Enochrus Thomson, 1859

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 : 26-29

Genus Enochrus Thomson, 1859

( Figs. 1 B–D, 3, 5, 6A, 13–28, 44B, 45B, 65A, 66A–C, 67A)

Species examined. Enochrus (Holcophilydrus) simulans ( Sharp, 1873) (L1–3), Enochrus (Holcophilydrus) umbratus Sharp, 1884 (L3), Enochrus (Methydrus) japonicus ( Sharp, 1873) (L1–3).

Diagnosis. Larvae of the genus Enochrus are similar to other genera of the Japanese Acidocerini , but may be distinguished from them by the following characters: 1) head capsule without long scale-like setae; 2) nasale serrate ( Figs. 13C , 26A ); 3) antennal sensorium (SE1) rather long (in first instar; Fig. 14A ) to small (in third instar; Fig. 17B ); 4) mandibles asymmetrical, left mandible with only one inner tooth or with one large and one small inner teeth (e.g., Figs. 17 C–D); 5) abdominal segments with spinose prolegs ( Fig. 28 ).

General morphology. Body slender, almost parallel-sided, widest between second and fourth abdominal segments ( Fig. 6A ).

Head. Head capsule subquadrate ( Fig. 25A ). Frontal lines nearly V-shaped ( Fig. 13A ) to weakly lyriform ( Fig. 19A ), fused at base of head capsule, coronal line short. Surface of head capsule smooth. Six stemmata on each anterolateral portion of head capsule. Clypeolabrum asymmetrical ( Fig. 20C ). Nasale serrate, sometimes with distinct tooth on both sides. Anterior margin of epistome asymmetrical, projecting anteriorly on inner portion, then straight to emarginate.

Antenna 3-segmented, short, stout (in first instar; Fig. 14A ) to rather slender (in third instar; Fig. 17B ) ( Fig. 65A ). Scape stout (first instar) to slender (third instar). Pedicel slightly shorter to longer than scape. Flagellum the shortest.

Mandibles asymmetrical ( Figs. 17 C–D, 26C–D); right mandible with two inner teeth of almost same size; left mandible with one large inner tooth.

Maxilla ( Figs. 14 D–E, 15F–G, 17E–F) 6-segmented (including cardo), distinctly longer than antenna. Cardo small, irregularly shaped. Stipes the longest, longer than palpomeres 1–4 combined; apical part of inner face with a small spine-like cuticular projection. Maxillary palpus 4-segmented; palpomere 1 the widest, completely sclerotised; inner process sclerotised.

Labium well developed ( Figs. 13B , 14 F–G, 15H–I, 17G–H). Submentum (e.g., Fig. 13B ) fused to head capsule, large, subpentagonal, wider than mentum. Mentum subquadrate, wider and longer than prementum, with small cuticular spines on dorsal surface. Prementum subquadrate, with narrow, short cuticular spines on intersegmental membrane between prementum and labial palpomere 1, close to base of palpomere 1. Ligula small, distinctly shorter than labial palpi, largely sclerotised. Labial palpi 2-segmented, covered with short cuticular spines on intersegmental membrane and palpomere 2.

Thorax. Prothorax wider than head capsule ( Fig. 6A ). Proscutum formed by large plate subdivided by fine sagittal line, anterior and posterior margins weakly sclerotised. Presternum rectangular, incompletely divided by fine sagittal line ( Fig. 44B ). Mesonotum incompletely divided by fine sagittal line, with two dorsal sclerites on each side, anterior one small and narrow, posterior one large, with setae of variable length. Metanotum with one large dorsal sclerite on each side; anterior part larger than posterior part, each sclerite with setae of variable length. Legs short, visible in dorsal view, 5-segmented, bearing short to rather long stout setae, lacking swimming hairs ( Figs. 45 B–C); all three pairs similar in shape.

Abdomen. Abdomen ( Fig. 6A ) 10-segmented, almost parallel-sided, tapering towards posterior end. Segments 1 to 7 similar in size and shape, covered with fine pubescence on membranous parts; segments 8 to 10 forming spiracular atrium ( Figs. 3 , 5 ). Segment 1 with four dorsal sclerites on each side; two of them situated on anteromedian part of segment: anterior one small, posterior one larger than anterior one; remaining two sclerites very small, sometimes undetectable, in posterior part of segment; each small sclerite bearing rather long seta; lateral sclerites of segment 1 very small ( Fig. 3 ). Segments 3 to 7 with spinose prolegs; shape of spines variable, from stout and strongly curved ( Figs. 28 A–D) to slender and not strongly curved ( Figs. 28 E–G).

Spiracular atrium ( Figs. 5 , 16B ): Segment 8 with large, oval dorsal plate; procercus incompletely sclerotised. Segment 9 trilobed, partially sclerotised; median lobe with one small, incompletely sclerotised projection on each side; each lateral lobe with short, membranous acrocercus; urogomphi short, one-segmented; prostyli reduced.

Primary chaetotaxy of head. Frontale altogether with 40–42 sensilla ( Figs. 13A, C , 22A, C ). Central part with three pairs of sensilla divergent posteriad; FR1 rather short seta close to frontal line; FR2 pore-like, situated anteromesally to FR1 at midlength between FR1 and FR3; FR3 short seta situated even more anteriorly and mesally than FR2, behind FR8. Setae FR5 and FR6 situated posteriorly to antennal socket; FR7 moderately long seta close to inner margin of antennal socket. FR4 pore-like, placed mesally to FR7. Nasale with a group of six stout, short setae (gFR1). Anterior margin of epistomal lobes with variable number of sensilla (gFR2). FR15 pore-like, behind nasale; seta FR8 long, situated posteriorly to FR15. Two setae (FR9 and FR10) situated medioanteriorly to antennal socket. FR11 and FR13–15 pore-like sensilla. FR11 and FR13, and short seta FR12 on inner part of epistome; location of the setae asymmetrical.

Parietale with 30 sensilla each ( Figs. 13 A–B, 22A–B). Dorsal surface with a group of five sensilla (PA1–5) situated posteriorly at midwidth of parietale, forming a longitudinal group; PA1–2 and PA4–5 short setae, PA3 pore-like; PA6 pore-like, located posteromesally close to joint of coronal and frontal lines, more distant from posterior margin of head than PA1. Very long seta PA7 and rather long seta PA12 on median part of dorsal surface of parietale; PA8 very long, situated posteriorly to antennal socket; pore-like sensillum PA10 at midlength between PA7 and PA8. Seta PA9 long, close to outer margin of antennal socket. Six sensilla (PA13–18) at midlength of lateral part; PA13, PA14, PA16 and PA18 setae, PA15 and PA17 pore-like sensilla; PA14–17 situated about anterior third; PA13 and PA18 on median part of parietale, PA13 situated more dorsally than PA18. Seta PA11 at about midlength between PA9 and PA13. Pore-like sensillum PA29 at about posterior fourth and midwidth of ventral surface of parietale; PA30 at about basal fourth of ventral surface of parietale, behind PA18. Anterior corner of epicranium with one pore-like sensillum (PA19) and three rather long setae (PA20–22); PA19 situated dorsally to remaining setae. Three pore-like sensilla (PA23–25) on anterior margin close to ventral mandibular acetabulum. Two long setae (PA26 and PA28) and one pore-like sensillum (PA27) situated ventrally at midlength of parietale; PA27 between PA26 and PA27.

Antenna ( Figs. 14A , 23A ): Antennomere 1 with five pore-like sensilla (AN1–5), AN1 at midlength; AN2 on distal part; AN3–5 on distal margin, AN3–4 dorsal, AN5 ventral. Antennomere 2 with one pore-like sensillum (AN6) situated dorsally on distal 0.26–0.28 of sclerite, three setae (AN7–9) on intersegmentary membrane between antennomeres 2 and 3, and on outer face of antenna next to sensorium (SE1); AN7 short, rounded apically, AN8 short, rather stout, AN9 minute; setae AN10 and AN11 situated apically or subapically on inner face of antennomere 2; AN10 long, AN11 short, stout; both setae close to each other. SE1 rather large, slightly shorter than antennomere 3. Antennomere 3 with apical sensilla (gAN) in apical membranous area.

Mandible ( Figs. 14 B–C, 23B–C) with two setae (MN1 and MN5) and four pore-like sensilla (MN2–4 and MN6). MN1 rather long, on outer face of mandible. Sensilla MN2–4 pore-like, forming a triangular group in midlength on dorsal surface; MN2 located posteriorly to line connecting MN3 and MN4; MN4 on lateral surface. Minute seta MN5 and pore-like sensilla MN6 situated subapically on apical mandibular tooth, MN5 on outer face, MN6 on inner face.

Maxilla ( Figs. 14 D–E, 23D–E): Cardo with one long ventral seta ( MX 1). Stipes with a row of five rather short, stout setae ( MX 7–11) situated dorsally along inner face; MX 7–11 equidistant, MX 7 shorter than others. Ventral surface of stipes with three pore-like sensilla ( MX 2–4) and two long setae ( MX 5–6); MX 2 and MX 3 located ventrally, MX 2 on about basal third, MX 3 on posterolateral part; MX 4–6 located subapically on outer face of sclerite; MX 5 between MX 4 and MX 6; MX 6 situated distally to MX 4–5. Palpomere 1 with one moderately short spiniform seta ( MX 16) situated basally on dorsal inner face. Pore-like sensillum ( MX 12) and two long setae ( MX 13–14) situated ventrally on anterior margin of sclerite; MX 13 on anterior part of outer face, between MX 12 and MX 14. Two pore-like sensilla ( MX 15 and MX 17) on membrane behind inner appendage: MX 17 dorsal, MX 15 ventral. Inner appendage with one rather long seta and two rather short setae (gAPP). Palpomere 2 with two pore-like sensilla ( MX 18–19) and minute seta ( MX 27); MX 18 on ventral surface of sclerite; MX 19 on inner face of intersegmental membrane between palpomeres 2 and 3; MX 27 at base of outer face. Palpomere 3 with two long setae ( MX 21 and MX 23) and two pore-like sensilla ( MX 20 and MX 22). Palpomere 4 with one long seta ( MX 24) on base of inner face, and with digitiform sensillum ( MX 25) and pore-like sensillum ( MX 26) situated subapically on outer face; MX 25 dorsal, MX 26 ventral. Apical membranous area of palpomere 4 with several minute setae (gMX).

Labium ( Figs. 13B , 14 F–G, 22B, 23F–G): Submentum ( Figs. 13B , 22B ) with two pairs of setae (LA1–2); LA1 very long on lateral corners, LA2 short on anterior lobes. Ventral surface of mentum with one pair of rather long setae (LA3) and one pair of pore-like sensilla (LA4) at midlength of lateral part of mentum; LA3 on distal 0.36; LA3 behind LA4. Dorsal surface of prementum with one pair of pore-like sensilla (LA8). Ventral surface of prementum with three pairs of sensilla (LA5–7) on lateral part; seta LA5 short, situated posterolaterally; LA6 very long, close to LA5; LA7 pore-like, close to distal margin of sclerite. Membrane between prementum and palpi with one pair of long setae (LA10) close to base of ligula, and one pair of minute setae (LA9) close to distal margin of premental sclerite. Ligula with one pair of small seta-like sensilla (LA12) on apical membranous area, and with one pair of pore-like sensilla (LA11) at base of ventral surface. Palpomere 1 with one pore-like sensillum (LA13) situated ventrally on outer part, close to basal margin of sclerite; LA14 situated dorsally on intersegmentary membrane between palpomeres 1 and 2. Palpomere 2 with one pore-like sensillum (PA15) situated subapically on outer part of dorsal surface of sclerite. Apical membranous area of palpomere 2 with several sensilla (gLA).

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. 13. Head capsule of Enochrus (Holcophilydrus) simulans (Sharp, 1873), first instar.A – dorsal view; B – ventral view; C – detail of anterior margin of head capsule, 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. 14. Head appendages of Enochrus (Holcophilydrus) simulans (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. 17. Enochrus (Holcophilydrus) simulans (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. 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. 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. 25. Enochrus (Methydrus) japonicus (Sharp, 1873), third instar. A – head, dorsal view; B – spiracular atrium, dorsal view.

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. 65. Examples of morphological transformations between instars in the Acidocerini and Hydrophilini larvae. A–B – antennae, dorsal view; C–D – mandibles, dorsal view. A – Enochrus (Methydrus) japonicus (Sharp, 1873), B – Hydrochara affinis (Sharp, 1873), C – Helochares (Hydrobaticus) nipponicus Hebauer, 1995, D – Hydrochara affinis.

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.

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. 16. Enochrus (Holcophilydrus) simulans (Sharp, 1873), third instar. A – head, dorsal view; B – spiracular atrium, dorsal 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.