Cercyon Leach, 1817

Suzumura, Alyssa Lee, Kobayashi, Norio & Ôhara, Masahiro, 2019, Revision of the Beach-Dwelling Species of Cercyon Leach (Coleoptera: Hydrophilidae) of the West Coast of North America, The Coleopterists Bulletin 73 (3), pp. 473-493 : 475

Genus Cercyon Leach, 1817

Diagnosis. Body variously oval, rather flat to strongly convex; pronotum and elytra forming a continuous curve in lateral view. Dorsal surface glabrous, with punctation and sometimes with microsculpture or indistinct pubescence; pronotum without longitudinal grooves or pit-like impressions except often with 2 lateroposterior depressions; color ranging from pale yellow to black. Ventral surface covered in dense, short pubescence except raised surfaces of meso- and metaventrites. Antennae 9-segmented, terminal 3 antennomeres forming compact club; bases visible dorsally, not hidden by lateral margin of head. Clypeus not separated from frons by a distinct suture, often truncate, extending anterior to eyes when viewed dorsally; mentum of variable shape, anterior margin subtruncate or emarginate, often impressed anteromedially. Maxillary palpi shorter than antennae, 2 nd palpomere swollen apically, length of apical segment equal to or slightly longer than penultimate; male maxilla bearing adhesion discs, though may be indistinct in smaller species. Elytra with distinct epipleura, at least in basal half. Middle portion of prosternum elevated to form a tectiform longitudinal keel, not differentiated laterally from antennal excavations. Median portion of mesoventrite elevated to form a median keel or plate; contacting metaventrite at a single point or separated from it by a narrow gap. Outer margin of profemora rounded; tarsal formula 5-5-5 with 1 st tarsomere always longer than 2 nd. Male genitalia of trilobed type, as in Fig. 7 . Female genitalia as in Fig. 13 .

Notes on Identification. Along with members of the Staphylinidae , Cercyon is one of the more common beach-dwelling beetle taxa. As it is the only beach-dwelling hydrophilid genus in most regions, excluding Australia and New Zealand, it can be easily differentiated from other beach beetles by its evenly oval habitus, uninterrupted outline between pronotum and elytra, presence of a mesoventral tablet, maxillary adhesion discs, and clubbed antennae.

Figures

Fig. 7. Cercyon species, male genitalia (parameres and basal piece, median lobe, and ninth ventrite, respectively). A–C) C. whuljensis (BC14MO-025), D–F) C. dux (BC14MO-030), G–I) C. fimbriatus (BC16MO-033), J–L) C. luniger (WA18AS-008), M–O) C. setulosus (WA16AS-006). Bp = basal piece; Co = corona; Ed = ejaculatory duct; Pm = parameres. Scale bars = 0.5 mm.

Fig. 13. Cercyon setulosus, female genitalia (WA17AS-004). b = bursa; g = gonocoxite; IX = ninth segment; ltg = laterotergite of segment VIII; s = spermatheca; sd = spermathecal duct; sg = spermathecal gland.