Limnoxenus niger, (GMELIN, 1790)

Archangelsky, Miguel, Martínez Román, Nicolás Rafael & Fikáček, Martin, 2021, Larval chaetotaxy and morphology are highly homoplastic yet phylogenetically informative in Hydrobiusini water scavenger beetles (Coleoptera: Hydrophilidae), Zoological Journal of the Linnean Society 192 (2), pp. 416-416 : 416-

LIMNOXENUS NIGER (GMELIN, 1790)

( FIGS 72–89 )

Diagnosis

Larval morphology: Head capsule subquadrate; frontal lines Y-shaped, coronal line short; nasale slightly asymmetrical, with five short teeth, median tooth the shortest (worn off in illustrated specimen), right tooth projecting farther than left one; lateral lobes of epistome almost symmetrical, bearing four setae and an inner area of strong and sharp cuticular projections, stronger on right lobe; posterior tentorial grooves close to midline, anterior, close to submentum. Cervical sclerites present. Antenna short, basal antennomere the longest and widest; sensory appendage short, 0.08–0.10 × as long as third antennomere. Mandibles symmetrical, with three inner teeth at midlength, basal tooth the smallest. Maxilla with large stipes, slightly longer than palpus, inner margin with an area of short and sharp cuticular projections basally, distally bearing a strong and sharp spine; first palpomere incompletely sclerotized dorsally, bearing an inner appendage longer than second palpomere, third palpomere the longest. Labium with submentum fused to head capsule, dorsally with sharp cuticular projections on mentum, second palpomere, and membranes between prementum and first palpomere and between first and second palpomeres, projections on membranes long; mentum subtrapezoidal, wider basally; prementum subquadrate, slightly wider basally; ligula well developed, longer than first palpomere; second palpomere the longest, 9–11 × as long as first palpomere. Pronotal plate large, subrectangular, with sagittal line; those of meso- and metathorax smaller, irregularly shaped, both with sagittal line. Abdomen mostly membranous ( Figs 10–12 ), segments one to seven with one anterior pair of small dorsal plates and three pairs of small plates bearing short projections each carrying a long distal seta, three additional pairs of projections on each lateral and lateroventral margin; dorsal plate of segment eight large, sub-oval, bilobate posteromesally; segment nine trilobed, sclerotized dorsally. Legs well developed, five-segmented; femur elongated, with numerous ventral (natatory?) setae. Morphometrics. See Table 3. Chaetotaxy. gFR2 with two inner pores and four setae projecting mediad; pore PA30 slightly posterior to pore PA29; A2 with two setae near SE1; seta MN1 posterior to pore MN2; pore MN4 equidistant to setae MN1 and MN5; inner margin of stipes with five setae, distal four bifid; pore LA11 located at on distal half of ligula.

Chaetotaxy

First instar larva (primary chaetotaxy): Head capsule ( Figs 72–74 ). Frontale with 30 sensilla: two very short setae on basal half, close to frontal lines (FR1); two pores (FR2) and two very short setae (FR3) on disc of frontale; three pairs of sensilla (pore FR4, short seta FR5 and long seta FR6) forming a triangle behind inner margin of antennal socket; short seta (FR7) on inner margin of antennal socket; two setae (FR10 short, FR9 long) close to inner margin of antennal socket, distally; anterior area of frontale with six pairs of sensilla, long seta (FR8) and pore (FR15) closer to midline, three pores (FR11, FR13, FR14) and very short seta (FR12) distal to inner margin of antennal socket. Nasale with eight setae (gFR1), six stout dorsal setae on anterior margin and two smaller setae ventrally, below two dorsal innermost setae of gFR1; each epistomal lobe with four setae projecting mediad and two small pores innermost to setae (gFR2). Each parietale with 30 sensilla. Dorsal surface: a basal longitudinal row of four very short setae (PA1, PA2, PA4, PA5) and one pore (PA3) close to PA2; one sub-basal pore (PA6) close to frontal lines; four long setae (PA7, PA12, PA13, PA14) arranged in a transverse row at about midlength (PA12 and PA14 slightly shorter), posterior to stemmata; one pore (PA10) and two setae (PA8 very long, PA11 short) surrounding innermost distal stemmata; three long setae (PA9, PA20, PA21) and one pore (PA19) on anterolateral corner of head capsule, PA9 near base of antennal socket, PA19 very close to PA20. Ventral surface with three pores (PA23, PA24, PA25) and one long seta (PA22) on anterolateral corner, close to mandibular acetabulum; two long setae (PA16, PA18) and two pores (PA15, PA30) along outer margin; one pore (PA17) close to seta PA16; two long setae (PA26, PA28) and two pores (PA27, PA29) forming a diagonal row on central area.

Antenna ( Fig. 78 ). A1 bare, with five pores, two dorsal (AN1 on basal third, AN2 on distal third), two lateral (AN3 apical, on outer margin, AN4 subapical, on inner margin), one ventral (AN5 apical). A2 with one dorsal pore (AN6), two distal setae on outer margin close to SE1 (AN7 and AN8, short), and two distal setae on inner margin (AN11 short, AN10 very long). A3 with an apical group of at least two short and two very long setae (gAN). SE1 very short.

Mandible ( Fig. 79 ). One long seta on outer margin at midlength (MN1), one minute seta on outer margin at distal third (MN5), three dorsal pores at level of distal retinaculum forming a triangle (MN2, MN3, MN4), pore MN6 apparently absent.

Maxilla ( Figs 80, 81 ). Cardo with one long seta ( MX 1). Stipes with an inner row of five setae, basal one spiniform ( MX 7), remaining four stouter, bifid apically ( MX 8– MX 11); ventrally with three pores, one close to inner margin ( MX 3), one subapical on outer margin ( MX 4) and one on disc at basal third ( MX 2); two subapical very long setae close to pore MX 4 on outer margin ( MX 5, MX 6). MP1 dorsally with subbasal spiniform seta ( MX 16) and one pore at base of appendage ( MX 17); ventrally two pores ( MX 15 at base of appendage, MX 12 apical on outer margin) and two long apical setae ( MX 13, MX 14); inner appendage with several short sensilla (gAPP). MP2 with two apical pores ( MX 19 dorsal, on membrane close to inner margin, MX 18 ventral, on outer margin), and one minute basal seta on outer margin ( MX 27). MP3 with two long apical setae ( MX 23 dorsal, on outer margin, MX 21 ventral) and two apical ventral pores ( MX 20 on outer margin, MX 22 below seta MX 21). MP4 with one short basal seta on inner margin ( MX 24), and two subapical pores on outer margin ( MX 25 digitiform and dorsal, MX 26 ventral); a group of several short sensilla constitute gMX1.

Labium ( Figs 74 , 86, 87 ). Submentum with two pairs of setae ( Fig. 74 ), one long close to outer corners (LA1), the other minute, on anterior corners (LA2). Mentum ventrally with one pair of subapical pores on outer margin (LA4) and two short subapical setae closer to midline (LA3). Prementum dorsally with one pair of basal pores (LA8), and one pair of minute setalike sensilla (LA9), distally on membrane connecting with palpi and ligula; ventrally with two short setae on basal corners (LA5), two long distal setae (LA6), and two pores close to distal corners (LA7). LP1 with one dorsal pore on membrane connecting with LP2 (LA14) and one minute ventral seta (LA13) basally; LP2 with one subapical pore on outer margin (LA15) and an apical group composed of several short sensilla and pores (gLA). Ligula dorsally with a pair of long basal setae (LA10) and a pair of apical pores (LA12); ventrally with a pair of subapical pores (LA11).

Second instar larva: Chaetotaxy more similar to the third instar than to the first. Frontale with one short secondary seta close to FR1. Each parietale with a row of three to four short secondary setae along frontal line, one short seta close to PA9, one short seta between seta PA8 and pore PA10. Antenna. A2 with a pair of basal scar-like structures on dorsal surface. Mandible. With two secondary setae basally, and few very tiny pores on outer face between these setae and MN1. Maxilla. Stipes with four to five secondary setae, two to three basally and two distally on outer margin. Labium. Mentum with a distal crown of two stout setae on each side; a short secondary seta present close to LA4.

Third instar larva. Head capsule ( Figs75–77 ). Frontale with a short secondary seta mesally of FR1. At least three short secondary setae behind PA7 demarcating each missing frontal line (probably belonging to the parietale). Each parietale with at least one secondary short seta near PA9 and one short secondary seta between PA8 and PA10. Antenna ( Fig. 82 ). A2 with two small basal pores on dorsal surface. Mandible ( Fig. 83 ). With several minute secondary pores and two setae on outer face; setae situated basally, four to five pores between this seta and MN1, c. two pores between MN4 and MN5. Maxilla ( Figs 84, 85 ). Stipes with two long and one short secondary setae on outer margin basally, and two setae situated in distal half. Labium ( Figs 77 , 88, 89 ). Mentum with four stout and blunt setae dorsally on anterolateral corner and a pair of secondary moderately long setae posteromesally of LA4; gLA with at least one long seta.

Figures

Figures 72–77. Larva of Limnoxenus niger, first instar (72–74) and third instar (75–77). 72, 75, labroclypeus. 73, 76, head capsule in dorsal view. 74, 77, head capsule in ventral view. Scale bars: Figs 72, 75: 0.05 mm, Figs 73–74, 76–59: 0.1 mm.

Figures 78–85. Larva of Limnoxenus niger, first instar (78–81) and third instar (82–85). 78, 82, antenna, dorsal view. 79, 83, right mandible, dorsal view. 80, 84, maxilla, dorsal view. 81, 85, maxilla, ventral view. Scale bars: 0.05 mm.

Figures 86–89. Larva of Limnoxenus niger, first instar (86–87) and third instar (88–89). 86, 88, mentum and prementum, dorsal view. 87, 89, mentum and prementum, ventral view. Scale bars: 0.05 mm.

Figures 1–12. Hydrobiusini larvae, third instar larval habitus in dorsal, lateral and ventral view: 1–3, Hydramara argentina. 4–6, Limnohydrobius melaenus. 7–9, Sperchopsis tessellata. 10–12, Limnoxenus niger. Scale bars: 2 mm.