Crephelochares abnormalis (Sharp, 1890)

Minoshima, Yûsuke N., Fikáček, Martin & Liu, Hsing-Che, 2023, Larval morphology of Crephelochares and Peltochares (Coleoptera: Hydrophilidae), Acta Entomologica Musei Nationalis Pragae (Acta. Ent. Mus. Natl. Pragae) 63 (2), pp. 305-322 : 307-311

Crephelochares abnormalis (Sharp, 1890)

( Figs 1–4 )

Third instar larva. General morphology. Body slender, almost parallel-sided until abdominal segment 5, then tapering posteriad ( Fig. 1A ). Nine pairs of spiracles, one on anterolateral area of mesothorax and eight on abdomen; last abdominal pair enclosed within spiracular atrium.

Colour. Body milky white with yellowish head and sclerotised parts on thorax and abdomen in ethanol-fixed specimen ( Fig. 1A ). In living specimens, sclerotized parts reddish brown, and body partly transparent, with contents of digestive system visible as dark stripe or spots ( Figs 1D–E ).

Head. Head capsule ( Figs 2A , 3 ) subquadrate. Frontal lines vestigial, only posterior part recognizable, converging basally; coronal line very short. Number of stemmata hardly recognizable but apparently six; anterior row with stemmata closely grouped; posterior row with three small stemmata. Clypeolabrum ( Fig. 3A ) asymmetrical. Nasale serrate with one small tooth on left side, not distinctly projecting anteriorly. Lateral lobes of epistome asymmetrical, weakly projecting anteriorly. Left epistomal lobe not projecting further than right lobe and nasale, anterior margin sinuate. Right epistomal lobe projecting further than nasale, anterior margin sinuate. Gular and submental sulci hardly visible. Posterior tentorial pits very small, near presumable junction of gular sulcus and submental sulcus. Cervical sclerites transverse, subdivided into small lateral sclerite and large mesal sclerite.

Antenna ( Fig. 4A ) 3-segmented, slender, moderately long. Antennomere 1 longest, slightly wider than antennomere 2. Antennomere 3 small, elongate, shortest and narrowest.

Mandibles ( Figs 4B–C ) distinctly asymmetrical, with different number of inner teeth. Left mandible with one inner tooth, apex of inner tooth weakly bifurcate; posterior two-thirds of incisor area distinctly serrate. Right mandible with two inner teeth; distal inner tooth bifurcate at apex, similar to inner tooth on left mandible but slightly narrower; proximal inner tooth simple, slightly smaller than distal one; posterior two-thirds of incisor area distinctly serrate.

Maxilla ( Figs 4D–E ). Counting cardo, 6-segmented, distinctly longer than antenna; stipes and palpus cylindrically sclerotized. Cardo small, irregularly triangular. Stipes longest and widest segment, longer than palpomeres 1–4 combined. Palpomere 1 wider than palpomeres 2 and 3, as long as palpomere 3. Palpomere 2 slightly wider than palpomere 3, slightly shorter than palpomere 4. Palpomere 4 narrowest. Inner process small, cylindrically sclerotized.

Labium ( Figs 4F–G ) well developed. Submentum large. Mentum subquadrate with rounded lateral face, widest at midlength. Dorsal surface bearing small but strong cuticular teeth on posteromesal area. Prementum trapezoid. Ligula slender, cylindrically sclerotized, distinctly shorter than labial palpus. Labial palpus slender, shorter than prementum; palpomere 1 short, palpomere 2 long.

Thorax. Prothorax slightly wider than head capsule ( Fig. 1A ). Proscutum formed by one large plate subdivided by fine sagittal line, anterior and posterior parts weakly sclerotised. Prosternum ( Fig. 2B ) subpentagonal, with incomplete sagittal line posteriorly. Meso- and metanotum with large, subquadrate plate divided by fine sagittal line. Legs ( Figs 1A , 2C ) well developed, visible from dorsal view, 5-segmented, bearing setae of variable length and pores but without swimming hairs; trochanter with one very long seta; setae on femur and tibiotarsus stout. All pairs similar in shape.

Abdomen ( Fig. 1A ) 10-segmented, mostly membranous; segments 1–7 similar in shape and size. Abdominal segment 1 with one pair of subquadrate dorsal sclerites moderate in size anteromesally. Ventral surface of abdomen with pair of well-developed prolegs composed by hook-like spines on segments 3–7. Prolegs on segments 3–5 present on projected area of abdomen ( Figs 1A–C ).

Spiracular atrium ( Figs 1B , 2D ). Segment 8 with oval dorsal plate with sinuate posterior margin. Procercus short, incompletely cylindrically sclerotized, with two short setae. Segment 9 trilobed, partly sclerotised; median lobe of spiracular atrium large; lateral lobes smaller than median lobe. Acrocercus membranous, very long. Urogomphi one segmented, short, sclerotised. Ventral face with pair of rather long lateral lobes; base of lobes connected by transverse ridge.

Chaetotaxy of head. Third instar. Frontale ( Figs 3A, C ). One pair of short setae (FR1) at midlength of central area of head capsule. Three pairs of sensilla (FR2, FR3, FR8) anteromesal to FR1, between antennal sockets; FR2 pore-like, posterior to FR3 and FR8; FR3 very short seta, between FR8 and FR2; FR8 seta (broken in the examined specimens) anterior to FR3. Two short setae (FR5, FR6) posteromesal to antennal socket; FR5 behind FR6. FR7 short seta on inner margin of antennal socket. Sensilla FR4, FR9, and FR10 close and mesal to antennal socket; FR4 pore, posteromesal to FR10; FR10 short seta between FR4 and FR9; FR9 rather short seta anterolateral to FR9. Pore FR14 anterolateral to FR9. Very short seta FR12 and pore-like sensillum FR13 on inner part of epistome; FR12 behind FR13. FR11 pore-like, behind lateral part of nasale. Pore-like sensillum FR15 behind nasale. Nasale with six short setae (gFR1); four setae arranged equidistant on anterior margin of nasale, two setae behind mesal two setae. Epistome with three setae (gFR2) on anterior margin; two small setae on inner area of epistome; one short scale-like seta at about midlength between inner two setae and mandibular articulation.

Parietale ( Figs 3A–B ). Dorsal surface with irregularly arranged longitudinal row of five sensilla (from posterior to anterior, PA1–5) posteriorly at midwidth between coronal line and lateral margin; PA3 pore-like, remaining four very short setae. PA6 pore-like sensillum, anteromesal to PA1. Three long setae (PA7, PA12–13) forming triangular group at midlength between antennal socket and posterior margin of head capsule; PA7 paddle-shaped, mesal to PA12 and PA13, PA13 behind PA12. One to two very short secondary setae mesal to PA7, close to frontal line. Three sensilla (PA8, PA10, and one secondary seta) behind antennal socket; PA8 long; PA10 pore-like, lateral to PA8; secondary seta anterior to PA8 and PA10. Seta PA9 rather short, paddle-shaped, on outer margin of antennal socket. One small secondary seta anterior to PA9. Anterolateral corner of head capsule with one pore-like sensillum (PA19), three setae (PA20–22) and three ventral pore-like sensilla (PA23–25) close to ventral mandibular acetabulum. PA19 dorsal, between PA9 and PA20; long seta PA20 and seta PA21 (broken in the examined specimens) situated laterally; long seta PA22 ventral. PA23 lateral to PA24 and PA25, PA24 between PA23 and PA25. One short secondary seta and one pore-like sensillum behind PA20. Two pore-like sensilla (PA15 and PA17) and six setae (PA11, PA14, PA16, and three secondary setae) laterally on anterior one-third of parietale; PA14 long, located more dorsally than remaining sensilla; PA15 lateral to PA11 and PA14; PA11 short, between PA14 and PA15; PA16 long, paddle-shaped, lateroventral; PA17 on ventral face, mesal to PA16; one very short secondary seta close to PA16; one short secondary seta in front of PA17; one short secondary seta behind PA17. Long seta PA18 and two lateroventral secondary setae at midlength of parietale; one short secondary seta anterior to PA18; one long secondary seta behind PA18. Pore-like sensillum PA30 lateroventral, on posterior third of parietale. Two ventral setae (PA26 and PA28) and one pore-like sensillum PA27 at midlength of parietale; PA26 very long, anterior to PA27 and PA28; PA28 long, paddle-shaped; PA27 between PA26 and PA28. Pore-like sensillum PA29 posteromesal to PA28.

Antenna ( Fig. 4A ). Antennomere 1 with five pore-like sensilla (AN1–5). AN1 dorsolateral on basal third; AN2 dorsal on anterior two-fifths; AN3–5 apical; AN3 on outer face, AN4 on inner face, AN5 on ventral face. Antennomere 2: pore-like sensillum AN6 dorsal on anterior fourth of sclerite. Six sensilla (AN7–11, SE1) on intersegmental membrane between antennomeres 2 and 3. Antennal sensorium SE1 and setae AN7–9 on outer face; SE1 slender, rounded apically, shorter than antennomere 3; AN7 short, stout, between SE1 and AN8; AN8 very short, posterior to AN7; AN9 small, mesal to AN8. Long, narrow paddle-shaped seta AN10 and very short seta AN11 on inner face. Antennomere 3 bearing a group of sensilla (gAN) on apical membranous area (most sensilla of gAN missing in the examined specimens).

Mandibles ( Figs 4B–C ). Mandible with five primary sensilla (MN1–5) and secondary setae. MN1 long, at about midlength on outer face. Pore-like sensilla (MN2–4) dorsally at midlength, forming triangular group. MN2 posteromesal to MN1, MN4 anterior to MN1 on outer face, MN3 mesal and slightly anterior to MN4. Seta MN5 very short, subapical on outer face. MN6 undetectable. Two short secondary setae behind MN1. Three rather short secondary setae basally on outer face.

Maxilla ( Figs 4D–E ). Cardo with one ventral seta ( MX 1) (broken in the examined specimens). Stipes with five stout setae ( MX 7–10) dorsally on inner face; MX 7–10 almost equidistant; MX 7 short, at base; MX 8–10 short, bearing one small subapical tooth; MX 11 much longer than MX 7–9. MX 2 and MX 3 pore-like, subbasal on ventral face; MX 2 on outer face, MX 3 on median part. Subapical area of ventral face bearing three primary sensilla ( MX 4–6) laterally; MX 4 pore-like, MX 5 long seta, wide paddle- -shaped, MX 6 long seta; MX 4 posterior to MX 6, MX 5 mesal to MX 4 and MX 6. Stipes with three short secondary setae; one subapical on inner part of ventral face; one on lateroventral face at posterior third; one basal on outer face. Palpomere 1 with three setae ( MX 13–14, MX 16) and three pore-like sensilla ( MX 12, MX 15, MX 17). MX 16 rather short basal seta on inner face. MX 12–14 apical; MX 12 and long seta MX 13 on lateroventral face; very long seta MX 14 mesal to MX 13. MX 15 and MX 17 on intersegmental membrane behind inner appendage; MX 17 dorsal, MX 15 ventral. Inner appendage with one long seta and few small sensilla (gAPP). Palpomere 2 with two apical pore-like sensilla ( MX 18, MX 19); MX 27 undetectable. MX 18 on lateroventral face; MX 19 dorsal on inner face of intersegmental membrane between palpomeres 2 and 3. Palpomere 3 with four primary sensilla ( MX 20–23); MX 21 moderately long seta, MX 22 pore-like, character states (seta or pore-like) of MX 20 and MX 23 unclear due to limited number of specimens, but MX 23 may be a seta (missing in the examined specimens) and MX 20 may be a pore based on comparison with other Acidocerinae genera. MX 23 dorsal and subapical, on lateral face; MX 20 lateral and apical; MX 21 and MX 22 on ventral face; MX 21 subapical; MX 22 behind MX 21. Palpomere 4 with two apical pore-like sensilla ( MX 25 and MX 26) on outer face and one short seta ( MX 24) at base on inner face; MX 25 dorsal, digitiform; MX 26 ventral. Apical membranous area of palpomere 4 with several small sensilla (gMX).

Labium ( Figs 3B , 4F–G ). Submentum with long seta LA1 on lateral area and very short seta LA2 on anterior corner. Mentum with two pairs of primary sensilla (LA3–4) on lateroventral face and six pairs of secondary setae. LA3 very short seta, on posterior third; LA4 pore-like, at about midlength. Four pairs of short, stout dorsal secondary setae; one pair posterior, three pairs on lateral face. One pair of short dorsoapical setae on median part; one pair of very short ventral secondary setae on anterolateral corners of mentum. Dorsal face of prementum with one pair of subbasal pore-like sensilla (LA8) on mesal area of sclerite; pore-like sensillum LA9 on median part of membrane between prementum and palpi; rather long seta LA10 at base of ligula. Ventral face of prementum with three lateral sensilla (LA5–7); LA5 very short basal seta; LA6 very long seta, anterior to LA5; LA7 pore-like, on anterior margin of sclerite. Ligula with one pair of small sensilla (LA12) on apical membranous area; one pair of ventral pore-like sensilla (LA11) on membrane in basal area of ligula. Palpomere 1 with very short seta LA13 and pore-like sensillum LA14; LA13 basal, on ventral face; LA14 dorsal, on intersegmental membrane between palpomeres 1 and 2. Palpomere 2 with apical pore-like sensillum LA15; gLA composed of setae of variable length on apical membranous area.

Biology. ANDERSON (1976) described the egg-case of Crephelochares nitescens , which is in a hollow made by the adult, bag-like and without mast and cap. This is an unusual type of egg-case in the Hydrophilidae . Unfortunately, we could not find any egg-case of C. abnormalis in the rearing box, this was possibly due to the unusual type of egg-case (see Discussion).

Live larvae of C. abnormalis had air bubbles in their alimentary canal as observed by ANDERSON (1976) in the first instar larva of C. nitescens . This behaviour has been observed in several previous studies; the bubbles may increase their buoyancy, which helps them float on water surface for respiration ( MINOSHIMA & HAYASHI 2015). Larvae were very active under rearing conditions: moving relatively quickly and stopping from time to time, keeping the middle and posterior parts of the abdomen in the same position, and actively exploring the area within reach of the anterior body of the larva ( Fig. 1E ).

Comparative notes. The larva of C. abnormalis described here corresponds well with the larval description of C. nitescens by ANDERSON (1976) except for the following differences: the nasale in C. abnormalis is irregularly serrated and has only one small lateral tooth on the left ( Fig. 3C ), whereas that of C. nitescens has five rather large teeth of equal size; the epistomal lobes are asymmetrical in C. abnormalis ( Fig. 3C ), whereas symmetrical in C. nitescens , and apex of distal inner tooth on right mandible is bifurcate in C. abnormalis ( Fig. 4B ), whereas simple in C. nitescens ( ANDERSON 1976: fig. 9).

Figures

Fig. 1. Third instar larvae of Crephelochares abnormalis (Sharp, 1890). A – habitus, dorsal, lateral, ventral view; B – abdominal apex, ventral view (a: proleg on segment 7, b: ventral lobe, c: acrocercus); C – left proleg, abdominal segment 6, ventral view; D, E – alive individual.

Fig. 2. Third instar larva of Crephelochares abnormalis (Sharp, 1890).A – head, dorsal view; B – prosternum, ventral view; C – mesothoracic leg; D – spiracular atrium, dorsal view, many setae missing in the specimen.

Fig. 3. Head capsule of third instar larva of Crephelochares abnormalis (Sharp, 1890). A – dorsal view; B – ventral view; C – detail of anterior margin, dorsal view.

Fig. 4. Head appendages of third instar larva of Crephelochares abnormalis (Sharp, 1890). A – antenna, dorsal view, most sensilla of gAN missing; B, C – mandibles, dorsal view; D, E – maxilla, dorsal view (D) and ventral view (E); F, G – labium, dorsal view (F) and ventral view (G).