Oosternum costatum (LeConte, 1855)

Archangelsky, Miguel, 2018, Larval chaetotaxy and morphometry of Oosternum costatum (Coleoptera: Hydrophilidae) including a discussion of larval characters with phylogenetic relevance, Acta Entomologica Musei Nationalis Pragae 58 (2), pp. 499-511 : 500-507

Oosternum costatum (LeConte, 1855)

( Figs 1–22 )

Material examined. Adults of Oosternum costatum were collected in USA: OHIO: Lawrence Co., v–vi.1995, Wayne National Forest, Sharps creek, Bluegrass trail, ex pig manure (P. W. Kovarik, M. Archangelsky leg.); larvae were obtained by rearing under laboratory conditions. For the descriptions two first instar (L1) and two third instar (L3) larvae were examined; one L 3 larva was saved for the habitus pictures; second instar larvae were not available.

First instar larva. Morphology. Measurements and ratios in Table 1. Color. Head capsule, head appendages, thoracic and abdominal sclerotized structures light brown; membranous areas whitish. Non-sclerotized integument covered by fine and short microtrichiae. Body. Subcylindrical, wider at about midlength, slightly narrowing towards anterior end (as in Figs 1, 2 of third instar larva); legs reduced.

Head capsule ( Figs 3–5 ). Subquadrate, broader than long; occipital foramen wide. Coronal line absent, area enclosed by frontal lines inversely bell-shaped. Stemmata closely aggregated. Clypeolabrum asymmetrical, right side slightly concave, left side with small notch bearing a patch of dense pubescence formed by fine microtrichae; lateral lobes of epistome short, rounded, not projecting farther than nasale; clypeolabrum bearing two sharp cuticular projections interpreted as egg-bursters ( Fig. 5 , EB). Antenna ( Fig. 6 ). Three-segmented, A1 the longest and widest segment; A2 short, narrower than A1, wider than long; A3 slightly longer than A2, very narrow; SE1 subequal to A 3 in length and width. Mandibles ( Figs 7–8 ). Strongly asymmetrical, right one slightly smaller than left one, bearing one strongly serrated inner tooth, distal inner margin finely serrated; left mandible lacking inner tooth, finely serrated distally on inner margin, serration becoming stronger on central and subbasal area of inner margin. Maxilla ( Figs 9–11 ). Six-segmented, longer than antenna, cardo subtriangular, partially covered by submentum and parietale. Stipites large, longer than palpus, strongly asymmetrical, left stipes broader than right one, dorsally bearing a band of sharp cuticular spines along distal margin. Palpus four-segmented, first palpomere the largest, subrectangular, with inner appendage reduced; second and third palpomeres shorter, wider than long; fourth palpomere the narrowest, slightly longer than wide. Labium ( Figs 4 , 12 ). Submentum large, subtriangular, fused to head capsule, with a pair of deep lateral, subapical notches. Mentum short, narrower than submentum, transversely subrectangular. Prementum reduced, as a wide and narrow subtriangular plate on ventral face. Palpi two-segmented, basal palpomere short, distal palpomere slightly longer. Ligula reduced, not sclerotized. Asymmetrical hypopharyngeal lobe as a membranous pubescent lobe developed on left side. Cervical sclerites ( Fig. 3 ) dorsal, narrow and transverse.

Thorax. Slightly wider than head capsule, pronotal plate semicircular, wider than long, covering most of prothorax, divided by fine sagittal line; mesonotum and metanotum lacking sclerotized plates. Prosternal plate subtriangular, transverse, lacking sagittal line, shorter than pronotal plate; meso- and metasternum membranous. Legs unsclerotized, reduced to a small membranous lobe bearing patch of short setae. Mesothoracic spiracles non-functional.

Abdomen. Ten-segmented, segments IX and X modified (as in Fig. 1 of third-instar larva). Segments I to VII subequal in size and shape, lacking sclerites, subdivided by transverse folds. Segment VIII with large dorsal plate, posteriorly lobed; with a short pair of procerci laterad to dorsal plate. Segment IX trilobed, with a pair of one-segmented urogomphi. Spiracles on segments I–VII biforous, non-functional; those on segment VIII large, annular.

Primary chaetotaxy. Head capsule ( Figs 2–12 ). Frontale with 28 sensilla: one pair of short setae on basal quarter close to frontal lines (FR1); two pores (FR2) and two very short setae (FR3) close to midline at about midlength; two pairs of setae laterad to FR3 closely aggregated (FR5 long, FR6 short); one pair of pores posterior to inner margin of antennal sockets (FR4); one pair of pores (FR15) posterior to egg-bursters ( EB); six pairs of sensilla distal to inner margin of antennal socket, four setae (FR8 short, FR9 broken, probably long as in third instar larva and FR10 and FR12 very short) and two pores (FR11, FR13); nasale with gFR1 bearing four dorsal short and rather stout setae, and a pair of small setae inserted ventrally; left two setae of gFR1 slightly shorter. Epistomal lobes lacking sensilla, therefore gFR2 absent. Seta FR7 apparently absent, pore FR14 present. Each parietale with 30 sensilla: dorsal surface with a basal group of four minute setae (PA1, PA2, PA4, PA5) and one pore (PA3) arranged in an almost L- or semicircular shape; one basal pore (PA6) almost touching frontal line; seta PA7 short, close to PA5; setae PA12, PA13, PA14 closely aggregated on lateral margin at about midlength, one very long and two shorter; setae PA8 (long), PA11 (short) and pore (PA10) in a transverse row past midlength; two setae (PA9 long, PA20 short) and pore (PA19) in a transverse row on anterolateral corner of head capsule. Ventral surface with three pores (PA23, PA24, PA25) and two setae (PA21 long, PA22 short) on anterolateral corner, close to mandibular acetabulum; one pore (PA15) on lateral surface at about midlength; setae PA16 (long) and PA26 (shorter), and pore PA17 closely grouped; pore PA27 and seta PA28 (broken, probably long) close to midline; pores PA29 and PA30, and long seta PA18 forming a transverse line at about basal third.

Antenna ( Fig. 6 ). A1 bare, with five pores, three dorsal ones on dorsal surface ( AN 1 basal, AN2 distal on midline, AN4 distal on inner margin) and two ventral ones close to inner surface ( AN 3 at midlength, AN5 distal). A2 with two pores, one distal ( AN 6) and one subbasal (■), and four setae, two very short subapical on outer face ( AN 7, AN 8) close to base of SE1, and two subapical on inner margin ( AN 10 long, AN11 very short). A3 with a distal group of at least three very short setae, one short sensorium and one long seta (gAN).

Mandibles ( Figs 7–8 ). Both mandibles with six sensilla; one short seta on outer margin at basal quarter ( MN 1), one minute seta on outer margin at distal quarter ( MN 5) in right mandible (closer to apex in left mandible), three dorsal pores ( MN 2, MN 3, MN 4) forming an oblique row at level of inner retinaculum of right mandible; pore MN 6 small, subapical on inner margin, difficult to see.

Maxilla ( Figs 9–11 ). Cardo with one short seta ( MX 1). Stipes with a row of seven stout and short setae along inner margin, basal seta of that row likely homologous to MX 7, remaining setae forming gMX2; left stipes with MX 7 and basal two setae of gMX2 arranged in an oblique row, distal four in a longitudinal row; ventral face with three pores, one on basal third ( MX 2), one on distal third close to inner margin ( MX 3) and one at midlength on outer margin ( MX 4), and two setae distal to pore MX 4 ( MX 5 long, MX 6 slightly shorter). MP 1 dorsally with one basal seta on inner face ( MX 16), pore MX 17 apparently absent; ventrally with two setae ( MX 13 long, MX 14 shorter) and two pores ( MX 12 on outer margin, MX 15 on inner margin at base of gAPP); inner appendage reduced, with several setae of different lengths (gAPP). MP 2 with two pores, one ventral and apical on outer margin ( MX 18) and one dorsal at inner margin on membrane connecting with MP 3 ( MX 19); one basal minute seta ( MX 27) on outer margin. MP 3 with two short setae and two pores, one seta dorsal, on outer margin ( MX 23), the other ventral on inner margin ( MX 21); both pores on ventral surface, one close to seta MX 21 ( MX 22), the other apical, close to outer margin ( MX 20). MP 4 with one short basal seta dorsally ( MX 24) and two dorsal subapical pores ( MX 25 central, digitiform, MX 26 closer to outer margin); a group of five or six very short sensilla constitute gMX1.

Labium ( Figs 4 , 12 ). Submentum with two pairs of setae ( Fig. 4 ), one long (LA1), the other minute, on anterior margin (LA2). Mentum ventrally with one pair of short setae (LA3) and one pair of lateral pores (LA4). Prementum with two pairs of setae (LA6 rather long, close to midline, LA5 minute, lateral) and one pair of pores (LA7) between LA5 and LA6. Sensilla associated with ligula absent (LA9–12) except for one pair of pores (LA12?), resembling LA12 placed on membranous lobe between labial palpi (behind palpi in ventral view). LP 1 with one minute seta (LA13, ventral) and one distal pore (LA14, on inner margin); LP 2 with one subapical pore on outer face (LA15) and a group of at least five short sensilla constituting gLA.

Third-instar larva. Morphology ( Figs 1–2 , 13–22 ). Measurements and ratios in Table 1. Head capsule, head appendages, thoracic and abdominal sclerotized structures darker. Frontal lines visible basally.Antennae more slender and longer, A2 longer than wide. Pronotal plate larger, subrectangular, with sagittal line; mesonotal and metanotal plates present, smaller; mesonotal plate ca. 1/2 the length of pronotal plate, metanotal plate ca. 1/2 the length of mesonotal plate. Pleural plates present on pronotum; prosternal plate larger, slightly shorter than pronotal plate. Legs with a membranous coxal area and a minute sclerotized segment. Right stipes with cuticular projections along inner and distal margins; left stipes with long cuticular projections along inner, distal and outer margins. MP 1–3 with short and sharp cuticular projections on dorsal face.

Chaetotaxy. Head capsule ( Figs 13–22 ). Frontale with one secondary short seta between FR3 and FR4; pore FR14 distinguishable, close to FR9; seta FR1 proportionally longer, FR5 proportionally shorter; gFR1 with seven setae, five dorsal and two ventral. Each parietale with three secondary sensilla; one minute seta between PA4 and PA5, one short seta on outer margin near PA11 and one pore between seta PA9 and pore PA19; seta PA7 very short; seta PA8 proportionally shorter, group of setae PA12–14 with two long and one short seta; seta PA22 proportionally longer. Antenna ( Fig. 16 ). Without secondary sensilla, sensory appendage distinctly shorter and slightly narrower than third antennomere. Mandibles ( Figs 17–18 ). Each mandible with three secondary setae, two short setae on basal outer face, behind MN 1, third one minute, close to pore MN 4. Maxilla ( Figs 19–21 ). Stipes without secondary setae on ventral face; dorsal face with two secondary setae on gMX2, basal seven setae of right gMX2 bifid apically, basal three setae of left gMX2 bifid; MX 7 on right stipes bifid; maxillary palpomeres lacking secondary sensilla. Labium ( Figs 14 , 22 ). Without secondary sensilla, LA6 proportionally longer; pore LA7 located laterad of LA5.

Figures

Figs 1–2. Habitus of third instar larva of O. costatum (LeConte, 1855). 1 – dorsal view; 2 – lateroventral view. Scale bar = 1 mm.

Figs 3–4. Chaetotaxy of first instar larva of O. costatum (LeConte, 1855).3 – head capsule, dorsal view; 4 – head capsule, ventral view.Scale bar = 0.05 mm.

Figs 5–8. Chaetotaxy of first instar larva of O. costatum (LeConte, 1855). 5 – detail of clypeolabrum; 6 – left antenna, dorsal view; 7 – left mandible, dorsal view; 8 – right mandible, dorsal view. Scale bars = 0.01 mm.

Figs 9–12. Chaetotaxy of first instar larva of O. costatum (LeConte, 1855). 9 – left maxilla, ventral view; 10 – left stipes, dorsal view; 11 – right maxilla, dorsal view; 12 – labium, ventral view. Scale bars = 0.01 mm.

Figs 13–14. Chaetotaxy of third instar larva of O. costatum (LeConte, 1855). 13 – head capsule, dorsal view; 14 – head capsule, ventral view. Scale bar = 0.05 mm.

Figs 15–18. Chaetotaxy of third instar larva of O. costatum (LeConte, 1855). 15 – detail of clypeolabrum; 16 – left antenna, dorsal view; 17 – left mandible, dorsal view; 18 – right mandible, dorsal view. Scale bars = 0.02 mm.

Figs 19–22. Chaetotaxy of third instar larva of O. costatum (LeConte, 1855). 19 – left maxilla, dorsal view; 20 – right stipes, dorsal view; 21 – left maxilla, ventral view; 22 – labium, ventral view (hypopharyngeal lobe not included). Scale bars = 0.02 mm.