Enochrus (Holcophilydrus) umbratus Sharp, 1884

Minoshima, Yûsuke & Hayashi, Masakazu, 2011, Larval morphology of the Japanese species of the tribes Acidocerini, Hydrobiusini and Hydrophilini (Coleoptera: Hydrophilidae), Acta Entomologica Musei Nationalis Pragae (suppl.) 51, pp. 1-118 : 37-41

Enochrus (Holcophilydrus) umbratus Sharp, 1884

( Figs. 1C , 6A , 19–21 , 28D , 44B , 45B )

Material examined. JAPAN: HOKKAIDÔ: 2 L3, Shinoro, Sapporo-shi , 26.vi.2009,YM ; 6 L3, Kamishinoro, Shinorochô, Sapporo-shi , N43°08.38´, E141°22.27´, 11.vii.2009, YM .

General morphology. Third instar. Colour. Head and sclerotised parts light yellowish brown; membranous parts milky to slightly greyish white ( Fig. 1C ).

Head ( Figs. 19A , 20 ). Frontal lines slightly lyriform. Nasale serrate. Right epistomal lobe strongly projecting about as far as nasale; left lobe strongly projecting, not projecting further than nasale; lateral sides of anterior margin of epistome emarginate.

Antenna ( Fig. 21A ) short, slightly slender. Scape slightly longer than pedicel. Pedicel with inner membranous area completely surrounded by sclerite situated on apical 0.18 of inner face of sclerite.

Mandibles ( Figs. 21 B–C) asymmetrical; left mandible with one moderately large inner tooth, the tooth slightly serrate; inner edge of mandibular apex serrate.

Maxilla ( Figs. 21 D–E): Maxillary palpomere 1 about as long as palpomere 4, palpomere 2 the shortest, palpomere 3 slightly shorter than palpomere 4.

Labium ( Figs. 21 F–G): Mentum subquadrate, with small cuticular spines on basal half of dorsal surface. Ligula about as long as labial palpomere 1. Labial palpi much shorter than mentum, covered with narrow cuticular spines on intersegmental membrane between palpomeres 1 and 2, and on dorsal surface of sclerite of palpomere 2.

Abdomen. Abdominal segments 2 to 7 similar to segment 1 but anteromedian part with only one small, oval dorsal sclerite on each side; lateral sclerites narrow, very small; segments 3 to 7 with spinose prolegs, spines of prolegs stout, strongly curved at apex ( Fig. 28D ).

Spiracular atrium ( Fig. 19B ): Segment 8 with large, oval dorsal plate; posterior edge of sclerite slightly emarginate medially; procercus incompletely sclerotised, with two long and one rather short setae.

Chaetotaxy of head. Third instar. Frontale altogether with 42 primary sensilla and two secondary sensilla ( Figs. 20A, C ). One short secondary seta at midlength between FR 1 and FR 5. Rather short setae ( FR 5 and FR 6) behind antennal socket. Second seta of gFR1 of right side shorter than others. Anterior margin of epistomal lobes each with two setae and one pore-like sensillum (gFR2). Two setae ( FR 9 and FR 10) and one pore-like sensillum ( FR 14) situated medioanteriorly to antennal socket; FR 9 moderately long; FR 10 rather short. Each epistomal lobe with three sensilla; location of sensilla asymmetrical; FR 12 between FR 11 and FR 13 on right side; FR 12 behind FR 13 on left side.

Parietale with 30 sensilla and several secondary sensilla each ( Figs. 20 A–B). PA 1–2 and PA 4–5 forming longitudinal row; PA 3 close to PA 4, lying laterally of the latter. Three to four short secondary setae situated along frontal line, between PA 6 and PA 7 but medially of line connecting PA 6 and PA 7. PA 10 situated laterally of line connecting PA 7 and PA 8. One moderately long secondary seta between PA 8 and PA 9; one rather short secondary seta situated slightly mediad of PA 8. One short secondary seta close to PA 10, but sometimes absent. Two rather short secondary setae close to outer margin of antennal socket. One short secondary seta slightly behind PA 21; one short secondary seta slightly behind PA 15; one rather short secondary seta slightly medioposteriorly to PA 13; two rather short setae close to PA 15–17, one between PA 15 and PA 16, the other anterior to PA 17; one rather short secondary seta located anteriorly to PA 18; one moderately long secondary seta between PA 18 and PA 28 close to PA 18. PA 14 rather short seta; PA 13, PA 16 and PA 18 long setae. PA 11 short seta. PA 23 more distant from PA 24–25 than the latter from each other, close to PA 22.

Antenna ( Fig. 21A ): AN 2 on distal fourth of antennomere 1. Antennomere 2 with one very small secondary sensillum on basal margin of dorsal surface of sclerite; setae AN 10–11 on inner membranous area surrounded by sclerite; SE 1 small. Apical sensilla (gAN) with one long trichoid seta, two rather long setae, and several short setae.

Mandible ( Figs. 21 B–C): Outer face of mandible with three small secondary setae; one on median part close to MN 4; two on basal fourth; three to four rather short secondary setae on basal part of mandible.

Maxilla ( Figs. 21 D–E): Outer face of stipes with two long secondary setae; one on apical part of sclerite, one on basal third of sclerite. MX 2 on basal third; MX 3 on basal two-fifths. MX 23 at midlength of outer face of sclerite; MX 20 on subapical part of outer face; MX 21–22 situated ventrally on inner part of sclerite; PA 22 behind PA 21.

Labium ( Figs. 21 F–G): Dorsal surface of mentum with four rather short, stout secondary setae on lateral face; one pair of short, stout secondary setae situated anteromedially close to distal margin; ventral surface with one short secondary seta on anterior corners. LA 8 at midwidth and on posterior third of prementum, on borderline of sclerite and membranous area between prementum and palpi. Apical membranous area of palpomere 2 with one rather long seta and several short setae (gLA).

Habitat. Standing or very slowly flowing water. Larvae were found in water.

Identification. This species is distributed in eastern Japan, and co-occurs with Enochrus (Methydrus) vilis ( Sharp, 1884) in the collecting site in Sapporo, Hokkaidô. The adults of E. umbratus are much larger than those of E. vilis and the same may be expected of their larvae. Hence, for this study we have used only the largest third instar larvae collected at the locality – they are all too large to belong to E. vilis and may therefore be clearly associated with E. umbratus .

Figures

Fig. 1. Habitus of the third instar larvae of Acidocerini and Hydrobiusini. A – Agraphydrus narusei (Satô, 1960), dorsal and lateral view; B – Enochrus (Holcophilydrus) simulans (Sharp, 1873), dorsal view; C – E. (H.) umbratus Sharp, 1884, dorsal view; D – E. (Methydrus) japonicus (Sharp, 1873), dorsal view; E – Helochares (Helochares) pallens (MacLeay, 1825), dorsal view; F – H.(Hydrobaticus) nipponicus Hebauer, 1995, dorsal view; G – Hydrobius pauper Sharp, 1884, dorsal and ventral view.

Fig. 6. Habitus of third instar larvae of the Acidocerini and Hydrobiusini, dorsal view.A – Enochrus umbratus Sharp, 1884; B – Helochares nipponicus Hebauer, 1995; C – Hydrobius pauper Sharp, 1884.

Fig. 19. Enochrus (Holcophilydrus) umbratus Sharp, 1884, third instar.A – head, dorsal view; B – spiracular atrium, dorsal view.

Fig. 20. Head capsule of Enochrus (Holcophilydrus) umbratus Sharp, 1884, third instar.A – dorsal view; B – ventral view; C – detail of anterior margin of head capsule, dorsal view.

Fig. 21. Enochrus (Holcophilydrus) umbratus Sharp, 1884, third instar.A – antenna, dorsal view; B–C – mandibles, dorsal view; D – maxilla, dorsal view; E – maxilla, ventral view; F – labium, dorsal view; G – labium, ventral view.

Fig. 28. Prolegs of Enochrus Thomson, 1859 speci- es, ventral view. A–C – Enochrus (Holcophilydrus) simulans (Sharp, 1873): A – first instar; B – second instar; C – third instar. D – Enochrus (Holcophilydrus) umbratus Sharp, 1884, third instar. E–G – Enochrus (Methydrus) japonicus (Sharp, 1873): E – first instar; F – second instar; G – third instar.Photographs were taken with compound light microscope.

Fig. 44. Prosterna of Acidocerini species, third instar, ventral view. A – Agraphydrus narusei (Satô, 1960); B – Enochrus umbratus Sharp, 1884; C – Helochares nipponicus Hebauer, 1995.

Fig. 45. Mesothoracic legs of Acidocerini species, third instar, anterior view.A – Agraphydrus narusei (Satô, 1960); B – Enochrus umbratus Sharp, 1884; C – Enochrus japonicus (Sharp, 1873); D – Helochares nipponicus Hebauer, 1995.