Enochrus (Methydrus) japonicus ( Sharp, 1873 )

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 : 41-48

Enochrus (Methydrus) japonicus ( Sharp, 1873)

( Figs. 1D , 3 , 5 , 22–27 , 28 E–G, 65A, 66B)

Material examined. JAPAN: HOKKAIDÔ: 1 L1, 1 L2, 3 L3 ( SEHU, EUMJ), Tôge-shita, Rumoi-shi , 18.vii.2007, H. Yoshitomi leg. ; 4 L2, Tôge-shita, Rumoi , 7.–10.vii.2009, YM ; 2 L1, 9 L2, 7 L3, 3.5km WSW of Okusawasuigenchi, pond near upstream of Katsunai-gawa river, Otaru-shi , alt. 360 m, N43°08.51´E140°56.20´, 2.vii.2009, YM & M. Ôhara leg.

General morphology. Third instar. Colour. Head and sclerotised parts dark brown to brown; membranous parts greyish white; abdominal segments with transverse rows of small blackish spots ( Fig. 1D ).

Head ( Figs. 25A , 26A ). Frontal lines slightly lyriform. Nasale serrate with one tooth on each side. Right epistomal lobe projecting further than nasale; left lobe weakly projecting, not reaching further than nasale; lateral sides of anterior margin of epistome emarginate.

Antenna ( Fig. 26B ) short, rather slender. Scape as long as pedicel. Pedicel with inner membranous area surrounded by sclerite on apical third.

Mandibles ( Figs. 26 C–D) asymmetrical; right mandible with two inner teeth, distal one slightly larger than basal one, inner faces of distal tooth and apex of right mandible serrate; left mandible with one large slightly serrate inner tooth; inner face of apex of left mandible serrate.

Maxilla ( Figs. 26 E–F): Maxillary palpomere 1 about as long as palpomeres 3 and 4, palpomere 2 the shortest; palpomere 2 as wide as palpomere 3; palpomere 4 the narrowest.

Labium ( Figs. 26 G–H): Mentum subquadrate, with small cuticular spines dorsally. Ligula about as long as labial palpomere 1. Labial palpi 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 ( Fig. 3 ). Abdominal segments with very small transverse setiferous projections on membranous parts. Segments 2 to 7 similar to segment 1 but anteromedian part with one small, oval dorsal sclerite on each side; lateral sclerites narrow, very small; segments 3 to 7 with spinose prolegs, spines of prolegs narrow, weakly curved towards apex ( Fig. 28G ).

Spiracular atrium ( Figs. 5 , 25B ): Segment 8 with moderately large, oval dorsal plate; the plate with a small emargination on median part of posterior edge; procercus incompletely sclerotised, with two long and one rather short setae.

Second instar. Similar to third instar larva, more weakly sclerotised than third instar.

Head. Frontal lines more distinctly than those in third instar.

Antenna ( Fig. 24B ) proportionally short, slender. Scape slightly shorter than pedicel.

Maxilla ( Figs. 24 E–F): Maxillary palpomere 2 slightly wider than palpomere 3.

First instar. Similar to second instar larva, more weakly sclerotised than second instar. Colour. Head and sclerotised parts brown to light brown.

Head. Antenna ( Fig. 23A ) proportionally short, stouter than that of second instar.

Labium ( Figs. 23 F–G): Labial palpi slightly shorter than mentum.

Abdomen. Spiracular atrium: Segment 8 with moderately large, oval dorsal plate; the plate with a small emargination and two projections on median part of posterior edge, projections with one short seta; posterior edge of the plate with one pair of setae.

Primary chaetotaxy of head. Frontale altogether with 40 sensilla ( Figs. 22A, C ). Rather long setae ( FR 5–6) behind antennal socket. Anterior margin of epistomal lobes with four sensilla altogether (gFR2); right lobe with one short seta and one pore-like sensillum; left lobe with two short setae. Two setae ( FR 9 and FR 10) and one pore-like sensillum ( FR 14) situated medioanteriorly to antennal socket; FR 9 very long; FR 10 short; FR 10 at midlength between FR 4 and FR 9. Each epistomal lobe with three sensilla; location of setae asymmetrical: FR 13 situated between FR 11 and FR 12, laterally of line connecting FR 11 and FR 12 on right side, FR 12 between FR 11 and FR 13 on left side.

Parietale ( Figs. 22 A–B): PA 1–5 forming longitudinal row; PA 3 between PA 2 and PA 4. PA 10 situated slightly laterally of line connecting PA 7 and PA 8. PA 13, PA 14, PA 16 and PA 18 long setae. PA 11 rather short seta. PA 23 more distant from PA 24–25 than the latter from each other, lying close to PA 22.

Antenna ( Fig. 23A ): AN 2 on distal 0.28 of antennomere 1. Setae AN 10–11 on inner membranous area surrounded by sclerite. Apical sensilla (gAN) with two long trichoid setae, others short.

Maxilla ( Figs. 23 D–E): MX 8–11 with subapical tooth. MX 2 located ventrally on basal 0.33 of sclerite; MX 3 placed on basal 0.44 of inner face. MX 23 in distal part of outer face of sclerite; MX 20 placed laterally, close to distal margin of ventral surface of sclerite; MX 21–22 situated on inner part of sclerite; MX 21 situated apically; MX 22 behind MX 21.

Labium ( Figs. 23 F–G): LA 8 situated dorsally at midwidth on borderline of sclerite and membranous area between prementum and palpi. Apical membranous area of palpomere 2 with one long setae and several short setae (gLA).

Secondary chaetotaxy of head. Second instar. Parietale ( Figs. 27 A–C): One very small sensilla close to PA 5, sometimes absent. Three to five rather short secondary setae situated along frontal line, between PA 6 and PA 7, mesally of line connecting PA 6 and PA 7. Two rather short secondary setae situated posteriorly to outer part of antennal socket; one between PA 8 and PA 9; one situated more medially than PA 8, at midlength between frontal line and PA 8. One pore-like secondary sensillum and one rather short secondary seta close to outer margin of antennal socket, seta situated more medially than pore-like sensillum. One short secondary seta located slightly more posteriorly to PA 21, sometimes absent; two rather short secondary setae close to PA 15, one ventral, the other dorsal; one rather short secondary seta lying slightly medioposteriorly to PA 13; one rather long secondary seta situated anteriorly to PA 18; two to three rather long secondary setae between PA 17 and PA 30.

Antenna ( Fig. 24B ): Antennal sensorium ( SE 1) proportionally smaller than in first instar.

Mandible ( Figs. 24 C–D): One pore-like secondary sensillum close to MN 1; five short setae on basal part of mandible, behind MN 1.

Maxilla ( Figs. 24 E–F): Outer face of stipes with two long secondary setae; one on apical part of sclerite, one on basal third of sclerite; inner face of stipes with four rather short, stout secondary setae, all secondary setae with small subapical tooth; setae on inner face of stipes undistinguishable from primary sensilla.

Labium ( Figs. 24 G–H): Dorsal surface of mentum with two rather short, stout secondary setae on lateral face; one pair of short, stout setae situated anteromedially, close to distal margin; ventral surface with one pair of short secondary setae on anterior corners.

Third instar. Similar to second instar larvae.

Antenna ( Fig. 26B ): Antennal sensorium ( SE 1) proportionally smaller than in second instar.

Mandible ( Figs. 26 C–D): Outer part of basal half of mandibles with six small to rather short setae situated behind MN 1.

Maxilla ( Figs. 26 E–F): Inner face of stipes with seven rather short, stout secondary setae, all secondary setae with small subapical tooth.

Labium ( Figs. 26 G–H): Dorsal surface of mentum with four to five rather short, stout secondary setae situated laterally.

Habitat. Standing water. Larvae were found in very shallow water, on the muddy banks of a pond, or in very slowly flowing water. They usually hide below the fallen leaves or in the mud.

Identification. No other closely related species occurs in the collecting sites and the morphology of the above larva moreover corresponds with the morphology of the larvae of E. simulans and E. umbratus , which are most similar to E. japonicus . Hence, the larvae were identified as E. japonicus .

Remarks. MORIOKA (1955) described an unidentified Enochrus larva collected in the field as Enochrus sp. He pointed out that the larvae were collected together with adults of E. japonicus , and no other hydrophilid species occurred in the collecting site. The larvae probably belong to E. japonicus , but we are not able to confirm the species-level identification as the description does not include enough details for a reliable comparison.

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. 3. Larva of Enochrus japonicus (Sharp, 1873) with basic morphological terms. A – dorsal view; B – lateral view; C – ventral view.

Fig. 5. Spiracular atrium of Enochrus japonicus (Sharp, 1873). A – dorsolateral view; B – lateral view.

Fig. 22. Head capsule of Enochrus (Methydrus) japonicus (Sharp, 1873), first instar. A – dorsal view; B – ventral view; C – detail of anterior margin of head capsule, dorsal view.

Fig. 23. Head appendages of Enochrus (Methydrus) japonicus (Sharp, 1873), first 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. 24. Enochrus (Methydrus) japonicus (Sharp, 1873), second instar.A – clypeolabrum, dorsal view; B – antenna, dorsal view; C–D – mandibles, dorsal view; E – maxilla, dorsal view; F – maxilla, ventral view; G – labium, dorsal view; H – labium, ventral view.

Fig. 25. Enochrus (Methydrus) japonicus (Sharp, 1873), third instar. A – head, dorsal view; B – spiracular atrium, dorsal view.

Fig. 26. Enochrus (Methydrus) japonicus (Sharp, 1873), third instar. A – clypeolabrum, dorsal view; B – antenna, dorsal view; C–D – mandibles, dorsal view; E – maxilla, dorsal view; F – maxilla, ventral view; G – labium, dorsal view; H – labium, ventral view.

Fig. 27. Enochrus (Methydrus) japonicus (Sharp, 1873), chaetotaxy of head capsule, dorsal view. A–C – second instar larva: A – median part of head capsule; B–C – intraspecific variation of secondary sensilla between PA6 and PA7. D–F – third instar larva. D – median part of head capsule; E–F – intraspecific variation of secondary sensilla between PA6 and PA7. Secondary sensilla are marked by a black rhombus.

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.