Hydrochara affinis ( 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 : 83-91

Hydrochara affinis ( Sharp, 1873)

( Figs. 2 A–B, 51A, 52–55, 64A–B, 65B, 67B)

Material examined. JAPAN: HONSHÛ: Shimane-ken : 29 L1, Nakanosu , Nadabun-chô, Izumo-shi, 26.v.2008 (egg cases collected in the field), 28.v.2008 (fixed), MH ; 1 L3, Midami-chô , Izumo-shi (paddy field), 31.v.2008, MH ; 1 L1, Nakanosu , Nadabun-chô, Izumo-shi, 14.v.2008, MH ; 1 L3, same locality, 22.v.2008, MH ; 2 L3, same locality, 24.v.2008, MH ; 1 L3, Nakanosu , Nadabun-chô, Izumo-shi (paddy field), 26.v.2008, MH ; 2 L3, same locality, 1.vi.2008, MH ; 1 L1, 4 L3, same locality, 11.vi.2008, MH ; 1 L3 ( HGF), Okinoshima , Sono-chô, Izumo-shi (paddy field), 10.vi.2007, MH .

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

Head. Head capsule subtrapezoidal, widest anteriorly ( Fig. 54A). Frontal lines almost V-shaped, fused at base of head capsule, coronal line very short. Surface of head capsule smooth. Six stemmata on each anterolateral portion of head capsule. Clypeolabrum almost symmetrical ( Fig. 55A). Nasale weakly rounded, slightly serrate. Epistomal lobes almost symmetrical, rounded, not projecting anteriad.

Antenna ( Figs. 55 B–C) 3-segmented, long, slender. Scape longer than pedicel and flagellum combined; inner face smooth. Pedicel with outer membranous area completely surrounded by sclerite, situated at midlength of sclerite. Flagellum slightly longer than pedicel.

Mandibles ( Figs. 55 D–E) symmetrical, with two inner teeth, distal one larger than basal one; inner teeth and inner edge of mandibular apex slightly serrate.

Maxilla ( Figs. 55 F–G) 6-segmented (including cardo), slightly longer than antenna. Cardo small, irregularly shaped. Stipes the longest, longer than palpomeres 1–4 combined; inner face bearing tiny spine-like cuticular projections between two basal-most setae. Maxillary palpus 4-segmented, palpomere 1 about as long as palpomere 3, palpomeres 1 and 3 each the longest, palpomere 2 the shortest, palpomere 4 slightly longer than palpomere 2; palpomere 1 the widest, completely sclerotised; inner process sclerotised.

Labium ( Figs. 55 H–I) well developed. Submentum fused to head capsule, large, subpentagonal, wider than mentum (e.g., Fig. 52B). Mentum subtrapezoidal in dorsal view, distinctly wider than prementum, dorsal surface bearing densely arranged, tiny cuticular projections on basal two-fifths and sparsely arranged, small tooth-like cuticular projections; lateral surface serrate. Prementum subrectangular; dorsal surface of sclerite with one pair of small membranous areas situated on anterior portion. Ligula stout, distinctly shorter than labial palpi, longer than labial palpomere 1, largely sclerotised, apical membranous area slightly bifid. Labial palpi shorter than mentum.

Thorax. Prothorax slightly wider than head capsule ( Fig. 51A). Proscutum formed by one large plate subdivided by fine sagittal line, anterior part rather weakly sclerotised; whole sclerite bearing densely arranged setae of variable length. Prosternum subpentagonal, with incomplete sagittal line ( Fig. 64A). Mesonotum with two sclerites on each side; anterior one small, narrow; posterior one large, anterior part larger than posterior part. Metanotum with one large sclerite each, anterior part subdivided by transverse ridge, posterior part smaller than anterior part. Legs ( Fig. 64B) long, visible in dorsal view, 5-segmented, bearing numerous long swimming hairs; all three pairs similar in shape.

Abdomen. Abdomen 10-segmented, tapering posteriad, covered with fine pubescence on membranous parts; segments 1 to 7 similar in shape and size ( Fig. 51A). Segment 1 with four small dorsal sclerites and two setiferous tubercles dorsally; two sclerites situated anteromedially, remaining two behind the latter; two setiferous tubercles on posterior portion behind sclerites, each bearing very long setae; lateral part of segment 1 with one setiferous tubercle and very long setiferous projection on each side. Segments 2 to 7 similar to segment 1, but with three small dorsal sclerites each, one on anterior part, two on posterior part; the sclerites smaller than those on first segment.

Spiracular atrium ( Fig. 54B): Segment 8 with oval dorsal plate; median part of posteri- or edge of plate with four strong projections, inner two larger than lateral ones; procercus incompletely sclerotised, with one long and two short setae. Segment 9 trilobed, partially sclerotised; each lateral lobe with short, incompletely sclerotised acrocercus; urogomphi short, one-segmented; prostyli very long.

First instar. Similar to third instar larva, more weakly sclerotised than second third ( Fig. 2A).

Head. Nasale almost straight.

Antenna: Inner face of scape with large cuticular teeth ( Fig. 53A). Flagellum about as long as pedicel.

Maxilla distinctly longer than antenna. Maxillary palpomere 4 the longest, slightly longer than palpomere 3 ( Figs. 53 D–E).

Labium: Mentum bearing small, strong cuticular spines on basal half of dorsal surface ( Figs. 53 F–G). Thorax. Prothorax narrower than head capsule ( Fig. 2A). Abdomen. Spiracular atrium: Segment 8 with oval dorsal plate, posterior edge of plate with two projections.

Primary chaetotaxy of head. Frontale altogether with 44 sensilla ( Figs. 52A, 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, between FR 1 and FR 3; FR 3 short and rather stout seta, situated more anteriorly and slightly more mesally than FR 2. Three rather short setae ( FR 5–7) and one pore-like sensillum ( FR 4) close to antennal socket; FR 4 located mesally to antennal socket, FR 5–6 situated posteromesally to antennal socket; FR 5 behind FR 6; FR 7 close to inner margin of antennal socket. Nasale with a group of six equidistant, stout and short setae (gFR1). Each epistomal lobe with a group of four setae; lateral two short, stout, mesal two small (gFR2). Pore-like sensillum FR 15 placed posteriorly to median part of nasale; rather long seta FR 8 behind FR 15. Two setae ( FR 9–10) situated anteromesally to antennal socket; FR 10 shorter than FR 9, behind FR 9; pore-like sensillum FR 14 situated anteriorly to antennal socket. Pore-like sensilla FR 11 and FR 13 and short seta FR 12 close to base of epistomal lobe; FR 11–12 close to lateral seta of nasale; FR 12 situated anteriorly and laterally to FR 11; FR 13 at midlength between FR 11 and FR 14.

Parietale with 30 sensilla each ( Figs. 52 A–B). Dorsal surface with a group of five sensilla ( PA 1–5) forming longitudinal row at midwidth in posterior part of parietale; PA 1–2 and 4–5 short setae, PA 3 pore-like. PA 6 pore-like, located posteromesally close to joint of coronal and frontal lines, more distant from posterior margin of head than PA 1. PA 7 rather long seta, situated anteriorly to PA 5, in about posterior two-fifths of parietale; two rather long setae ( PA 12–13) and one long seta ( PA 14) on median part of parietale, forming a triangular group; PA 12 situated medially of line connecting FR 13 and FR 14; FR 14 situated anteriorly to remaining sensilla. Seta PA 8 rather long, behind antennal socket close to frontal line; seta PA 9 long, situated posterolaterally to antennal socket. Pore-like sensillum PA 10 situated laterally of line connecting PA 8 and 14, rather close to PA 8. PA 11 long seta, located slightly laterally of line connecting PA 9 and PA 18. Pore-like sensillum PA 15 and rather long seta PA 18 in anterior third of lateral surface; PA 18 behind PA 15; moderately long seta PA 16 and porelike sensillum PA 17 on lateral surface of parietale, located anteriorly to PA 15; PA 17 situated mesally of 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 remaining setae; PA 20 between PA 19 and PA 21; PA 22 situated ventrally to remaining sensilla. 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 closely aggregated, on inner face; PA 24 behind PA 25. Two long setae ( PA 26 and PA 28) and one pore-like sensillum ( PA 27) situated ventrally on median part of parietale; PA 27 between PA 26 and PA 28, close to PA 28. Two pore-like sensilla ( PA 29–30) close to basal margin of ventral parietale; PA 29 situated mesally to PA 30; PA 30 on posterolateral corner.

Antenna ( Fig. 53A): Antennomere 1 with five pore-like sensilla ( AN 1–5), AN 1 situated dorsally on basal 0.27, AN 2 dorsally on basal 0.35, AN 3–5 on distal margin; AN 3 and AN 4 lateral, AN 5 ventral. Antennomere 2 with one pore-like sensillum ( AN 6) situated dorsally on basal 0.36 of sclerite; three setae ( AN 7–9) and sensorium SE 1 on small membranous area completely surrounded by sclerite and situated at midlength of antennomere 2; AN 7 short, stout; AN 8 short, between AN 7 and AN 9; AN 9 small; SE 1 behind remaining sensilla; AN 10–11 on inner face of intersegmentary membrane between antennomeres 2 and 3, AN 10 rather short, 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 rather long setae and a few short setae.

Mandible ( Figs. 53 B–C) with two setae ( MN 1 and MN 5) and four pore-like sensilla ( MN 2–4 and MN 6). MN 1 rather short, on outer face of mandible. Sensilla MN 2–4 pore-like, on 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. MN 5 minute, situated subapically on outer face of mandible; MN 6 situated subapically on inner face of mandible.

Maxilla ( Figs. 53 D–E): Cardo with one rather short ventral seta ( MX 1). Stipes with a row of five short to rather long setae ( MX 7–11) situated dorsally along inner face; MX 7–9 and MX 9–11 equidistant from each other; MX 7–8 short, rather stout; MX 9 shorter than MX 8, stout; MX 10 slightly longer than MX 8, stout; MX 11 the longest, trichoid. Pore-like sensillum MX 2 situated ventrally on basal 0.14 of sclerite; pore-like sensillum MX 3 on basal two-fifths of inner face; one pore-like sensillum ( MX 4) and two rather long setae ( MX 5–6) located ventrally on outer face of apical part of sclerite; MX 4 situated on lateral face; MX 5 between MX 4 and MX 6, longer than MX 6. Dorsal surface of palpomere 1 with one moderately long seta ( MX 16) situated basally on dorsal inner face; ventral surface of sclerite of palpomere 1 with three sensilla ( MX 12–14) close to distal margin of sclerite; MX 12 pore-like, MX 13 rather short seta, MX 14 moderately long seta; MX 12 on lateral face; MX 13 between MX 12 and MX 14; MX 14 mesally. 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 and MX 19) and one minute seta ( MX 27); MX 18 situated ventrally on median part of distal margin of sclerite; MX 19 situated dorsally on inner part of intersegmental membrane between palpomeres 2 and 3; MX 27 on base of outer face. Palpomere 3 with two rather short setae ( MX 21 and MX 23) and two pore-like sensilla ( MX 20 and MX 22); MX 20 on outer face close to distal margin of sclerite; MX 21 situated ventrally on distal margin of sclerite; MX 22 behind MX 21; MX 23 situated dorsally on outer part close to distal margin. 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. 52B, 53 F–G): Submentum with two pairs of setae ( LA 1–2); LA 1 moderately long, in each lateral corner, LA 2 short, situated anteromesally to LA 1 ( Fig. 52B). Ventral surface of mentum with one pair of rather short setae ( LA 3) and pore-like sensilla ( LA 4) on distal part; LA 3 behind LA 4, LA 4 close to distal margin. Dorsal surface of prementum with two pairs of pore-like sensilla ( LA 8–9); LA 8 at midwidth of prementum close to basal margin; LA 9 on 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 very short on basal margin of sclerite; long seta LA 6 and pore-like sensillum LA 7 on distal margin of sclerite; LA 6 situated mesally to PA 7. Membrane between 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 on apical membranous area; LA 12 dorsal, LA 11ventral. Palpomere 1 with one minute seta ( LA 13) situated ventrally on basal margin of sclerite; LA 14 on dorsal face of intersegmentary membrane between palpomeres 1 and 2. Palpomere 2 with one subapical pore-like sensillum ( LA 15) on outer face. Apical membranous area of palpomere 2 with several short setae (gLA).

Secondary chaetotaxy of head. Third instar. Parietale bearing numerous, rather long secondary setae on lateral surface.

Antenna: Antennomere 1 bearing moderately long to small, densely arranged secondary setae, setation more dense in distal two-thirds of inner face ( Fig. 55B).

Mandible: Outer part of mandible bearing numerous small secondary setae ( Figs. 55 D– E).

Maxilla ( Figs. 55 F–G): Outer face of stipes with a row of long to rather short secondary setae; one long secondary seta close to MX 5–6; dorsal surface of sclerite of stipes with a longitudinal row of secondary setae; basal two rather short, stout, remaining setae very short; ventral surface of sclerite of stipes with rather irregular row of moderately long to short secondary setae. Palpomere 1 with three to four short to rather long secondary setae on distal end of sclerite.

Labium ( Figs. 55 H–I): Dorsal surface of mentum with about 25 short secondary setae below tooth-like cuticular projections; lateral face of mentum with about six to nine small secondary setae on each lateral projection; ventral surface of mentum with about 6 to 11 rather short, secondary setae on each lateromedian part of ventral surface, LA 3 undistinguishable.

Biology. Larvae were found in standing water ( HAYASHI 2009a). The biology of the species has been reported by previous studies (e.g., for the life history see HOSOI (1947); for the construction of the egg case see YOSHIMURA (1992); and for the construction of egg case in regards to leaf preference during case construction in paddy fields see JINGUJI & TSUYUZAKI (2008)). HOSOI (1947) reported a fourth larval instar of H. affinis , however in the present study we consider the species as having three instars as it is usual for other hydrophilids. Three instars are also present in the western Palaearctic Hydrochara caraboides ( Linnaeus, 1758) (M. FIKÁČEK, pers. comm. 2011).

Identification. No other closely related species occurs in the collecting sites where the examined larvae were collected; hence, the larvae clearly belong to H. affinis .