Description of the larva of Cybister (Melanectes) brevis Aubé, 1838 (Coleoptera, Dytiscidae, Cybistrinae)

137 Description of the larva of Cybister (Melanectes) brevis Aubé, 1838 (Coleoptera, Dytiscidae, Cybistrinae) Kohei Watanabe1, Masakazu Hayashi2 1 Ishikawa Insect Museum, Hakusan, Ishikawa, 920–2113 Japan 2 Hoshizaki Green Foundation, Izumo, Shimane, 691–0076 Japan Corresponding author: Kohei Watanabe (koutar[email protected]) Copyright: © Kohei Watanabe & Masakazu Hayashi. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract Cybister (Melanectes) brevis Aubé, 1838 is an endangered species in Japan. Here, we present the first description of C. (M.) brevis larvae, emphasizing the chaetotaxy of the cephalic capsule, head appendages, legs, last abdominal segment, and urogomphi. Morphological differences between the subgenera Cybister Curtis, 1827 and Melanectes Brinck, 1945 include (1) the width of notches between medial and lateral projections of frontoclypeus, (2) the shape of egg bursters, and (3) the appearance of the clypeal lamellae. Key words: Chaetotaxy, diving beetle, larval stage, Red List, water beetle Introduction Study of larval morphology of the beetle family Dytiscidae Leach, 1815 has garnered increasing attention since the 1990s whilst emphasis was put on chaetotaxy and porotaxy analyses (Alarie and Michat 2023). Such descriptive approach resulted in remarkable advancements allowing discovery of new character sets that have enhanced comparisons among taxa. Cybistrinae Sharp, 1880 are large to very large-bodied Dytiscidae, comprising 12 genera worldwide (Miller et al. 2024; Nilsson and Hájek 2025). Larval descriptions have been recorded for 9 of these 12 genera—Bifurcitus Brinck, 1945, Cybister Curtis, 1827, Megadytes Sharp, 1882, Metaxydytes Miller, Michat & Ferreira, 2024, Onychohydrus Schaum & White, 1847, Paramegadytes Trémouilles & Bachmann, 1980, Spencerhydrus Sharp, 1882, Sternhydrus, Brinck, 1945, and Trifurcitus Brinck, 1945 (Miller et al. 2024). This paper is meant to be a contribution towards the study of the larval morphology of the genus Cybister, which is comprised of four subgenera: Cybister Curtis, 1827, Megadytoides Brinck, 1945, Melanectes Brinck, 1945, and Neocybister Miller, Bergsten & Whiting, 2007 (Nilsson and Hájek 2025). At the present time, larval descriptions are only available for selected members of the subgenus Cybister (Fiori 1949; Watts 1964; Alarie et al. 2011; Watanabe and Hayashi 2024). More specifically, this contribution aims at formally describing the larvae of a species of the subgenus Melanectes, namely that of Cybister (M.) brevis Aubé, 1838, which is distributed across Japan, China, Taiwan, and Academic editor: Mariano Michat Received: 17 August 2025 Accepted: 13 October 2025 Published: 29 October 2025 ZooBank: https://zoobank. org/8360A184-9D76-45DD-ADAE46C037EBA2D0 Citation: Watanabe K, Hayashi M (2025) Description of the larva of Cybister (Melanectes) brevis Aubé, 1838 (Coleoptera, Dytiscidae, Cybistrinae). ZooKeys 1257: 137–151. https://doi.org/10.3897/ zookeys.1257.168982 ZooKeys 1257: 137–151 (2025) DOI: 10.3897/zookeys.1257.168982 138 ZooKeys 1257: 137–151 (2025), DOI: 10.3897/zookeys.1257.168982 Kohei Watanabe & Masakazu Hayashi: Description of Cybister brevis larvae South Korea (Nakajima et al. 2020; Watanabe and Yoshitomi 2022; Jiang et al. 2023). Whereas previously superficially described (Ichikawa 1984; Kamite 2008; Mitamura et al. 2017; Inoda et al. 2022; Watanabe and Hayashi 2023; Watanabe, 2024), this species would benefit from being described according to the new descriptive format including chaetotaxy and porotaxy analyses (Alarie and Michat 2023) thereof facilitating comparison with larvae of other known Cybistrinae larvae. It is worth noting that Cybister (M.) brevis is listed as “Near Threatened” on the Japanese Red List (Ministry of the Environment of Japan 2015). Materials and methods Seven larvae used for the description were obtained through rearing, following the methods described by Watanabe et al. (2021, 2023). An additional 10 specimens were collected in the field. The larvae were fixed in 70% ethanol and subsequently mounted on slides in 70% ethanol or on HS-slides (Shirayama et al. 1993) with Euparal. The specimens were observed under an optical microscope (Nikon ECLIPSE E400) up to 400× magnification and sketched using a Nikon Y-IDT drawing tube. After scanning the sketches, line drawings were prepared using an iPad Pro 11-inch (4th generation). Photographs of the living larvae were captured using a Nikon D500 digital camera equipped with a Nikon AF-S Micro NIKKOR 60 mm f/2.8G ED lens. Measurements were obtained using a stereomicroscope (Leica M205C, Planapo 1.0×) with a transmitted light base (Leica TL3000 Ergo), a camera (Leica DFC420), and LAS software (v. 3.3.1). Fine structural details of the specimens were observed using a scanning electron microscope (SEM; JEOL JCM-6000 Neoscope Scanning Electron). The larvae were freeze-dried and coated with ultrathin gold layers using high-vacuum evaporation. The examined larvae have been deposited in the larval collections of the Ishikawa Insect Museum (IIM), Ishikawa, Japan, the Kohei Watanabe Collection (KWC), Ishikawa, Japan, and the Hoshizaki Green Foundation (HOWP), Shimane, Japan. The observation methods used in the study followed Watanabe and Hayashi (2024). The measurements and notation of primary setae and pores used in this study follow Alarie et al. (2011), Michat et al. (2015, 2019), and Watanabe and Hayashi (2024). The terms: A and AN, antenna; AB and LAS, abdominal segment VIII (last abdominal segment); CL, longest claw; CO, coxa; COL, coronal line length; FE, femur; FR, frontoclypeus; FRL, frontoclypeus length; HL, head length; HW, maximum head width; L, leg; LA, labium; LP, labial palpi; MN, mandible; MNL, mandible length; MNW, mandible width; MP, maxillary palpi; MX, maxilla; OCW, occipital foramen width; PA, parietal; PPF, maxillary palpifer; PT, pretarsus; TA, tarsus; TI, tibia; TL, total length; TR, trochanter; U and UR, urogomphus. Results Description of the larvae of Cybister (Melanectes) brevis Aubé, 1838 Cybister (Melanectes) brevis Aubé, 1838 Figs 1–33, Table 1 Source of material. Descriptions were based on five instar I and two instar III specimens (HOWP, IIM) reared ex ovo in the laboratory at the Ishikawa Insect Museum from adults collected at the following locality: Japan • Hakusan-shi, 139 ZooKeys 1257: 137–151 (2025), DOI: 10.3897/zookeys.1257.168982 Kohei Watanabe & Masakazu Hayashi: Description of Cybister brevis larvae Figures 1–5. Cybister (Melanectes) brevis Aubé, 1838. 1. Instar I; 2, 3. Instar II; 4, 5. Instar III. 140 ZooKeys 1257: 137–151 (2025), DOI: 10.3897/zookeys.1257.168982 Kohei Watanabe & Masakazu Hayashi: Description of Cybister brevis larvae Ishikawa Prefecture; 21.VI.2023; K. Watanabe leg. Additional specimens were collected in association with adults at the following localities: Japan • three instar I (KWC); Awazu, Misaki-machi, Suzu-shi, Ishikawa Prefecture; 9.VII.2023; K. Watanabe leg.; • one instar II (KWC); Araya-machi, Nomi-shi, Ishikawa Prefecture; 24.VI.2021; K. Watanabe leg.; • one instar I, two instar II (KWC); Yawata-machi, Hakusan-shi, Ishikawa Prefecture; 21.VI.2023; K. Watanabe leg.; • two instar II (KWC); Fushimi, Misaki-machi, Suzu-shi, Ishikawa Prefecture; 25.VI.2023; K. Watanabe leg.; • one instar III (KWC); idem; 9.VII.2023; K. Watanabe leg. Diagnosis. Larvae of C. (M.) brevis can be distinguished from those of other Cybister species by the following combination of characteristics: egg burster spiniform; lamellae clypeales with weak longitudinal unevenness and split into several hair-like projections; lateral projections of frontoclypeus narrow; proximal article of A2 dark brown; distal article of A1 approximately < 1.3 times longer than basal article; primary seta AN3 absent; some individuals with almost entirely black (instar II) or dark brown (instar III) head capsule; pronotum with two dark-brown longitudinal stripes dorsally (instars II and III). Description. Instar I (Figs 1, 6–33). Color (Fig. 1). Head capsule yellow-brown; frontoclypeus and parietal region internal to stemmata lighter; with several small brown maculae; stemmata brown; antennae pale yellow except for first article of A2, apical half of third article of A3 and A4 dark brown; mandible pale yellow except for distal portion light yellow to red-brown, thick row of small setae on inner margin brown; maxilla pale yellow except for third article of palpomere 3 dark brown; labium pale yellow; thoracic tergites yellow-brown with several small brown maculae and pair of slightly larger brown maculae on anterior quarter of protergite; Table 1. Measurements and ratios for the larvae of Cybister (Melanectes) brevis Aubé, 1838. N = number of specimens examined. Measure C. brevis Measure C. brevis Instar I (N = 3) Instar II (N = 3) Instar III (N = 3) Instar I (N = 3) Instar II (N = 3) Instar III (N = 3) HL (mm) 2.33–2.38 3.46–3.54 4.84–5.15 PPF/MP1 0.33–0.36 0.41–0.46 0.46–0.48 HW (mm) 1.63–1.65 2.49–2.52 3.50–3.70 MP1/MP2 1.68–1.75 1.62–1.80 1.69–1.88 FRL (mm) 1.07–1.09 1.48–1.53 1.88–1.98 MP3/MP2 1.52–1.59 1.31–1.37 1.24–1.29 OCW (mm) 0.70–0.76 1.19–1.26 1.84–2.04 MP/LP 1.95–2.10 2.04–2.16 1.68–1.94 HL/HW 1.43–1.45 1.39–1.41 1.36–1.39 LP2/LP1 0.69–0.81 0.71 0.51–0.60 HW/OCW 2.17–2.34 2.00–2.10 1.80–1.90 L3 (mm) 4.84–4.94 7.17–7.35 9.46–9.79 COL/HL 0.53–0.55 0.57–0.58 0.61–0.62 L3/L1 1.26–1.30 1.28–1.29 1.26–1.30 FRL/HL 0.45–0.47 0.42–0.43 0.38–0.39 L3/L2 1.13–1.14 1.14–1.15 1.12–1.16 A/HW 1.18–1.22 1.06–1.08 0.91–0.98 L3/HW 2.94–3.03 2.84–2.93 2.59–2.80 A2/A1 1.01–1.05 0.94–0.95 0.78–0.88 L3 (CO/FE) 1.00–1.14 0.99–1.02 0.97–0.98 A3/A1 0.71–0.74 0.60–0.67 0.50–0.54 L3 (TI/FE) 0.69–0.78 0.70–0.72 0.66–0.68 A4/A3 0.13–0.14 0.10–0.11 0.08–0.09 L3 (TA/FE) 0.81–0.89 0.71–0.73 0.63–0.64 A3’/A4 0.63–0.79 0.79–0.88 0.72–0.85 L3 (CL/TA) 0.35–0.36 0.32–0.33 0.26–0.29 MNL/MNW 2.75–2.87 2.73–2.81 2.84–2.92 LAS (mm) 4.05–4.13 5.85–5.94 7.44–7.58 MNL/HL 0.43–0.46 0.44 0.42–0.45 LAS/HW 2.46–2.51 2.34–2.37 2.04–2.12 A/MP 1.31–1.39 1.36–1.38 1.31–1.36 U (mm) 0.02 0.03 0.06–0.07 141 ZooKeys 1257: 137–151 (2025), DOI: 10.3897/zookeys.1257.168982 Kohei Watanabe & Masakazu Hayashi: Description of Cybister brevis larvae prosternal sclerites yellow-brown; base of setae on each posterior side of membranous region of mesoand metathorax brown; abdominal tergites I–VI yellow-brown with several small brown maculae; base of setae on lateromedial and ventral membranous regions brown; abdominal tergites VII–VIII yellow-brown with several small brown maculae; base of setae on ventral membranous region of abdominal tergite VII brown; legs pale yellow; urogomphus yellow-brown. Body (Fig. 1). Elongate, subcylindrical; measurements and body shape ratios are shown in Table 1. Head (Figs 6–14, 20–27). Cephalic capsule (Figs 6, 7, 20, 26). Flattened, subquadrate, longer than broad; maximum width at level of anterior stemmata, constricted at level of occipital region (Fig. 20), lacking temporal spiniform setae; occipital suture present; ecdysial line well marked, COL long; occipital foramen deeply emarginate ventrally (Figs 7, 26); tentorial pits visible slightly above middle of ventral midline (Figs 7, 26, 27); frontoclypeus subtriangular, anterior margin projected forward, divided into three well-developed projections, medial projection slightly longer than lateral projections, medial projection triangular, narrow and of same length on both sides, lateral projections triangular, slightly broader than medial projection, with inner length longer than outer length, inner margin slightly curved, notches between medial and lateral projections wide; anterolateral lobes rounded, not projecting beyond lamellae clypeales; EB present, large, spiniform (Figs 6, 24), Figures 6, 7. First-instar larva of Cybister (Melanectes) brevis Aubé, 1838, cephalic capsule. 6. Dorsal aspect; 7. Ventral aspect. EB, egg burster; LC, lamellae clypeales; TP, tentorial pit. 142 ZooKeys 1257: 137–151 (2025), DOI: 10.3897/zookeys.1257.168982 Kohei Watanabe & Masakazu Hayashi: Description of Cybister brevis larvae near ecdysial line at level of seta PA8; six stemmata on each side, four dorsal, two ventral. Antenna (Figs 8, 9, 25). Elongate, slender, slightly longer than HW, composed of four antennomeres; A1 and A2 subequal in length, A1 subdivided into two articles, distal article approximately 1.3 times longer than basal article; A2 subdivided into three articles: first article shortest, slightly shorter than third article, Figures 8–10. First-instar larva of Cybister (Melanectes) brevis Aubé, 1838. 8, 9. Antenna; 10. Mandible; 8, 10. Dorsal aspect; 9. Ventral aspect. sp, spinula. 143 ZooKeys 1257: 137–151 (2025), DOI: 10.3897/zookeys.1257.168982 Kohei Watanabe & Masakazu Hayashi: Description of Cybister brevis larvae second article longest; A3 shorter than A2, subdivided into three articles: first article shortest, slightly shorter than second article, third article longest; A3’ elongate, slender, slightly shorter than A4; A4 shortest. Mandible (Figs 10, 21, 22). Strong, falciform, broadest at base, narrowing to apex, abruptly narrowed toward apex from pore MNc; mandibular channel present. Maxilla (Figs 11, 12, 26). Premaxillary lobes well developed; cardo well developed, subovate with slightly concave apex, stipes elongate, slender, subcylindrical; galea absent; PPF elongate, slender, palpomere-like; MP elongate, slender, shorter than antenna, composed of three palpomeres, MP1 longest, MP2 shortest; MP1 subdivided into three subequal articles, MP2 subdivided into two articles, distal article longer than basal article; MP3 subdivided into three articles, first and second articles subequal in length, third article longest. Labium (Figs 13, 14, 26). Prementum broader than long, not sclerotized ventromedially, anterodorsal margin well projected forward into rounded median process; LP approximately half as long as MP, composed of two palpomeres; LP1 longer than LP2; LP1 subdivided into two articles subequal in length; LP2 subdivided into two articles, distal article longer than basal article. Figures 11–14. First-instar larva of Cybister (Melanectes) brevis Aubé, 1838. 11, 12. Maxilla; 13, 14. Labium; 11, 13. Dorsal aspect; 12, 14. Ventral aspect. 144 ZooKeys 1257: 137–151 (2025), DOI: 10.3897/zookeys.1257.168982 Kohei Watanabe & Masakazu Hayashi: Description of Cybister brevis larvae Thorax (Figs 1, 15, 16, 28–31). Pro-, meso-, and metanotum convex, with subequal width; protergite longer than broad, twice as long as mesotergite, subrectangular, lateral margins emarginate at about mid-length, anterior and posterior margins straight; mesoand metatergite small, broader than long, subequal in length, subtrapezoidal, posterior margin emarginate medially; sagittal line present on all tergites; sternum of prothorax membranous except for one pair of small subtriangular sclerites, sterna of mesoand metathorax membranous; spiracles absent. Legs (Figs 15, 16, 28–31). Long, composed of six segments, L1 shortest, L3 longest; CO, FE, TI, and TA subcylindrical, elongate, slender; TR short, divided into two parts by an annulus; PT with two long, curved claws; TA of L1 with apically multifid setae forming a dense patch (cleaning device), these setae larger and flat on posteroventral region (Figs 28, 29), smaller on anteroventral region (Figs 30, 31). Figures 15, 16. First-instar larva of Cybister (Melanectes) brevis Aubé, 1838, metathoracic leg. 15. Anterior aspect; 16. Posterior aspect. 145 ZooKeys 1257: 137–151 (2025), DOI: 10.3897/zookeys.1257.168982 Kohei Watanabe & Masakazu Hayashi: Description of Cybister brevis larvae Abdomen (Figs 1, 17–19, 32, 33). Eight-segmented; segments I–VI subequal in length, mostly membranous with a small tergite on anterodorsal region, tergites I–VI subrectangular, without anterior carina, sagittal line present; sterna Figures 17–19. First-instar larva of Cybister (Melanectes) brevis Aubé, 1838. 17, 18. Abdominal segment VIII; 19. Urogomphus; 17. Dorsal aspect; 18, 19. Ventral aspect.