Hydrophilus (Hydrophilus) acuminatus Motschulsky, 1854

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 : 91-98

Hydrophilus (Hydrophilus) acuminatus Motschulsky, 1854

( Figs. 2C , 56–59 , 64C , 66D )

Material examined. JAPAN: HOKKAIDÔ: 5 L1, Tôge-shita, Rumoi , vi.2008, H. Yoshitomi leg. ; 8 L1, 6 L2, 2 L3, same locality, vii.2008, H. Yoshitomi leg. ; 2 L1, same locality, 6.-11.vi.2009 (fixed) (egg cases collected in the field), YM .

General morphology. Third instar. Body rather thick ( Fig. 2C ). Colour. Brown to light yellowish brown, sclerotised parts darker ( Fig. 2C ).

Head. Head capsule ( Fig. 58A ) semicircular. Frontal lines U-shaped, transverse posteriorly, fused at base of head capsule, coronal line short. Surface of head capsule smooth. Six stemmata on each anterolateral portion of head capsule. Clypeolabrum (e.g., Fig. 56C ) almost symmetrical. Nasale smooth, slightly rounded. Epistomal lobes rounded, almost symmetrical, projecting further than nasale.

Antenna ( Figs. 59 A–C) 4-segmented, slender, long. Scape the longest, longer than pedicel and flagellum combined. Pedicel subdivided by narrow membranous area; basal segment small, apical segment longer than basal segment. Flagellum about as long as pedicel.

Mandibles strong asymmetrical (e.g., Figs. 57 B–C, 58A); right mandible with two closely aggregated inner teeth, basal tooth slightly larger than distal one; left mandible with one inner tooth.

Maxilla ( Figs. 59 D–E) 6-segmented (including cardo), slender, distinctly longer than antenna. Cardo small, irregularly shaped. Stipes the longest, slightly longer than palpomeres 1–4 combined. Maxillary palpus 4-segmented, palpomere 1 the longest and widest, palpomere 4 the shortest and narrowest; inner process sclerotised.

Labium (e.g., Figs. 56B , 57 F–G) well developed. Submentum fused to head capsule, large, subpentagonal, wider than mentum ( Fig. 56B ). Mentum subtrapezoidal in dorsal view, more than twice wider than prementum, densely bearing small cuticular projections on dorsal surface, anterior corners strongly projecting anteriad. Prementum subquadrate; dorsal surface of sclerite with one pair of small membranous areas completely surrounded by sclerite. Ligula distinctly shorter than labial palpi, median part sclerotised. Labial palpi shorter than mentum.

Thorax. Membranous parts of thoracic segments with densely arranged small cuticular asperities and small stout setae. Prothorax wider than head capsule ( Fig. 2C ). Proscutum with two large, irregularly shaped subrectangular plates, each bearing numerous short to rather long setae; the plates closely attached to each other medially ( Fig. 2C ). Prosternum subtrapezoidal, rather narrow. Mesonotum with two sclerites on each side; anterior one on medioanterior part, small, narrow; posterior one on median part, large but smaller than those on proscutum, subtriangular, bearing sparsely arranged setae of variable length. Metanotum with one subtriangular sclerite on each side, sclerite smaller than mesothoracic tergum, bearing sparsely arranged setae. Legs (e.g., Fig. 64C ) long, visible in dorsal view, 5-segmented; trochanter and femur bearing long, densely arranged swimming hairs; all three pairs similar in shape.

Abdomen. Abdomen 10-segmented, tapering posteriad ( Fig. 2C ); membranous area covered with densely arranged small cuticular asperities and small stout setae as those on membranous areas of thorax. Segments 1 to 7 similar in shape, with four small tubercles bearing long seta on posterior portion of each segment; segments 1 to 8 with one short setiferous projection on each side, projection on segment 8 smaller than those on segments 1 to 7.

Spiracular atrium ( Fig. 58B ): Segment 8 with two dorsal sclerites, sclerites rather slender, subtriangular, widest posteriorly; segment 9 trilobed, partially sclerotised; each lateral lobe with two sclerites, one on inner part, one on outer face, and with short incompletely sclerotised acrocercus; median lobe smaller than lateral lobe, with two sclerites; urogomphi short, one segmented; prostyli long, membranous.

Second instar. Very similar to third instar.

First instar. Similar to second instar, but more weakly sclerotised.

Head. Antenna 3-segmented, long ( Fig. 57A ). Scape about as long as pedicel and flagellum combined. Flagellum longer than pedicel.

Maxilla: Maxillary palpus 4-segmented, all palpomeres subequal in length; palpomere 1 the widest ( Figs. 57 D–E).

Labium: Ligula mostly membranous ( Figs. 57 F–G).

Thorax. Mesothoracic tergum bearing dense and fine pubescence. Membranous area covered by fine pubescence without cuticular asperities.

Abdomen. Abdomen covered by fine pubescence as on thorax, without cuticular asperities.

Primary chaetotaxy of head. Frontale with 50 sensilla altogether ( Figs. 56A, C ). Seta FR 1 rather short in basal third of frontale; one pore-like sensillum ( FR 2) and one rather short seta ( FR 3) on distal third of frontale, FR 2 between FR 1 and FR 3, FR 3 more medially and more anteriorly than FR 2. Two rather short setae ( FR 5–6) situated posteromesally to antennal socket; FR 6 behind FR 5. Pore-like sensillum ( FR 4) and short seta ( FR 7) close to inner margin of antennal socket; FR 7 behind FR 4. Nasale with a group of six equidistant, short setae and two small setae; small setae situated mesally (gFR1). Each epistomal lobe with a group of six sensilla; lateral three short setae, median two minute setae, inner one pore-like sensillum (gFR2). Pore-like sensillum FR 15 located posteriorly to median setae of nasale; short seta FR 8 situated posterolateral to FR 15. Two setae ( FR 9–10) and one pore-like sensillum ( FR 13) placed anteromesally to antennal socket. FR 9 rather shot but longer than FR 10; FR 10 situated slightly laterally of FR 9; position of FR 10 slightly asymmetrical; pore-like sensillum FR 11 at about midlength between FR 13 and FR 8; FR 13 situated anterolaterally to PA 10, mesally of line connecting FR 12 and FR 14. Pore-like sensillum ( FR 14) located anteriorly to antennal socket; short seta ( FR 12) close to groove between nasale and epistomal lobe.

Parietale with 33 sensilla each ( Figs. 56 A–B). Dorsal surface with group of five sensilla ( PA 1–5) situated posteriorly at midwidth, forming slightly irregular longitudinal row; PA 1–2 and 4–5 short setae, PA 3 pore-like. Pore-like sensillum PA 6 located posteromesally close to frontal line. PA 7 rather short seta, at midlength between basal margin of parietale and antennal socket, close to frontal line; one rather short additional seta between PA 6 and PA 7, close to PA 7. Two setae ( PA 12–13) on median part of parietale; PA 12 rather long, situated mesally to PA 13; PA 13 rather short. Seta PA 8 rather long, behind antennal socket, between PA 7 and PA 9, close to frontal line; seta PA 9 rather long, placed posterolaterally to antennal socket. Pore-like sensilla PA 10 situated at midlength between PA 9 and PA 12 but laterally of line connecting PA 9 and PA 12. Two rather long setae ( PA 11 and PA 14) and one pore-like sensillum ( PA 15) located on about anterior fifth of lateral surface of parietale; PA 15 situated laterally of PA 11 and PA 14; PA 14 behind PA 11 and PA 15. Anterior corner of epicranium with one pore-like sensillum ( PA 19) and two rather long setae ( PA 20–21) dorsally; PA 20 between PA 19 and PA 21; PA 21 placed laterally of PA 20 and PA 19; ventral surface with one rather long seta ( PA 22) and three pore-like sensilla ( PA 23–25) close to mandibular acetabulum; PA 22 and PA 23 on lateral margin of mandibular acetabulum, PA 22 behind PA 23; PA 24 between PA 22 and PA 25, close to PA 25; PA 25 on inner margin of mandibular acetabulum. Two long setae ( PA 16 and PA 18), one rather short additional seta, and one pore-like sensillum ( PA 17) situated ventrally behind PA 21, forming a row; PA 18 on lateral surface; PA 16 between PA 17 and PA 18; PA 17 between additional setae and PA 16; PA 16–18 and one additional seta equidistant. Two rather long setae ( PA 26 and PA 28) and one pore-like sensillum ( PA 27) situated ventrally on median part of parietale, divergent posteriad, PA 28 behind PA 27, closer to PA 27 than to PA 26; PA 26 more anteriorly and more laterally than PA 27 and PA 28. Two pore-like sensilla ( PA 29–30) on basal part of ventral surface of parietale; PA 30 on lateral surface; one pore-like additional sensilla situated ventrally on lateral surface of posterior 0.24 of parietale.

Antenna ( Fig. 57A ):Antennomere 1 with five pore-like sensilla ( AN 1–5), bearing numerous short to moderately long additional setae on inner face; AN 1 on basal 0.25; AN 2 on basal 0.36, AN 3–5 on distal margin; AN 1–4 dorsal, AN 5 ventral. Antennomere 2 with one porelike sensillum ( AN 6) situated dorsally on basal 0.18 of sclerite, and four setae ( AN 7–8 and AN 10–11) on intersegmentary membrane between antennomeres 2 and 3; AN 7–8 on outer face of antenna next to sensorium SE 1; AN 7 short, AN 8 shorter than AN 7; setae AN 10–11 on inner face of antenna; AN 10 rather short; AN 11 short, both setae close to each other. SE 1 small, rounded. Antennomere 3 with one pore-like additional sensillum situated dorsally on distal 0.39; apical sensilla (gAN) in apical membranous area; gAN with two rather long setae and a few short to minute setae.

Mandible ( Figs. 57 B–C) with two setae ( MN 1 and MN 5) and three pore-like sensilla ( MN 2–4 and MN 6), sparsely covered with very small pore-like additional sensilla on outer face. MN 1 rather short, on outer face of mandible. Sensilla MN 2–4 situated dorsally on median part; MN 2 at midlength of line connecting MN 3 and MN 4; MN 4 on outer face close to MX 1. Seta MN 5 minute, situated subapically on outer face. Pore-like sensillum MN 6 situated subapically on inner face.

Maxilla ( Figs. 57 D–E): Cardo with one rather long ventral seta ( MX 1). Stipes with a row of five moderately short, rather stout setae ( MX 7–11) situated dorsally on inner face; MX 8–10 equidistant, separated by distance larger than between MX 7 and MX 8 and between MX 10 and MX 11; MX 7 the shortest; MX 9 the longest. Ventral surface of stipes with three pore-like sensilla ( MX 2–4) and two rather long setae ( MX 5–6); MX 2 located mesally on basal 0.25; MX 3 on basal 0.30 of inner face; MX 4–6 located lateroventrally on distal 0.15 of sclerite; MX 6 situated posteriorly to remaining sensilla, MX 5 between MX 6 and MX 4. Dorsal surface of palpomere 1 with one moderately short spiniform seta ( MX 16) situated basally on dorsal inner face; ventral surface of palpomere 1 with one pore-like sensillum ( MX 12) and two rather long setae ( MX 13–14); MX 12–13 on outer face, close to distal margin of sclerite; MX 13 close to MX 12, situated mesally of MX 12; MX 14 situated mesally on distal fourth. 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 on middle of borderline between sclerite and intersegmental membrane between palpomeres 2 and 3; MX 19 on lateroventral surface of intersegmental membrane between palpomeres 2 and 3; MX 27 situated basally on outer surface. Palpomere 3 with two rather long setae ( MX 21 and MX 23) and two pore-like sensilla ( MX 20 and MX 22); MX 20 on outer face of distal margin of sclerite; MX 21 on inner face of intersegmental membrane between palpomeres 3 and 4; MX 22 at midwidth of anterior third of ventral surface of sclerite; MX 23 on inner part close to distal margin of ventral surface of sclerite. Palpomere 4 with one rather short seta ( MX 24) situated basally on 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. 56B , 57 F–G): Submentum ( Fig. 56B ) with two pairs of setae ( LA 1–2); LA 1 long in lateral corners, LA 2 short on anterior margin. Ventral surface of mentum with one pair of long 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 sensilla ( LA 8–9); pore-like sensillum LA 8 situated basally at midwidth of prementum; small seta 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 part; seta LA 5 short, on basal portion; LA 6 long, situated close to distal margin of sclerite and LA 7, laterally of LA 7; LA 7 pore-like, on borderline between sclerite and membranous area between prementum and palpi. Membrane between sclerite of prementum and palpi with one pair of rather long setae ( LA 10) close to base of ligula. Ligula with two pairs of pore-like sensilla ( LA 11 and LA 12) situated apically; LA 12 dorsal, LA 11 ventral. Palpomere 1 with one minute seta ( LA 13) situated 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 subapically on laterodorsal face of sclerite. Apical membranous area of palpomere 2 with several short setae (gLA).

Secondary chaetotaxy of head. Second instar. Frontale bearing about 50 secondary sensilla on anterior two-thirds of median part, between FR 1 and gFR1.

Parietale bearing numerous secondary sensilla.

Mandible: Mandibles with numerous small secondary sensilla on outer face.

Maxilla: (e.g., Figs. 59 D–E): Stipes covered with small secondary setae, the setae on ventral surface denser than those on dorsal surface; inner face with four short, stout secondary setae; outer face bearing five rather long trichoid secondary setae; three long secondary setae situated ventrally on basal part, behind MX 2–3; four rather short, stout setae located basally on outer part of dorsal surface of sclerite. Palpomere 1 with four rather long secondary setae. Two setae situated dorsally at midwidth of sclerite, one on anterior third, one at midlength. One seta situated subapically on outer face of sclerite, close to MX 12, the seta forming a group of setae with MX 12–13 situated dorsally to MX 12. One seta situated ventrally on median part of sclerite.

Labium: Mentum with sparsely arranged rather long secondary setae on lateral faces; dorsal surface of mentum covered with rather densely arranged small secondary setae.

Third instar. Very similar to second instar.

Antenna ( Figs. 59 A–C): Minute seta AN 9 present, on intersegmental membrane between palpomeres 2 and 3, close to AN 7–8 and SE 1.

Biology. Larvae are found in standing water ( HAYASHI 2009a). The biology of this species has been reported by previous studies. HOSOI (1939) studied the life history of the species. INODA et al. (2003) reported on the feeding effectiveness of asymmetrical mandibles of H. acuminatus in different prey: larvae preferred the right-handed shells of the snail, Austropeplea ollula ( Gould, 1859) ( Gastropoda: Lymnaeidae ), to the left-handed shells of Physa acuta Draparnaud, 1805 ( Gastropoda: Physidae ). They demonstrated that asymmetrical mandibles of H. acuminatus evolved as an adaptation for handling the right-handed snails.

Identification. Two species of Hydrophilus are known from Hokkaidô, Japan: Hydrophilus acuminatus and Hydrophilus dauricus Mannerheim, 1852 . However, H dauricus is only distributed in eastern Hokkaidô, whereas the collecting sites are situated in western Hokkaidô where only H. acuminatus occurs. Hence, we identified the examined larvae as H. acuminatus .

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. 56. Head capsule of Hydrophilus acuminatus Motschulsky, 1854, first instar.A – dorsal view; B – ventral view; C – detail of anterior margin of head capsule, dorsal view.

Fig. 57. Head appendages of Hydrophilus acuminatus Motschulsky, 1854, 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. 58. Hydrophilus acuminatus Motschulsky, 1854, third instar. A – head, dorsal view; B – spiracular atrium, dorsal view.

Fig. 59. Head appendages of Hydrophilus acuminatus Motschulsky, 1854, third instar. A – antenna, dorsal view; B – antenna, ventral view; C – detail of inner surface of intersegmental membrane between pedicel and flagellum; D – maxilla, dorsal view; E – maxilla, ventral 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. 66. Examples of morphological transformations between instars in the maxilla of the Acidocerini and Hydrophilini larvae. A–B – dorsal view, C – ventral view, D – dorsal (left side) and ventral (right side) views. A – Enochrus (Holcophilydrus) simulans (Sharp, 1873), B – Enochrus (Methydrus) japonicus (Sharp, 1873), C – E. simulans, D – Hydrophilus (Hydrophilus) acuminatus Motschulsky, 1854.