Medon calculosus Ahn & Jeon, 2025, sp. nov.

Medon calculosus sp. nov. Figs 1 A, B, 2, 3 Type specimens. Holotype • labeled as follows: “ Korea: Gyeongbuk Prov., Pohang-si, Janggi-myeon, Gaewon-ri 445-12 35.855905°N, 129.524668°E, 31 X 2023, KJ Ahn, pebble beach in low-tide zone; Holotype, Medon calculosus Ahn and Jeon, Desig. K. - J. Ahn 2024, deposited in CNUIC, Daejeon, Korea ”. Paratypes • 3 exx., same data as holotype • 2 exx., Korea: Gyeongbuk Prov., Pohang-si, Janggi-myeon, Gaewon-ri 445-12 35.855905°N, 129.524668°E, 2 XI 2023, KJ Ahn, pebble beach in mid-tide zone • 2 exx., same locality as above, 24 III 2023, KJ Ahn, pebble beach in low-tide zone • 2 exx., Gyeongju-si, Yangnam-myeon, Suryeom-ri, Jigyeong-beach, 35.652814°N, 129.450041°E, 2 XI 2023, KJ Ahn, pebble beach in low-tide zone • 2 exx., Naa-ri, Naa-beach, 35°41′57.31″N, 129°28′26.73″E, 7 m, 13 VII 2018, IS Yoo, JS Lee, JG Jung, in gravels covered with seaweed in high-supratidal zone, flotation • 2 exx., Yeongduk-gun, Byonggok-myeon, Byonggok-ri, 36.602581°N, 129.415695°E, 24 IX 2022, KJ Ahn, flotation on pebble beach. Description. Male. Body length 3.0– 3.3 mm. Body slender, more or less parallel-sided, flattened, and densely pubescent. Head, pronotum, elytra, antennae and legs brown to reddish-brown; abdomen black (Fig. 1). Head. Subquadrate, about 1.03 times as long as wide, widest basal 1 / 5 and about 1.15 times wider than pronotum, dorsal surface covered with dense pubescence; eye small, about 0.26 times as long as temple; antenna very long, extending to middle of elytron; inserted under side of front head, insertion invisible from dorsal view; all antennomeres elongate, scape longest and widest, pedicel to antennomere 7 distinctly longer than wide, antennomeres 8–10 subglobular, antennomere 11 water drop-shaped, relative length of 11 antennomeres 21: 14: 14: 13: 14: 13: 13: 11: 11: 11: 14; gular sutures narrowly separated and converged posteriorly. Neck moderately wide, about 1 / 3 as wide as head. Mouthparts. Labrum (Fig. 2 A) broad, anterior margin with two pairs of tooth-like processes, medial one longer and slender; mandibles (Fig. 2 B) asymmetrical, with 3–4 internal teeth; maxilla (Fig. 2 C) with galea and lacinia fringed with long setae; maxillary palpomere 1 small, about 3.18 times as long as 2; palpomere 2 expanded apically, little curved inwardly, about 0.97 times as long as 3; palpomere 3 droplet-shaped, widest apical third; palpomere 4 minute, about 0.88 times as long as 1; labium (Fig. 2 D) with palpomere 1 small, about 0.46 times as long as 2; palpomere 2 widest near middle, about 1.87 times as long as 3; palpomere 3 small and thin, about 1.13 times as long as 1. Thorax. Pronotum more or less rectangular, about 1.17 times as long as wide; narrower than head, almost as long as head, widest apical 1 / 5 and narrowed posteriorly, yellow short setae densely present, impunctate central region present; prosternum well developed, median area upheaved with transverse carina, prosternal process acute and very long, hypomeral projection well developed and triangular. Elytra about 1.09 times as wide as pronotum, almost as long as pronotum, elytron 2.37 times as long as wide, distinct and shallow punctures present, covered with yellow setae, lateral margin straight and parallel. Metendosternite Y-shaped. Hind wings short but longer than elytron. Tarsal formula 5-5 - 5, front, middle and hind tarsomeres 1–5 each strongly widened. Abdomen. Almost parallel-sided and broadened posteriorly after segment VI. Tergites III – VI with shallow and transverse basal depression. Sternite III with basal transverse carina, medially pointed; apical margin of tergite VII subtruncate (Fig. 3 A) and sternite VIII slightly emarginate (Fig. 3 B); apical margin of sternite IX (Fig. 3 C) slightly emarginate; tergites IX and X (Fig. 3 D) covered with scattered setae, apical margin with numerous long and brown setae. Aedeagus. Median lobe symmetrical, comprising about half sclerotized part and membraneous part; more or less triangular lobe protruded in dorsal view (Fig. 3 E); apical process narrow and well sclerotized in lateral view (Fig. 3 F). Female. Similar to male, but apical margin of sternite VIII rounded. Distribution. Korea (South). Etymology. The name is derived from the Latin calculus meaning ‘ pebble’, referring to the species’ marine coastal habitat. Remarks. This species is extremely similar to M. tomokoae in external morphological characters (Figs 3 A – D, 4 A – D) but can be distinguished by the different external form and internal structure of the aedeagus (Figs 3 E, F, 4 E, F) and by the characters listed in Table 1.