Paracymus aeneus
Creators: name: Minoshima, Yûsuke N.
name: Watanabe, Kohei
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Coleoptera
Hydrophilidae
Paracymus
Paracymus aeneus

description: Paracymus aeneus (Germar, 1823)
(Figs 1–6)
Description of egg-case. Egg-case (Fig. 1A, B, E). Whitish, sac-shaped, composed by globular part and long, slender mast. Mast narrowing apically.
Description of larvae. First instar. General morphology similar to second instar larva. Thorax slightly wider than head capsule (Fig. 1C, F, G). Spiracles on mesothorax and abdominal segments 1–7 proportionally more strongly protruded than in second instar (Fig. 1P). Head. Frontal lines similar to second instar (Fig. 2A). Epistomal lobe weakly angulate at apex (Fig. 2C). Maxilla (Fig. 3C). Borderline between sclerite and membrane of head appendages hardly visible in several cases, but generally identical with later instar. Cuticular scale-like structures on dorsal membrane of stipes present but very indistinct (Fig. 3C). Labium (Fig. 3D). Anterior membranous part of mentum bearing cuticular spines laterally.
Primary chaetotaxy of head. Frontale (Fig. 2A, C). Median part with three pairs of sensilla (FR1–3); FR1 long seta on midlength, close to frontal line; FR2 pore-like, anteromesal to FR1; FR3 very short seta, anterior to FR2. Three sensilla (FR4–6) closely aggregated behind inner margin of antennal socket; FR4 pore-like, mesal to FR6; FR5 short seta behind FR6; FR6 long seta. FR7 short seta on inner margin of antennal socket. Short seta FR8 and pore-like sensillum FR15 behind nasale, between antennal sockets. Sensilla FR9–14 on epistome; FR11 pore-like, mesal, very close to lateral seta of gFR1; FR12 very short seta, lateral to FR11; FR13 pore-like behind FR12; FR9 long seta, very close and posterolateral to FR13; FR10 short seta behind FR9 and FR13; FR14 pore-like, posterolateral to FR9. Nasale with eight setae (gFR1); one short seta on each lateral area; two short, stout mesal setae on anterior margin; two very short, stout setae behind mesal setae; two very short ventral setae on anterior margin. Apex of epistomal lobe with two very short, rather stout setae (gFR2).
Parietale (Fig. 2A–B). Dorsal surface with five sensilla (PA1–5) in posterior area, irregularly longitudinally distributed; PA1, PA2, PA4 and PA5 minute setae, PA3 pore-like; PA1 behind PA3; PA3 lateral to PA2; PA2 posterior to PA4; PA4 posteromesal to PA5; PA5 between PA4 and PA7. PA6 pore-like sensillum, basal, close to frontal line, mesal to PA2. PA7, PA12–17 forming transverse row at midlength on lateral area of head capsule; from dorsal to ventral, PA7 (very long seta), PA12 (short seta), PA13 (very long seta), PA14 (very long seta), PA15 (pore-like), PA16 (very long seta), PA17 (pore-like). PA10 pore-like sensillum between PA7 and PA8. PA8 very long seta behind antennal socket, close to frontal line. PA9 very long seta behind lateral margin of antennal socket. PA11 short seta between PA9 and PA13. Anterior corner of head capsule with seven sensilla (PA19–25). PA19 pore-like, dorsal; PA20 somewhat short seta on lateral face, lateral to PA19; PA21 long seta, lateral, ventral to PA20; PA22 long seta, lateroventral, mesal to PA21. Pore-like sensilla PA23–25 close to ventral mandibular articulation; PA23 lateral to PA24; PA24 between PA23 and PA25; PA25 on mesal margin. PA26 long seta on anterior third, mesal to PA17. Long seta PA18 ventral, on midlength of lateral area, posterior to PA17. PA30 pore-like, posterior to PA18. PA27 and PA28 on midlength and midwidth of ventral parietale, close to each other; PA27 pore-like, anterior to very long seta PA28. Pore-like sensillum PA29 on posterior fifth, somewhat close to gular sulcus.
Antenna (Fig. 3A). Antennomere 1 with five pore-like sensilla (AN1–5). AN1 dorsal on posterior third; AN2– 5 subapical; AN2 dorsal; AN3 on outer face; AN4 on inner face; AN5 ventral. Antennomere 2 with six sensilla (AN6–8, AN10, AN11 and SE1). AN6 pore-like, on subapical area of sclerite. AN7, AN8, AN10, AN11 and SE1 on intersegmental membrane between antennomeres 2 and 3; AN7 very short seta, on outer face, behind SE1; AN8 minute seta, dorsolateral, behind SE1; AN9 absent; AN10 and AN11 on inner face; AN10 very long seta, anterior to very short seta AN11. Sensorium SE1 on outer face, long, rather stout, weakly curved, as long as or slightly longer than antennomere 3. Antennomere 3 with six apical sensilla of variable length and shape (gAN) on apical membranous area.
Mandible (Fig. 3B). Mandible with two setae (MN1 and MN5) and three pore-like sensilla (MN2–4); MN6 undetectable. MN1–4 at midlength; somewhat short seta MN1 on posterior third of lateral margin, posterolateral to MN2; MN2–4 forming a triangle; MN2 between MN1 and MN3; MN3 mesal to MN2; MN4 on lateral margin, anterior to MN1. MN5 very short seta, on anterior third of lateral margin.
Maxilla (Fig. 3C). Cardo with one short ventral seta (MX 1). Stipes with a row of five setae (MX 7–11) along dorsal inner face; MX 7 short, basal; MX 8–11 short, stout, almost equidistant. MX 2 and MX 3 ventral pore-like sensilla at midlength; MX 2 on median area; MX 3 on inner area. MX 4–6 lateral, subapical; MX 4 pore-like, behind MX 6; MX 5 very long seta, ventral to MX 6; MX 6 long seta. Dorsal surface of palpomere 1 with one somewhat long, stout seta (MX 16) on inner face. Lateroventral area with three sensilla (MX 12–14) closely aggregated; MX 12 pore-like, lateral to MX 14; MX 13 very long seta, posterior to MX 12 and MX 14; MX 14 long seta. MX 15 and MX 17 pore-like, on membrane behind inner appendage; MX 17 dorsal; MX 15 ventral. Inner appendage with one somewhat long seta and a few (possibly four) setae (gAPP). Palpomere 2 with two pore-like sensilla (MX 18 and MX 19) and one very short seta (MX 27). MX 18 ventral on lateral face; MX 19 dorsal on inner face of intersegmental membrane between palpomeres 2 and 3; MX 27 basal on outer face. Palpomere 3 with four sensilla (MX 20–23). Pore-like sensillum MX 20 and long seta MX 23 on outer face; MX 20 ventral to MX 23. Long seta MX 21 and pore-like sensillum MX 22 on inner face; MX 22 posterior to MX 21. Palpomere 4 with one somewhat long seta (MX 24) at base of inner face and with digitiform (MX 25) and pore-like (MX 26) sensilla on subapical part of outer face; MX 25 dorsal; MX 26 ventral. Apical membranous area of palpomere 4 with several very short setae of variable shape (gMX).
Labium (Figs 2B, 3D). Submentum (Fig. 2B) with two pairs of setae (LA1–2); LA1 very long on lateral corner of submentum; LA2 short on anterolateral area. Mentum (Fig. 3D) with two pairs of ventral sensilla (LA3 and LA4) on median part of anterior margin; LA3 short seta, lateral to pore LA4. Prementum with four pairs of sensilla (LA8–11) on dorsal surface, three pairs (LA5–7) on ventral surface. LA8 pore-like on median part, on borderline of sclerite. LA9–11 on median area of anterior membranous area, behind ligula; LA9 pore-like, small, behind LA10; LA10 long seta, between LA11 and LA9; LA11 pore-like, very close to or on base of ligula. LA5 very short seta on basal margin of lateral face. LA6 and LA7 on anterior margin; LA6 somewhat long, mesal to LA7; LA7 pore-like. LA12 elongate pore-like, on dorsal apex of ligula. Labial palpomere 1 with two sensilla (LA13 and LA14); LA13 very short seta, ventral, on basal margin; LA14 pore-like, dorsal, on intersegmental membrane of palpomeres 1 and 2. Palpomere 2 with pore-like sensillum LA15 on midlength of outer face and group of apical sensilla (gLA) on anterior membranous area.
Second instar. General morphology similar to third instar. Frontal lines of head capsule clearly visible, lyriform, not convergent at base; coronal line absent (see first instar, Fig. 2A). Labium (Fig. 4E). Cuticular spines on anterior membrane of mentum reduced in size and number, often undetectable.
Chaetotaxy of head, compared with first instar (see third instar, Fig. 5C). Head capsule. Frontale: one rather long secondary seta behind FR1. Parietale: one or two rather long secondary setae anterior to PA6; two short secondary setae close to PA8, one behind PA8, one between PA9 and PA8; one short secondary seta lateral to PA9; one rather short secondary seta anterior to PA16.
Mandible (Fig. 4C) with four secondary setae on outer face; two short setae on basal part; one minute seta anterior to short setae; one minute seta behind MN1. Maxilla (Fig. 4D). Stipes with two long lateral secondary setae, one close to MX 5, one on subbasal part. Labium (Fig. 4E). Mentum with three to four pairs of stout, short secondary setae on lateral part of membrane between mentum and prementum.
Third instar. General morphology. Colour light, transparent yellowish white to grey with yellowish brown sclerites in living individuals (Fig. 1I); thoracic and abdominal tubercles slightly darker in ethanol fixed specimens (Fig. 1D). Spiracles: mesothoracic spiracle on lateroventral face, moderately protuberant; first seven abdominal spiracles on dorsolateral face, moderately protuberant (Fig. 1R). Last pair on segment 8 large and enclosed within spiracular atrium (Fig. 1S).
Head. Head capsule subquadrate; cervical sclerites very small, irregularly shaped (Fig. 5A). Frontal lines not visible. Gular sulcus only recognized by internal ridge (Fig. 5D). Surface of head capsule smooth. Six stemmata on each anterolateral corner of head capsule (Fig. 5A, also see Fig. 1F–I). Clypeolabrum (Fig. 5B) slightly asymmetrical. Nasale slightly asymmetrical with three distinct teeth aggregated mesally, almost equal in size and shape. Lateral lobes of epistome slightly asymmetrical, rounded anteriorly, both lobes projecting anteriorly as far as nasale teeth. Anterolateral part of lobes membranous. Both lobes bearing a few small cuticular teeth on anteromesal part, mesal to gFR2. Ventral anterior margin of head capsule symmetrical.
Antenna (Fig. 6A) three-segmented, short, rather slender. Antennomere 1 longest and stoutest, slightly longer than antennomeres 2 and 3 combined; antennomere 2 twice as long as antennomere 3; antennomere 3 narrowest and shortest.
Mandibles (Fig. 6B) symmetrical, stout, strongly curved in apical half. Mandible with three inner teeth; anterior two teeth equal in size and shape, anterior margin simple, posterior margin finely serrate; posterior tooth smaller. Incisor area finely serrated.
Maxilla (Fig. 6C) six-segmented, longer than antenna. Cardo large, subtriangular. Stipes the longest and widest, distinctly longer than palpomeres 1–4 combined. Dorsal surface of stipes mostly membranous, borderline between sclerite and membrane indistinct or hardly visible. Lateral and ventral surface sclerotized excluding anterior area. Sclerotized part of dorsal surface reduced to an anterior transverse strip connecting inner and outer faces and a basal narrow, transverse area extending from base of inner face. Dorsal surface of stipes bearing fine cuticular projections; inner and anterior part bearing spine-like projections; mesal part bearing fine scale-like structures, hardly recognizable (see comments below). Maxillary palpus four-segmented. Palpomere 1 incompletely cylindrically sclerotized dorsally, the widest, slightly longer than wide, slightly shorter than palpomeres 2 to 4 combined. Inner appendage partly sclerotized. Palpomere 2 short, as long as and wider than palpomere 3. Palpomere 3 wider than palpomere 4. Palpomere 4 longer than palpomeres 2 and 3.
Labium (Figs 5D, 6D) developed, but with reduced ligula. Submentum (Fig. 5D) fused to head capsule, subpentagonal, wider than mentum. Submental sulcus hardly visible. Mentum (Fig. 6D) longer than wide, subquadrate in dorsal view, subtrapezoidal in lateral view (see second instar, Fig. 4E), wider than prementum; dorsal surface bearing ca. five teeth medially; short cuticular spine on anterior corner may be absent (see comments below). Prementum subquadrate; anterior membranous area with a few short lateral cuticular spines dorsally. Ligula reduced as small protuberance, partly sclerotized. Labial palpus rather long. Palpomere 1 short, two-thirds the length of palpomere 2; intersegmental membrane between palpomeres 1 and 2 bearing a few short cuticular spines dorsally. Palpomere 2 almost straight.
Thorax. Surface of thoracic membrane covered with fine cuticular asperities (e.g., Fig. 1N). Prothorax wider than head capsule (Fig. 1D, I). Proscutum formed by one large plate subdivided by fine sagittal line, with sparsely arranged transverse row of short setae anteriorly, densely covered with fine spine-like cuticular projections (Fig. 1M); anterior part weakly sclerotized. Prosternal sclerite with a pair of subquadrate plates (Fig. 5E). Mesonotum with three pairs of sclerites on each side (Fig. 5F); two transverse sclerites mesally on anterior margin; two small lateral sclerites on anterior margin; two large, posterior, transverse subtriangular sclerites. Two small, pigmented tubercles on lateral face; lateral one behind mesonotal spiracle, larger than mesal one. Metanotum with a pair of narrow, transverse sclerites on median area of anterior margin (Fig. 5F). Five pairs of tubercles on posterior part of dorsal and lateral face of mesothorax, forming a transverse row. Mesal-most tubercle very small, indistinct; lateral four small, pigmented, from mesal to lateral: transverse tubercle, two circular tubercles, lateral one larger than remaining ones. Legs short, five-segmented (Fig. 5H).
Abdomen 10-segmented, weakly lobed laterally (Fig. 1D, I); surface of membrane covered with fine cuticular asperities (Fig. 1N). Segments 1 to 7 similar in shape and size, bearing a pair of very small dorsal sclerites (Fig. 5G) on median part of anterior margin and small, pigmented tubercles behind dorsal sclerites; tubercles forming three transverse rows on dorsal to lateral faces. Lateral face with large tubercles.
Spiracular atrium developed (Fig. 5I). Segment 8 as wide as posterior part of segment 7, bearing one small, pigmented tubercle on each side, with one large subquadrate dorsal plate partly covered with fine spine-like cuticular projections and bearing several setae of variable length (Fig. 1O); posterior margin of dorsal plate partly membranous medially. Procercus incompletely sclerotized. Segment 9 trilobed, partly sclerotized, without acrocercus; urogomphi short, one-segmented; prostyli absent.
Chaetotaxy of head capsule and head appendages (Figs 5B–D, 6) very similar to second instar. Anterior corner of mentum with four (left) and two (right) setae in the examined specimen (see below Note).
Notes on description. The larvae of P.aeneus are small, and some of the head appendages are partly membranous. This posed some limitations on their descriptions.
The membranous part of the stipes is prone to distortion during slide preparation,obscuring character observations. Owing to their size and delicate structure, dorsal cuticular structures on the stipes are often challenging to observe, examination of multiple specimens is essential for an understanding of these characters.
The number of secondary setae on the anterior corner of the mentum seem to vary intraspecifically. In this study, the examined third instar larva shows four setae on the left and two on the right, although it is possible that some setae on the right side could be broken.
Biology. Adults constructed eggs-cases on algae and stones as substrates (Fig. 1E). Three larvae hatched from an egg-case.
Larvae were observed walking on algae and water surface, breathing by exposing the tip of the abdomen to the water surface (Fig. 1F). Larvae spent most of their time on wet land. Because they could not feed well in the water, they moved to land while holding the prey and lifted their head and ate the prey as reported in other hydrophilids (Fig. 1J, K) (e.g., Archangelsky 1997). Larval cannibalism was frequently observed (Fig. 1J). In containers containing water fleas, larval midgut content was coloured, suggesting that small invertebrates in the water also served as potential prey. Larvae that ceased feeding burrowed through the soil; however, all individuals died without reaching the adult stage.
Since adults commenced egg laying three days after rearing began, June is presumed to be part of the reproductive season of this species. Adults can walk under the water surface using a ventral air bubble, a behaviour reported in the other hydrophilids (Takahashi 1922; Kurosawa 1987; Matsuno 1989; Gould & Valdez 2021) (Fig. 1L).
description: Published as part of Minoshima, Yûsuke N. & Watanabe, Kohei, 2024, Morphology of the egg-case and larva of Paracymus aeneus (Germar) (Coleoptera: Hydrophilidae), pp. 200-214 in Zootaxa 5541 (2) on pages 202-210, DOI: 10.11646/zootaxa.5541.2.5, http://zenodo.org/record/14240956