The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepidion gertschi (Insecta, Zygentoma)

Matushkina, N.

doi:10.15407/zoo2026.04.383

The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepidion gertschi (Insecta, Zygentoma)

Tricholepidion gertschi (Lepidotrichidae) has an unresolved phylogenetic position, being placed as sister group to Dicondylia (Zygentoma + Pterygota) or within Zygentoma, making it the only insect species whose ordinal status remains uncertain. In this work, the sclerite composition of the femal...

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Datum:	2026
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Автори та афіліації:	N. Matushkina — Department of Ecology and Zoology, Institute of Biology and Medicine, Taras Shevchenko National University of Kyiv, Ukraine
1. Verfasser:	Matushkina, N.
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author	Matushkina, N.
author_facet	Matushkina, N.
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author_sort	Matushkina, N.
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description	Tricholepidion gertschi (Lepidotrichidae) has an unresolved phylogenetic position, being placed as sister group to Dicondylia (Zygentoma + Pterygota) or within Zygentoma, making it the only insect species whose ordinal status remains uncertain. In this work, the sclerite composition of the female postabdomen of T. gertschi was investigated using scanning electron microscopy. A transverse sclerotized bridge between the gonangula, interpreted as a derivative of sternite 9, was found. Comparative analysis with published data shows that the presence of distinct sternal sclerotisations in segments 8 and 9 of T. gertschi is a plesiomorphic condition that has not been found in Zygentoma so far. This finding confirms the unique morphology of this species. It is suggested that transverse sternal sclerotisations in T. gertschi enhance mechanical stabilisation of the ovipositor during egg laying in hard substrates such as wood.
doi_str_mv	10.15407/zoo2026.04.383
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first_indexed	2026-08-27T01:00:38Z
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fulltext	DOI 10.15407/zoo2026.04.383 UDC 595.713:591.464 THE SCLERITE COMPOSITION OF THE FEMALE GENITALIC REGION IN AN ENIGMATIC SILVERFISH TRICHOLEPIDION GERTSCHI (INSECTA, ZYGENTOMA) N. Matushkina Department of Ecology and Zoology, Institute of Biology and Medicine, Taras Shevchenko National University of Kyiv, vul. Volodymyrska, 64/13, Kyiv, 01033 Ukraine E-mail: natalymatushkina@knu.ua N. Matushkina (https://orcid.org/0000-0002-5426-8016) urn:lsid:zoobank.org:pub:0C875001-EA67-421A-ACA3-A265DEB1BA76 The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepid- ion gertschi (Insecta, Zygentoma). Matushkina, N. — Tricholepidion gertschi (Lepidotrichidae) has an unresolved phylogenetic position, being placed as sister group to Dicondylia (Zygentoma + Pterygota) or within Zygentoma, making it the only insect species whose ordinal status remains uncertain. In this work, the sclerite composition of the female postabdomen of T. gertschi was investigated using scanning electron microscopy. A transverse sclerotized bridge between the go- nangula, interpreted as a derivative of sternite 9, was found. Comparative analysis with published data shows that the presence of distinct sternal sclerotisations in segments 8 and 9 of T. gertschi is a plesiomorphic condition that has not been found in Zygentoma so far. This finding confirms the unique morphology of this species. It is suggested that transverse sternal sclerotisations in T.gertschi enhance mechanical stabilisation of the ovipositor during egg laying in hard substrates such as wood. Ke y words : cuticle, SEM, insect morphology, homology. Introduction Insects exhibit a particularly intricate organisation of the postabdomen were genita- lia are developed (Scudder, 1961; Klass & Ulbricht, 2009; Matushkina & Stetsun, 2025). This region includes the venters of abdominal segments 7 (posterior part only), 8, and 9 (Snodgrass, 1933; Scudder, 1961; Rousset, 1973; Bitsch 1974 a; Klass 2008; Klass & Matushkina, 2012, 2018). The ovipositor which consists of valves is the principal structure of the female external genitalia (Matushkina & Stetsun, 2025). Entomology Zoodiversity, 60(4): 383–390, 2026 © Publisher Publishing House “Akademperiodyka” of the NAS of Ukraine, 2026. The article is published under an open access license CC BY-NC-ND (https://creativecommons.org/licenses/ by-nc-nd/4.0/) ISSN 2707-725X. Zoodiversity. 2026. Vol. 60, No. 4 N. Matushkina ISSN 2707-725X. Zoodiversity. 2026. Vol. 60, No. 4 384 The overall complexity of the region is largely attributable to the sclerites surround- ing the bases of valves and an elaborate musculature. The extant species Tricholepidion gertschi Wygodzinsky, 1961 — regarded as a ‘living fossil’ by Wygodzinsky (1961) and some authors listed below — is the sole surviving representative of the silverfish family Lepidotrichidae Silvestri, 1913, which was previously known only from Eocene fossils. Although originally as- signed to the order Zygentoma in the original taxonomic description, the phyloge- netic position of T. gertschi has long been contentious owing to its distinctive com- bination of plesiomorphic and apomorphic morphological features (Engel, 2006). Competing hypotheses recognize T. gertschi as the sister group of the Dicondylia (i. e., Zygentoma + Pterygota) (e. g., Bitsch & Bitsch, 2000; Staniczek, 2000), as the basalmost lineage within Zygentoma (Kristensen, 1981; Koch, 2003; Engel, 2006), or as closely related to certain zygentoman subgroups (e. g. Nicoletiidae by Smith, 1998; Nicoletiidae + Ateluridae by Dallai et al., 2004, see discussion therein). Mo- lecular data have yet to resolve the placement of T. gertschi unambiguously (Giri- bet et al., 2004; see also Nardi et al., 2003), leaving the relationships of T. gertschi — and by extension the entire family Lepidotrichidae — among basal hexapods one of the most debated issues in hexapod phylogeny. Moreover, Tricholepidion was considered as a “the most promising candidate for a new insect ‘order’ (or, in other words, the only insect species whose ordinal classification remains uncertain)” (Zrzavý, 2008: p. 220). In the present study, the sclerite composition of the female genitalic region in T. gertschi was analyzed using scanning electron microscopy (SEM). By comparing these original observations with published morphological data for Archaeognatha and Zygentoma, homologous elements were identified and the structural organisation of the female genitalic region was clarified. This comparative approach aims to refine the understanding of female genitalic evolution at the base of the insect tree of life and to contribute to ongoing discussions about the systematic placement of T. gertschi. Material and Methods A single female postabdomen of Tricholepidion gertschi was provided by Markus Koch (University of Bonn, Germany) with the following label information: Heath and Marjorie Angelo Coast Range Reserve, Northern California, USA (M. Koch leg). The materials were preserved in 100% ethanol. For scanning electron microscopy (SEM), the postabdomen of the specimen was rinsed with water, dissected with fine tweezers and microscissors, and then macerated in 10% KOH solution for 10–12 h at room temperature. The resulting cuticular frag- ments were thoroughly washed in distilled water, dehydrated through a graded ethanol and acetone series, and dried using a critical-point dryer (OM CPD 7501). Dried cuti- cle parts were mounted on the stub probe holder using an adhesive tab, sputter-coated with gold–palladium (OM-SC7640), and examined using a Zeiss EVO 50 SEM (Muse- um of Zoology, Senckenberg Natural History Collections, Dresden, Germany). The nomenclature of abdominal sclerotisations is based on conditions recorded from Archaeognatha (Klass & Matushkina, 2012, 2018; Matushkina & Klass, 2020). Abbreviations: BS (+ number) = basal sclerite of coxal vesicle, topographically ho- The Sclerite Composition of The Female Genitalic Region in an Enigmatic Silverfish ISSN 2707-725X. Zoodiversity. 2026. Vol. 60, No. 4 385 mologous with basal sclerite of the gonapophysis (number = segment); CX (+ num- ber) = coxa/−ite (number = segment); GP (+ number) = sclerotisation of gonapoph- ysis (number = segment); LC9 = laterocoxa/−ite of segment 9 (number = segment); SL (+number) = stylus (number = segment); ST (+number) = sternum/−ite (num- ber = segment); TG (+number) = tergum/−ite (number = segment); vs (+ number) = coxal vesicle (number = segment). Results The female postabdomen of T. gertschi is cylindrical, with a laterally compressed true ovipositor consisting of the ventral valves of the 8th abdominal segment (gonapo- physes 8) and the dorsal valves of the 9th segment (gonapophyses 9) (Fig. 1). All gonapophyses remain unfused at their bases (Fig. 2, a, b). The basal part of gonapo- physis 8 (= basal sclerite) is separated from the remainder of the gonapophysis by an annulation and differs in its microsculpture, which more closely resembles the setose microsculpture of the adjacent coxite 8 than the essentially smooth surface of the distal gonapophysis 8 (Fig. 2, c). The basal region of the gonapophysis 9 is morpho- logically indistinct, showing no peculiarities in microsculpture. Coxites 8 are separated medially. The approximately triangular laterocoxites 8, which adjoin the anterolateral margins of coxites 8, are connected medially via a narrow transverse sclerite (= sternite 8) that is clearly visible both externally and internally (Fig. 2, a). Coxites 9 form the third valves of the ovipositor, cover- ing the base of the true ovipositor laterally. Anterolateral to coxites 9 lie the lon- Fig. 1. Female postabdomen of Tricholepidion gertschi (Zygentoma), semi-diagrammatic lateral view with caudal appendages partly removed. Abbreviations of abdominal structures are given in Materials and Methods GP9 TG10 TG9 TG8 TG7 CX7CX8CX9 GP8 SL7SL8 SL9 vs7 N. Matushkina ISSN 2707-725X. Zoodiversity. 2026. Vol. 60, No. 4 386 gitudinally elongated sclerites — the gonangula (laterocoxites 9). Each gonangu- lum articulates with tergite 9 via its anterior lateral margin, with the gonapoph- ysis 8 via its ventral medial margin, and with coxite 9 via its posterior margin. A rod-like sclerotized transverse bridge lies between the medial surfaces of the two gonangula, deepening in the space between gonapophyses 8 and gonapophyses 9 (Fig. 2, a, d). This bridge is completely hidden under gonapophyses, and thus not visible externally. In cross section the bridge is semicircular and composed predominantly of distinctly layered cuticle with clear fiber network (Figs 2, e, f). According to the principles of topographic homology, this transverse bridge is interpreted as a derivative of sternite 9. Fig. 2. Microstructure of the basal part of the ovipositor of Tricholepidion gertschi from the inside (a, b, d–f) and outside (c), SEM: a — base of the ovipositor; b — base of gonapophyses 8 and 9; c — base of gonapophysis 8 and its basal sclerite; d–f — transverse sclerotized bridge between gonangula at different magnifications, note layered cuticle consisting of fiber network. Abbreviations of abdominal structures are given in Materials and Methods The Sclerite Composition of The Female Genitalic Region in an Enigmatic Silverfish ISSN 2707-725X. Zoodiversity. 2026. Vol. 60, No. 4 387 Discussion Wingless insects (Archaeognatha and Zygentoma) share a plesiomorphic morphol- ogy of the generalized abdominal segment. In Archaeognatha, a typical pregenital segment bears a tergite, as an entire U-shaped sclerite, dominating the dorsal sur- face, and a set of ventral sclerotisations of varying size, including one or two median sternal sclerotisations, paired coxites (with styli and, in anterior segments, eversible vesicles), and paired laterocoxites (sclerites anterolateral to coxites) (Fig. 3, a) (Sturm & Machida, 2001; Klass & Matushkina, 2012). In the female postabdomen, where the ovipositor develops, the eversible vesicles on the coxites are replaced by gonapophy- ses (ovipositor valves), the coxites become more widely spaced (coxite 9 is also en- larged), and the sternal sclerotisations either fuse with the adjacent coxites (segment 8) or are weakly sclerotized (segment 9) (Klass & Matushkina, 2012). The female postabdomen of Zygentoma conforms to this general pattern but exhibit a more consolidated ventral region and a flatter tergite. The sclerite composi- tion of the female postabdomen in Zygentoma has been originally described in de- tail for two species: Thermobia domestica (Packard, 1837) (Lepismatidae) and Nico- letia sp. (Nicoletiidae) (Rousset, 1973). In T. domestica, the seventh abdominal seg- ment bears a single ventral sclerotisation encompassing both the sternal and coxal regions, referred to as the coxosternite. The ventrum of the eighth segment displays Fig. 3. Sclerite composition of female postabdomen of Tricholepidion gertschi (Zygentoma) (b–d) in comparison with typical pregenital ventrum of Archae- ognatha (a), schematic reconstructions: a — generalized pregenital abdominal ventrum of Archaeognatha (based on data from Klass & Matushkina, 2012), b–d — sclerite composition of 7th, 8th, and 9th abdominal ventrums of T. gerts- chi, respectively. Abbreviations of ab- dominal structures are given in Mate- rials and Methods a c b d N. Matushkina ISSN 2707-725X. Zoodiversity. 2026. Vol. 60, No. 4 388 two separate bilateral sclerotisations — the coxites (each bearing a stylus and a coxal vesicle transformed into a gonapophysis) — whereas the ninth segment, in addition to the coxites, gonapophyses, and styli, also bears an elongated laterocoxite 9, the gonangulum. This sclerite articulates anteroventrally with gonapophysis 8, antero- dorsally with tergite 9, and posteriorly with coxite 9. Distinct sternal sclerotisations are absent in the eighth and ninth segments. By contrast, the composition of female postabdominal sclerites in Nicoletia more closely resembles that of Archaeognatha. The ventrum of the seventh seg- ment bears a trapezoidal median sternal sclerotisation (the (eu)sternite after Klass & Matushkina, 2018), surrounded by paired lateral regions (the coxites) each con- taining an eversible vesicle and a stylus. Anteriorly, there is a transverse presternal sclerotisation (the intersternite after Klass & Matushkina, 2018) and, on each side, a small laterocoxal sclerite. The ventrum of the eighth segment in Nicoletia does not differ in composition from that in Thermobia. In the ninth segment, sternal sclerotisations are absent, and the very large gonangulum partially substitutes the coxal sclerotisation, being separated from it by a membrane rather than a joint (this condition was erroneously interpreted by Rousset as the gonangulum is “not separated from the coxite”). Compared with these taxa, the composition of female postabdominal sclerites in Tricholepidion is unique within Zygentoma studied in this respect, as it retains distinct sternal sclerotisations connecting the laterocoxites in segments 8 and 9 (Figs 3, c–d). The presence of sternal sclerotisation(s) in segment 8, char- acteristic of Archaeognatha, can — given the well-established sister-group rela- tionship between Archaeognatha and Dicondylia (e. g., Misof et al., 2014) — be interpreted as a plesiomorphic condition. However, sternal sclerotisation in seg- ment 9 represents a feature not reported in any other representative of wingless insects. This transverse sclerotisation, forming a seemingly rigid cuticular bridge, may restrict the movements of the separated right and left gonapophyses 9 rela- tive to each other. Notably, in the studied representatives of Archaeognatha (Klass & Matushkina, 2018: Petrobiellus takunagae), as well as in Thermobia and Nicoletia (Rousset, 1973), gonapophyses 9 show varying degrees of fusion, whereas gonapophyses of Tricholepidion are free at their bases (Matushkina, 2011). Thus, Tricholepidion displays a unique adaptation for mechanical stabili- sation of the bases of gonapophyses 9. In terms of serial homology, presence of sternal sclerotisation of the 9th abdominal segment considered a plesiomorphic state as it occurs in unspecialized pregenital segments of Archaeognatha and pregenital and 8th abdominal segments of some Zygentoma. In a functional context, this peculiar sclerite composition of the female genital segments in Tricholepidion may relate to oviposition in relatively hard substrates such as wood, as suggested by the presence of a blade-like, pointed, denticulated ovipositor (Wygodzinsky, 1961; Matushkina, 2011; Matushkina & Stetsun, 2025). The present observations confirm considerable variation in consolidation of ventral sclerites within Zygentoma, with Tricholepidion retaining several plesiomorphic fea- tures and thus representing a unique condition among wingless insects. Although this pattern appears consistent with a shift from a multi-elemental to a more inte- grated configuration, the actual direction of sclerite reorganisation within the order The Sclerite Composition of The Female Genitalic Region in an Enigmatic Silverfish ISSN 2707-725X. Zoodiversity. 2026. Vol. 60, No. 4 389 remains uncertain, and similar reorganisations may have occurred independently in multiple lineages. Such modifications may have functional implications for oviposi- tor functioning. Further comparative studies across wingless insect lineages are therefore needed to clarify the tendencies of evolutionary changes of the sclerite composition of the female postabdomen. Acknowledgments. The material examined in this study was kindly provided by Markus Koch. The author sincerely thanks Klaus-Dieter Klass for his general sup- port and valuable comments on the hexapod phylogeny. 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spelling	oai:ojs.akademperiodyka.org.ua:article-8752026-08-26T09:51:17Z The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepidion gertschi (Insecta, Zygentoma) Matushkina, N. cuticle SEM insect morphology homology Tricholepidion gertschi (Lepidotrichidae) has an unresolved phylogenetic position, being placed as sister group to Dicondylia (Zygentoma + Pterygota) or within Zygentoma, making it the only insect species whose ordinal status remains uncertain. In this work, the sclerite composition of the female postabdomen of T. gertschi was investigated using scanning electron microscopy. A transverse sclerotized bridge between the gonangula, interpreted as a derivative of sternite 9, was found. Comparative analysis with published data shows that the presence of distinct sternal sclerotisations in segments 8 and 9 of T. gertschi is a plesiomorphic condition that has not been found in Zygentoma so far. This finding confirms the unique morphology of this species. It is suggested that transverse sternal sclerotisations in T. gertschi enhance mechanical stabilisation of the ovipositor during egg laying in hard substrates such as wood. Publishing House "Akademperiodyka" of the National Academy of Sciences of Ukraine 2026-06-23 Article Article application/pdf https://ojs.akademperiodyka.org.ua/index.php/Zoodiversity/article/view/875 10.15407/zoo2026.04.383 Zoodiversity; Vol. 60 No. 4 (2026): Zoodiversity Zoodiversity (Vestnik Zoologii); Том 60 № 4 (2026): Zoodiversity 2707-7268 2707-725X 10.15407/zoo2026.04 en https://ojs.akademperiodyka.org.ua/index.php/Zoodiversity/article/view/875/408 Copyright (c) 2026 Natalia Matushkina
spellingShingle	Matushkina, N. The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepidion gertschi (Insecta, Zygentoma)
title	The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepidion gertschi (Insecta, Zygentoma)
title_full	The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepidion gertschi (Insecta, Zygentoma)
title_fullStr	The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepidion gertschi (Insecta, Zygentoma)
title_full_unstemmed	The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepidion gertschi (Insecta, Zygentoma)
title_short	The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepidion gertschi (Insecta, Zygentoma)
title_sort	sclerite composition of the female genitalic region in an enigmatic silverfish tricholepidion gertschi (insecta, zygentoma)
topic_facet	cuticle SEM insect morphology homology
url	https://ojs.akademperiodyka.org.ua/index.php/Zoodiversity/article/view/875
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The sclerite composition of the female genitalic region in an enigmatic silverfish Tricholepidion gertschi (Insecta, Zygentoma)

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