificepithetreferstothespine-shapedepiproctoftheholotype. Type material Holotype (Fig. 1A–B) LITHUANIA•♂;Lithuanianamber,Eocene,BartoniantoPriabonian,ca34–38Ma;beachcollected; CZT-PLE-BA6, ICJUST. Description Male Body. Slender (Fig. 1C–D), length (excluding antennae) ca 3.0 mm. Macropterous; generally dark brown. Head. Dark and rounded, much wider than pronotum (Fig. 1C). Triocellate; compound eyes large and protruded.Antennaefiliformanddarkbrown,preservedsegmentsalmostequalwithbodylength,with about 29 segments plus scapus and pedicellus, each segment covered with short bristles. Maxillary palp four-segmented, basal segment shortest, apical three segments subequal in size. Labial palp extremely short. THorax. Pronotum rectangular and dark brown, much longer than its width (Fig. 1C). Mesothorax and metathorax darkly sclerotized, much wider than prothorax. Legs generally brown (Fig. 1C–D); tibia European Journal of Taxonomy 1026: 107–122 (2025) 110 thinner and slightly longer than femur, ventroapically with two giant spurs. Tarsus with three segments; firstandthirdsegmentssubequalinlength,secondsegmentshortest. Wings. Forewings length ca 4.0 mm (Fig. 1C–D). In forewings, ScP reaches RA before ra-rp; RP originated from near base of RA, forked before ra-rp and at near half length of the wing; M forked before the fork of RP; Cu basally forked to CuA and CuP; area between M and CuA with six crossveins in left forewing and seven crossveins in right forewing; area between CuA and CuP with ten crossveins in left forewing and seven in right forewing; AA1 curved, AA2 forked. Hind wings rolled, veins invisible. aBdomen.Short,near⅓ofbodylength(Fig.1C–D).Terga1–8generallypale.Terga9–10sclerotized laterally and membranous medially (Fig. 2A–D). Posterior margin of tergum 9 projected with pale long spine, length of spine near half of tergum 9. Central area of tergum 10 generally pale, without obvious sclerite;lateralpartsoftergum10roundedandunmodified,withoutprojections.Epiprocthook-shaped Fig. 1. †Palaeopsole spinosasp.nov.,♂,holotype(CZT-PLE-BA6).A. Amber piece, dorsal view. B. Amber piece, ventral view. C. Habitus, dorsal view. D. Habitus, ventral view. Arrowheads = tarsal segments. Abbreviations: see Material and methods. Scale bars = 1 mm. CHEN Z.-T., New species of Eocene stoneflies (Plecoptera) 111 Fig. 2. †Palaeopsole spinosasp.nov.,♂,holotype(CZT-PLE-BA6). A. Photo of terminalia, dorsal view. B. Drawing of terminalia, dorsal view. C. Drawing of epiproct, lateral view. D. Lateral part of abdominal tergum 10, lateral view. Abbreviations: see Material and methods. Scale bars = 0.1 mm. Fig. 3. †Palaeopsole spinosasp.nov.,♂,holotype(CZT-PLE-BA6). A. Photo of terminalia, ventral view. B. Drawing of terminalia, ventral view. Abbreviations: see Material and methods. Scale bars = 0.1 mm. European Journal of Taxonomy 1026: 107–122 (2025) 112 with sharp apex, abruptly curved upwards; basal cushion of epiproct broad and wide, laterally with two bilobed plates. Paraprocts (subanal lobes) long, visible apical half semielliptical with sharp apex. Sternum 9 sclerotized, apically with distinct subgenital plate as wide as long and rounded apically, basally with oval vesicle (ventral lobe), which with length 1.5 × as long as its width (Fig. 3A–B). Cerci short and near cylindrical, apical half strongly sclerotized, hemispherical in shape. Remarks The slightly rolled wings, absence of the X-pattern of crossveins at the cord, and the unsegmented cerci collectively assign †P. spinosa sp. nov. to the family Leuctridae. The species’ small and simple epiproctexcludesitsaffiliationwiththesubfamilyMegaleuctrinae(Zwick2000).Itshighmorphological similarity to the extant genus Rhopalopsole Klapálek, 1912 suggests that it belongs to the subfamily Leuctrinae. The presence of a posteromedial spine on abdominal tergum 9, combined with its Eocene origin,confirms†P. spinosa as a member of the extinct genus †Palaeopsole Caruso & Wichard, 2011. Theoriginaldefinitionof†Palaeopsole was based solely on the type species, †P. weiterschani, and several characters of †P. spinosa do not align with the type. Notably, the two long, triangular paraprocts of †P. spinosa resemble those in Leuctra, in which males have two slender median specilla adapted for sperm transfer (Pardo & Zwick 1993; Zwick 2000). The apical half of the cercus in †P. spinosa is strongly sclerotized into a distinct hemisphere, a feature absent in all known Leuctridae, including its only congener, †P. weiterschani. Family Nemouridae Billberg, 1820 Subfamily Nemourinae Billberg, 1820 Genus Podmosta Ricker, 1952 Type species Nemoura decepta (Frison, 1942). †Podmosta biloba sp. nov. urn:lsid:zoobank.org:act:1850D232-C1D5-4A84-B1C3-F9EE6017FD40 Figs 4–7 Diagnosis Vein ra-rp joining anterior branch of RP after fork point in both forewing and hind wings, without typical ‘X-pattern’ at cord. Posterior margin of abdominal sternum 7 membranous, extended backwards. Median sclerite on female abdominal sternum 8 subtriangular basally, apical half bilobed, each lobe slender, extending along lateral margin of sternum 8’s anteromedial indentation. Etymology Thespecificepithetreferstothebilobedmedianscleriteonabdominalsternum8. Type material Holotype LITHUANIA•♀;Lithuanianamber,Eocene,BartoniantoPriabonian,ca34–38Ma;beachcollected; CZT-PLE-BA1, ICJUST. CHEN Z.-T., New species of Eocene stoneflies (Plecoptera) 113 Fig. 4. †Podmosta biloba sp. nov., ♀, holotype (CZT-PLE-BA1). A. Photo of wings in right view. B. Drawing of wings in right view. Inset = joints of ra-rp and RP branches. Abbreviations: see Material and methods. Scale bars = 1 mm. European Journal of Taxonomy 1026: 107–122 (2025) 114 Description Female Wings. In forewing (Fig. 4A–B), terminal costal crossvein c-r joining RA near ra-rp; ScP reaching RA before ra-rp; RP forked; ra-rp joining anterior branch of RP, distal to RP fork point; CuA and CuP unforked. In hind wings (Figs 4A–B, 5A–B), terminal costal crossvein c-r joining RA shortly distal to a-rp; ScP reaching RA before ra-rp; RP forked; ra-rp joining anterior branch of RP, distal to RP fork point;CuAandCuPunforked;AA1simple;AA2withatleastfivebranches. Terminalia. Posterior margin of abdominal sternum 7 membranous, extended backwards (Figs 6A– B, 7A). Sternum 8 deeply indented anteromedially, posterior half folded, notched near posteromedial margin; median sclerite subtriangular basally, apical half bilobed, each lobe slender, extending along Fig. 5. †Podmosta biloba sp. nov., ♀, holotype (CZT-PLE-BA1). A. Photo of wings in left view. B. Drawing of wing in left view. Abbreviations: see Material and methods. Scale bars = 1 mm. CHEN Z.-T., New species of Eocene stoneflies (Plecoptera) 115 lateral margin of sternum 8’s anteromedial indentation. Sternum 9 darkly sclerotized, slightly shorter than sternum 8, posteromedial margin slightly extended. Sternum 10 darkly sclerotized, near half length ofsternum9,posteriormarginunmodified.Paraproctssubtriangular,withroundedcorners.Cerciweakly sclerotized, 2 × as long as wide; apex rounded. Remarks †Podmosta bilobasp.nov.iseasilydistinguishedfromthefiveextantspeciesofPodmosta (Fig. 7) by its uniquely shaped median sclerite on the female abdominal sternum 8 (Frison 1936; Ricker 1952; Baumann 1975; Chen 2018a; Grubbs & Baumann 2023). It differs from †P. attenuata in the structure of the ra-rp vein, which joins the anterior branch of RP after the fork point in both the forewing and hindwing, resulting in the absence of the typical X-pattern in the cord area. In contrast, in †P. attenuata, the ra-rp vein joins RP before the fork point in both wings, forming the characteristic X venation pattern found in extant species of Podmosta (Frison 1936; Caruso & Wichard 2010; Grubbs & Baumann 2023). Fig. 6. †Podmosta biloba sp. nov., ♀, holotype (CZT-PLE-BA1). A. Photo of terminalia, ventrolateral view. B. Drawing of terminalia, ventrolateral view. Abbreviations: see Material and methods. Scale bars = 0.1 mm. 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The other members of the consortium are: Meise Botanic Garden, Meise, Belgium; Natural History Museum of Denmark, Copenhagen, Denmark; Naturalis Biodiversity Center, Leiden, the Netherlands; Museo Nacional de Ciencias Naturales-CSIC, Madrid, Spain; Leibniz Institute for the Analysis of Biodiversity Change, Bonn – Hamburg, Germany; National Museum of the Czech Republic, Prague, Czech Republic; The Steinhardt Museum of Natural History, Tel Aviv, Israël.