Limnohydrobius melaenus (Germar, 1824)

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-

LIMNOHYDROBIUS MELAENUS (GERMAR, 1924)

( FIGS 54–71 )

Diagnosis

Larval morphology: Head capsule subquadrate; frontal lines Y-shaped, coronal line short; nasale slightly asymmetrical, with five short teeth, median tooth the shortest, right tooth projecting slightly farther than left one; lateral lobes of epistome symmetrical, bearing eight stout setae and an inner area of strong and sharp cuticular projections; posterior tentorial grooves close to midline, anterior, close to submentum. Cervical sclerites present. Antenna short, basal antennomere slightly longer and wider than second; second antennomere bearing an inner area of short and sharp cuticular spines on basal half; sensory appendage short 0.2 × 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 slightly longer than second palpomere, third palpomere the longest; membrane between second and third palpomeres lacking cuticular projections. Labium with submentum fused to head capsule, dorsally with sharp cuticular projections on mentum, both palpomeres, and membranes between prementum and first palpomere and between first and second palpomeres, projections short, except those on membrane between both palpomeres; mentum subtrapezoidal, wider basally; prementum subquadrate; ligula well developed, three times longer than first palpomere; second palpomere 4.0 × times as long as first palpomere. Pronotal plate large, subrectangular, with posterior borders rounded and sagittal line; those of meso- and metathorax smaller, irregularly shaped, both with sagittal line. Abdomen mostly membranous ( Figs 4–6 ), segments one to seven with one anterior pair of small dorsal plates and two pairs of small lobes each carrying a long distal seta, two additional lobes on each lateral and lateroventral margin; dorsal plate of segment eight large, suboval; segment nine trilobed, sclerotized dorsally. Legs well developed, five-segmented. Morphometrics. See Table 3. Chaetotaxy. gFR2 with eight stout setae projecting mediad; pore PA30 behind pore PA29; A2 with two setae near SE1, seta AN9 apparently absent; seta MN1 posterior to pore MN2; pore MN4 closer to seta MN1 than to seta MN5; seta MN6 apparently absent; setae MX 7– MX 11 distinguishable; pore LA11 located at base of ligula.

Chaetotaxy

First instar larva (primary chaetotaxy): Head capsule ( Figs 54–56 ). Frontale with 30 sensilla: two very short setae at about 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, long 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 long setae (FR10, FR9) close to inner margin of antennal socket; distal area of frontale with six pairs of sensilla, long seta (FR8) and pore (FR15) close 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 on anterior margin and two smaller ventrally, below dorsal two innermost setae of gFR1; each epistomal lobe with eight large setae projecting mediad (gFR2). Each parietale with 30 sensilla. Dorsal surface: very short setae (PA1, PA2, PA4, PA5) and pore PA3 arranged in a longitudinal row, PA3 closer to PA2; pore PA6 close to frontal lines; three long setae (PA7, PA13, PA14) and a short to medium length seta (PA12) arranged in a transverse row at about midlength, posterior to stemmata; pore PA10 and long seta PA8 close to basal and distal margins of innermost distal stemma, respectively; short seta PA11 close to outermost posterior stemma; two long setae (PA9, PA20) and pore PA19 on anterolateral corner of head capsule, PA9 near base of antennal socket, PA19 very close to PA20. Ventral surface anteriorly with three pores on mandibular acetabulum (PA23, PA24, PA25) and two long setae on anterolateral corner (PA21, PA22); two long setae (PA16, PA18) and two pores (PA15, PA30) along outer margin at midlength (PA15 and PA16 close to each other); one pore (PA17) near seta PA16; two long setae (PA26, PA28) and two pores (PA28, PA29) forming a diagonal row on inner margin.

Antenna ( Fig. 60 ). A1 bare, with five pores, two dorsal (AN1 on basal third, AN2 on distal third), two lateral (AN4 subapical, on inner margin, AN3 apical, on outer margin), one ventral and apical (AN5). A2 with one dorsal pore (AN6), two distal setae on outer corner (AN7 short, AN8 minute; AN9 apparently absent), two distal setae on inner corner (AN11 short, AN10 long). A3 with an apical group of at least five setae, two long and three short (gAN). SE1 very short.

Mandible ( Fig. 61 ). One long seta on outer margin close to 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 62, 63 ). Cardo with one long seta ( MX 1). Stipes with an inner row of five setae, basal one spiniform ( MX 7), remaining four stout, bifid ( MX 8, MX 9, MX 10, MX 11); ventrally with three pores, one close to inner margin at midlength ( MX 3), one subapical on outer margin ( MX 4) and one at basal third ( MX 2); two long subapical setae close to MX 4 on outer margin ( MX 5, MX 6). MP1 dorsally with sub-basal stout and bifid seta ( MX 16) and one pore at base of appendage ( MX 17); ventrally with two pores ( MX 12 apical on outer margin, MX 15 at base of appendage) and two long apical setae ( MX 13, MX 14); inner appendage with one long seta and three or four short sensilla (gAPP). MP2 with two apical pores ( MX 19 dorsal on membrane at inner margin, MX 18 ventral, close to outer margin). MP3 with two apical setae ( MX 23 dorsal, long, on outer margin; MX 21 ventral, shorter) and two ventral pores ( MX 20 apical, on outer margin, MX 20 subapical, closer to inner margin). MP4 with one long 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 very short sensilla constitute gMX1.

Labium ( Figs 56 , 68, 69 ). Submentum with two pairs of setae ( Fig. 56 ), 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 one pair of short setae closer to midline (LA3). Prementum dorsally with one pair of basal pores (LA8) and one pair of minute seta-like sensilla (LA9) distally on membrane connecting with palpi and ligula; ventrally with two short setae on basal corners (LA5), two pores on anterolateral corners (LA7) and one pair of long distal setae (LA6). LP1 dorsally with one distal pore on membrane connecting with LP2 (LA14) and one minute basal seta ventrally (LA13); LP2 with one subapical pore on outer margin (LA15) and one 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 basal pores (LA11).

Third instar larva: Head capsule ( Figs 57–59 ). Frontale with at least four secondary setae close to FR1; a row of 8–9 short secondary setae demarcating each missing frontal line (it is difficult to assign these setae either to the frontale or to the parietale). Each parietale with three secondary setae, one short dorsal seta near PA9 and two setae on outer margin near PA18 (one short and one long). Antenna ( Fig. 64 ). A2 with two small basal pores on dorsal surface. Mandible ( Fig. 65 ). With several secondary setae arranged as follows: a sub-basal patch of nine short setae on outer and ventral margins, a row of at least 11 minute setae on outer margin behind MN1 and two minute setae distal to MN1 on outer margin, MN5 cannot be distinguished from these secondary setae (as marked on Fig. 65 ). Maxilla ( Figs 66, 67 ). Stipes with three secondary setae on outer margin, one long in dorsal view close to pore MX 4, two short in ventral view, one at midlength and one sub-basal; seta MX 7 long and slender; MP1 with MX 16 bifid. Labium ( Figs 59 , 70, 71 ). Mentum with a distal crown of ten stout and blunt setae and five or six more slender setae ventrally.

Figures

Figures 54–59. Larva of Limnohydrobius melaenus, first instar (54–56) and third instar (57–59). 54, 57, labroclypeus. 55, 58, head capsule in dorsal view. 56, 59, head capsule in ventral view. Scale bars: Figs 54, 57: 0.05 mm, Figs 55–56, 58–59: 0.1 mm.

Figures 60–67. Larva of Limnohydrobius melaenus, first instar (60–63) and third instar (64–67). 60, 64, antenna, dorsal view. 61, 65, right mandible, dorsal view. 62, 66, maxilla, dorsal view. 63, 67, maxilla, ventral view. Scale bars: 0.05 mm.

Figures 68–71. Larva of Limnohydrobius melaenus, first instar (68–69) and third instar (70–71). 68, 70, mentum and prementum, dorsal view. 69, 71, 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.