Sternolophus (Sternolophus) rufipes ( Fabricius, 1792 )

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 : 99-107

Sternolophus (Sternolophus) rufipes ( Fabricius, 1792)

( Figs. 2 D–E, 51B, 60–63, 64D, 65D)

Material examined. JAPAN: HONSHÛ: Shimane-ken: 1 L1, 13 L2, 1 L3, mouth of Kando-gawa river , Izumo-shi, 10.ix.2007, MH ; 1 L2, 1 L3, Nakanosu, Nadabun-chô, Izumo-shi (ponding fallow field), 23.viii.2007, MH; 3 L3, Nakanosu, Nadabun-chô, Izumo-shi, 10.ix.2007, MH; 7 L2, 1 L3, Okinoshima, Sono-chô, Izumo-shi (paddy field), 10.vi.2007, MH; 2 L2, 2 L2, Okinoshima, Sono-chô, Izumo-shi (artificial pond for conservation purpose), 11.vi.2007, MH; 1 L2, 1 L3, same locality, 13.ix.2007, MH ; 1 L3, Shakunouchi-kôen, Kisuki-chô, Un-nan-shi, 23.ix.2007, MH.

General morphology. Third instar. Body rather thick ( Fig. 51B ). Colour. Head and sclerotised parts yellowish brown; non-sclerotised parts greyish white to light yellowish brown ( Figs. 2 D–E).

Head. Head capsule ( Fig. 62A ) subtrapezoidal, widest anteriorly. Frontal lines almost V-shaped, fused at base of head capsule, coronal line short. Surface of head capsule smooth. Six stemmata on each anterolateral portion of head capsule. Clypeolabrum ( Fig. 62B ) slightly asymmetrical. Nasale with five teeth, four on right side more closely placed and projecting further than left one. Epistomal lobes almost symmetrical, weakly rounded, projecting as far as nasale; lateral part of anterior margin of epistomal lobes weakly serrate.

Antenna ( Figs. 63 A–B) 3-segmented, long, slender. Scape the widest, longer than pedicel and flagellum combined. Pedicel with outer membranous area completely surrounded by sclerite, situated on subapical part of outer face. Flagellum the narrowest, about as long as pedicel.

Mandibles ( Figs. 63 C–D) symmetrical, with three inner teeth, distal two large, basal one small; inner face of distal two teeth slightly serrate; basal half of inner face of mandibular apex serrate, apical portion smooth.

Maxilla ( Figs. 63 E–F) 6-segmented (including cardo), long, slender, distinctly longer than antenna. Cardo small, subtriangular. Stipes the longest, slightly longer than palpomeres 1–4 combined; one subapical spine-like cuticular projection on inner face; base of inner face bearing numerous small cuticular projections, between MX 7 and MX 8. Maxillary palpus 4-segmented; dorsal surface of palpomere 1 completely sclerotised; palpomere 1 about as long as palpomere 3; palpomere 2 the shortest; palpomere 4 slightly shorter than palpomere 3; palpomere 1 the widest; inner process sclerotised.

Labium ( Figs. 63 G–H) well developed. Submentum (e.g., Fig. 60B ) fused to head capsule, large, subpentagonal, apically rounded, wider than mentum. Mentum more than twice as wide as prementum, subquadrate, each anterior corner strongly projecting anteriad; dorsal surface of mentum bearing densely arranged small cuticular spines except for anterior-most part. Prementum subquadrate, longer than wide; dorsal surface of sclerite with one pair of small membranous areas completely surrounded by sclerite. Ligula distinctly shorter than labial palpi, longer than labial palpomere 1, partly sclerotised, basal and apical parts membranous. Labial palpi about as long as prementum.

Thorax. Prothorax wider than head capsule, widest at posterior end ( Fig. 51B ). Proscutum formed by one large plate subdivided by fine sagittal line, anterior and posterior margins weakly sclerotised; proscutum bearing numerous, rather short, fine setae. Prosternum subpentagonal, with fine, almost complete sagittal line. Mesonotum with two dorsal sclerites on each side; anterior one small, narrow; posterior one large, subtriangular, its anterior part subdivided by fine transverse ridge. Metanotum with one dorsal sclerite on each side, anterior part subtriangular, large, subdivided by fine transverse ridge, posterior part small. Legs ( Fig. 64D ) long, visible in dorsal view, 5-segmented, bearing long swimming hairs; all three pairs similar in shape.

Abdomen. Abdomen 10-segmented, tapering posteriad, densely covered with fine pubescence on membranous parts; segments 1 to 7 similar in size and shape ( Fig. 51B ). Segment 1 with one small dorsal sclerite on anteromedian part on each side, and with small setiferous projections laterally; segments 2 to 7 similar to segment 1 but dorsal sclerites smaller than those on segment 1.

Spiracular atrium ( Fig. 62C ): Segment 8 with large suboval dorsal plate, plate wider than long; posterior edge of segment 8 with four setiferous, stout and lobe-like almost membranous projections; procercus incompletely sclerotised, with three setae; urogomphi short, one segmented, with three setae, two long on median part, one very long on apex. Segment 9 trilobed, partially sclerotised; each lateral lobe with small, incompletely sclerotised acrocercus.

Second instar. Similar to third instar larva ( Fig. 2D ). Pubescence covering membranous parts of thoracic and abdominal segments sparser than in third instar. Swimming hairs of legs denser than in first instar.

First instar. Similar to third instar larva but more weakly sclerotised.

Head. Antenna long, rather slender ( Fig. 61A ).

Mandibles: Inner face of mandibles almost smooth ( Figs. 61 B–C).

Maxilla: Maxillary palpomere 4 the longest, slightly longer than palpomere 3 ( Figs. 61 D–E).

Labium: Labial palpi longer than prementum ( Figs. 61 F–G).

Primary chaetotaxy of head. Frontale altogether with 50 sensilla ( Figs. 60A, C ). Central part with three pairs of sensilla divergent posteriad; FR 1 short seta, rather close to frontal line; FR 2 pore-like, situated more anteriorly and more mesally than FR 1, between FR 1 and FR 3; FR 3 short and rather stout seta, situated even more anteriorly and slightly more mesally than FR 2. Two moderately short setae ( FR 5 and FR 6) and one pore-like sensillum ( FR 4) situated posteromesally to antennal socket; FR 5 and FR 6 more laterally than FR 4; FR 5 situated slightly posteriorly and laterally to FR 6. FR 7 rather long seta close to inner margin of antennal soc- ket. Nasale with a group of six equidistant stout and short setae and with two small setae on middle of ventral surface (gFR1), lateral-most seta of gFR1 on each side located in grooves between nasale and epistomal lobe. Each epistomal lobe with a group of six sensilla (gFR2); lateral two short and rather stout setae, middle two small setae, inner two pore-like. FR 15 pore-like, behind median setae of nasale; seta FR 8 rather long, situated posteriorly to FR 15. Two setae ( FR 9–10) situated mesally to antennal socket; FR 10 shorter than FR 9, behind FR 9; pore-like sensillum FR 14 situated medioanteriorly to antennal socket, posteriorly to FR 7. Pore-like sensilla FR 11and FR 13, and short seta FR 12 on inner part of each epistomal lobe; FR 11 close to lateral seta of nasale, FR 12 close to FR 13; FR 13 behind FR 12.

Parietale with 30 sensilla each ( Figs. 60 A–B). Dorsal surface with a group of five sensilla ( PA 1–5) situated posteriorly at midwidth, forming a slightly irregular longitudinal row; PA 1–2 and 4–5 short setae, PA 3 pore-like; PA 6 pore-like sensilla, located posteromesally close to joint of coronal and frontal lines, slightly more distant from posterior margin of head than PA 1. Long setae PA 7 and PA 12 and rather long seta PA 13 situated dorsally on median part of parietale; PA 7 located medially to remaining sensilla; PA 12 situated anteriorly to line connecting PA 7 and PA 13, more close to PA 13 than PA 7. Seta PA 8 long, behind antennal socket close to frontal line. Seta PA 9 long, close to outer margin of antennal socket. Long setae PA 11 and PA 14 and pore-like sensillum PA 10 in about anterior fourth of dorsal surface of parietale, forming rather large triangular group; PA 10 between PA 8 and PA 14; PA 11 on lateral surface; PA 14 between PA 7 and PA 11. Pore-like sensillum FR 15 and rather long seta PA 18 situated laterally on median part of parietale; PA 18 behind PA 15. Moderately long seta PA 16 and pore-like sensillum PR17 on lateroventral surface of parietale close to PA 11; PA 17 situated posteromesally to PA 16. Anterior corner of epicranium with one pore-like sensillum ( PA 19) and three long setae ( PA 20–22); PA 19 situated dorsally to the remaining setae; PA 20 between PA 19 and PA 21; PA 22 situated ventrally to remaining sensilla. Ventral surface with three pore-like sensilla ( PA 23–25) on anterior margin close to ventral mandibular acetabulum; PA 23 on outer face; PA 24 and PA 25 on inner part; PA 24 between PA 23 and PA 25. Two long setae ( PA 26 and PA 28) and one pore-like sensillum ( PA 27) situated ventrally on median part of parietale, almost equidistant, PA 26 situated anteriorly to remaining sensilla, PA 27 between PA 26 and PA 28. Two pore-like sensilla ( PA 29–30) located ventrally on basal part of parietale; PA 29 situated mesally to PA 30.

Antenna ( Fig. 61A ): Antennomere 1 with five pore-like sensilla ( AN 1–5); AN 1–2 in basal 0.35, AN 3–5 on distal margin; AN 1–4 dorsal, AN 5 ventral. Antennomere 2 with one dorsal pore-like sensillum ( AN 6) in distal 0.37 of sclerite, two setae ( AN 7–8) and sensorium SE 1 in small lateral membranous area completely surrounded by sclerite situated in distal 0.22 of sclerite; AN 7 short, behind AN 8 and SE 1; AN 8 minute; AN 10–11 on inner face of intersegmentary membrane between antennomeres 2 and 3, AN 10 long, AN 11 short, stout, both setae close to each other. SE 1 small, rounded. Antennomere 3 with apical sensilla (gAN) in apical membranous area; gAN with two long setae and a few short setae.

Mandible ( Figs. 61 B–C) with two setae ( MN 1 and MN 5) and four pore-like sensilla ( MN 2–4 and MN 6). MN 1 rather long on outer face of mandible. Sensilla MN 2–4 pore-like, at midlength of dorsal surface; MN 2 located laterally of line connecting MN 3 and MN 4, close to MN 1; MN 4 on lateral face. Seta MN 5 minute, situated subapically on outer face of mandible. MN 6 situated subapically on inner face of mandible.

Maxilla ( Figs. 61 D–E): Cardo with one rather 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 8–11 with subapical tooth; MX 7 shorter than others. Ventral surface of stipes with three pore-like sensilla ( MX 2–4) and two rather long setae ( MX 5–6); MX 2 close to outer face in basal 0.30 of sclerite; MX 3 close to inner face in distal 0.42 of sclerite; MX 4–6 subapical, on outer face of sclerite; MX 5 and MX 6 more distally than MX 4, MX 4–5 more laterally than MX 6. Dorsal surface of palpomere 1 with one moderately short spiniform seta ( MX 16) on base of inner face; ventral surface of palpomere 1 with one pore-like sensillum ( MX 12) and two rather long setae ( MX 13–14) close to distal margin of sclerite; MX 12 on lateral face; MX 13 close to MX 12, between MX 12 and MX 14; MX 14 situated medially. 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 a few short setae (gAPP). Palpomere 2 with two pore-like sensilla ( MX 18–19) and one minute seta ( MX 27); MX 18 situated ventrally at midwidth of palpomere 2, on borderline between sclerite and intersegmental membrane between palpomeres 2 and 3; MX 19 on inner face of intersegmental membrane between palpomeres 2 and 3; MX 27 at base of outer face. Palpomere 3 with two rather long setae ( MX 21 and MX 23) and two pore-like sensilla ( MX 20 and MX 22); MX 20 situated laterodorsally on distal margin of sclerite; MX 21 on middle of dorsal surface, on distal margin of sclerite; MX 22 behind MX 21; MX 23 on laterodorsal surface of sclerite, close to distal margin. Palpomere 4 with one rather 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. 60B , 61 F–G): Submentum ( Fig. 60B ) with two pairs of setae ( LA 1–2); LA 1 moderately long, in lateral corners; LA 2 short on anterior margin. Ventral surface of mentum with one pair of rather short setae ( LA 3) and one pair of pore-like sensilla ( LA 4) close to distal margin; LA 3 behind LA 4. Dorsal surface of prementum with two pairs of pore-like sensilla ( LA 8–9); LA 8 situated basally at midwidth; LA 9 in small membranous area completely surrounded by sclerite of prementum. Ventral surface of prementum with three pairs of sensilla ( LA 5–7) on lateral portion; seta LA 5 short, on basal margin of sclerite; LA 6 long seta, situated on distal margin of sclerite but more mesally than LA 7; LA 7 pore-like, on borderline between sclerite and membranous area between prementum and palpi. Membrane between prementum and palpi with one pair of long setae ( LA 10) at base of ligula. Ligula with two pairs of porelike sensilla; LA 12 dorsal, on apical membranous area, LA 11 subapical, on ventral surface of sclerite. Palpomere 1 with one minute seta ( LA 13) located ventrally, close to basal margin of sclerite; LA 14 situated dorsally on intersegmental membrane between palpomeres 1 and 2. Palpomere 2 with one pore-like sensillum ( LA 15) situated laterally on subapical margin. Apical membranous area of palpomere 2 with short to minute setae (gLA).

Secondary chaetotaxy of head. Second instar. Frontale with one short secondary seta close to FR 1, more posterolaterally than FR 1.

Parietale: Two to seven short secondary setae between PA 6 and PA 8 along frontal line. Three short secondary setae close to PA 7 and PA 13, situated laterally of line connecting PA 5 and PA 13. Two short secondary setae situated posteriorly to antennal socket close to PA 9 and PA 10; one more mesally than PA 9; the other more mesally than PA 10. One rather short secondary seta close to outer margin of antennal socket. One rather short secondary seta between PA 11 and PA 15. Lateral part bearing nine to eleven rather short secondary setae. One short secondary seta situated ventrally, rather close but more mesally than PA 17. PA 26 situated more anteriorly than that of first instar.

Antenna: Scape with a transverse row of setae formed by ca. seven rather long secondary setae, and four rather long secondary setae on subapical portion, behind AN 3–5 (e.g., Fig. 63A ).

Mandible (e.g., Figs. 63 C–D): Outer face of mandible bearing numerous small setae; base of mandible without or with one to two short secondary setae.

Maxilla (e.g., Figs. 63 E–F): Stipes with nine rather long secondary setae on outer face, and two short, stout secondary setae on basal part of outer face; one rather long seta on subapical part of ventral surface of sclerite.

Labium (e.g., Figs. 63 G–H): Dorsal surface of mentum with 12–17 short, stout secondary setae on anterior part of sclerite (e.g., Fig. 63G ); ventral surface of mentum with one long secondary seta close to but more laterally than LA 3.

Third instar. Similar to second instar.

Parietale: Five to seven rather short secondary setae between PA 6 and PA 8. Nine to 12 rather long secondary setae on lateral part.

Antenna ( Fig. 63A ): Antennomere 1 with 13–14 rather long secondary setae on apical part, behind AN 3–5.

Biology. Larvae are found in standing water ( HAYASHI 2009a). The life history of this species has been reported by HOSOI (1952) and HUANG & PU (1984).

Identification. Two species of the Sternolophus are known in Japan, but only one of them, S. rufipes , is distributed in Honshû. Hence we identified examined larvae as S. rufipes .

Remarks. YOSHIMURA (1959) described a Japanese hydrophilid larva collected in the field as Hydrocyclus sp. B. Judging from the description, figures, and the locality where the specimens were collected, the larvae described by him belong to Sternolophus rufipes .

Figures

Fig. 2. Habitus of the larvae of Hydrophilini, dorsal view. A–B – Hydrochara affinis (Sharp, 1873): A – first instar; B – third instar. C – Hydrophilus acuminatus Motschulsky, 1854, third instar. D–E – Sternolophus (Sternolophus) rufipes (Fabricius, 1792): D – second instar; E – third instar.

Fig. 51. Habitus of Hydrophilini species, third instar, dorsal view. A – Hydrochara affinis (Sharp, 1873); B – Sternolophus rufipes (Fabricius, 1792).

Fig. 62. Sternolophus (Sternolophus) rufipes (Fabricius, 1792), third instar, dorsal view. A – head; B – detail of anterior margin of head capsule; C – spiracular atrium.

Fig. 63. Head appendages of Sternolophus (Sternolophus) rufipes (Fabricius, 1792), third instar. A – antenna, dorsal view; B – detail of outer surface of pedicel, lateral view; C–D – mandibles, dorsal view; E – maxilla, dorsal view; F – maxilla, ventral view; G – labium, dorsal view; H – labium, ventral view.

Fig. 60. Head capsule of Sternolophus (Sternolophus) rufipes (Fabricius, 1792), first instar. A – dorsal view; B – ventral view; C – detail of anterior margin of head capsule, dorsal view.

Fig. 64. Prosternum and mesothoracic legs of Hydrophilini species.A – prosternum of Hydrochara affinis (Sharp, 1873), third instar, ventral view. B–D – mesothoracic legs, first instar, anterior view: B – Hydrochara affinis (Sharp, 1873); C – Hydrophilus acuminatus Motschulsky, 1854; D – Sternolophus rufipes (Fabricius, 1792).

Fig. 61. Head appendages of Sternolophus (Sternolophus) rufipes (Fabricius, 1792), 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.