Austrolimnius nyctelioides

Román, Nicolás Martínez & Archangelsky, Miguel, 2017, Description of the mature larva of Austrolimnius nyctelioides (Germain, 1892) and A. elatus Hinton, 1941 (Coleoptera: Elmidae), Zootaxa 4216 (6), pp. 585-595 : 586-593

Austrolimnius nyctelioides , mature larva

Body ( Figs 1, 2 ) elongate, slightly flattened dorsoventrally, widest at thorax, abdominal segments narrowing towards posterior end; body subtriangular in cross-section. Posterior margin of body segments with setiferous tubercles, dorsal ones 3.56 times as long as wide. Dorsal surface of thorax and abdomen with tubercles arranged in longitudinal rows (partially arranged in prothorax). Color yellowish to light brown. Length: 3.0– 3.5 mm; maximum width: 0.6–0.7 mm.

Head capsule ( Fig. 5 ) exposed to moderately covered by pronotum, anterior margin lacking tooth between base of antenna and clypeus. Surface with several ramose setae, mostly on disc of frons, and short slender setae on margin of parietale; various long ramose setae surrounding stemmata and two long blunt setae between frontal line and stemmata. Coronal line very short and broad, frontal lines long, extending to inner margin of antennal sockets. Frontoclypeal suture feeble; clypeal margin smooth, slightly convex. Gula subtrapezoidal, narrower than maxillolabial complex; basal margin wider and concave, distal margin narrower and convex; gular sutures poorly defined. Eyes with five stemmata on each side of head behind base of antenna.

Labrum subrectangular, slightly wider in anterior third; anterior margin slightly convex, anterolateral margins rounded, each with a row of three strong dorsal setae, outer two ramose, inner one stout and blunt; dorsal surface with six strong ramose setae arranged in a transverse row on anterior third. Ventral surface with anterior row of ramose setae, rest of ventral surface covered by short pubescence oriented mediad and posteriorly.

Antennae ( Fig. 6 ) short, with three antennomeres, located on anterolateral corners of head capsule. Basal antennomere short, conical, wider than long, apically with a crown of ramose setae; second antennomere the longest, cylindrical, with a few short distal setae, bearing a long sensorium, subequal in length to second antennomere. Third antennomere the shortest, much shorter than sensorium of second antennomere, bearing a long apical seta.

Mandibles ( Fig. 7 ) symmetrical, subtriangular, as long as wide. Apex with three blunt teeth. Dorsal surface with inner margin straight and sharp. Ventral surface with inner margin slightly concave, bearing a comb of long stout submarginal setae. Inner margin with long plumose prostheca; outer margin lacking ramose setae close to midlength.

Maxillae ( Fig. 8 ) with cardo short, irregularly suboval, transverse, 2.30–2.72 times wider than long, with stout ramose seta close to outer margin. Stipes the largest part, subrectangular, 1.60–1.74 times as long as wide, distal third with several setae distributed as follows: two long setae on outer margin (distal one ramose and shorter than basal one), inner margin with one short stout seta, distal margin with one ramose seta close to base of lacinia. Lacinia and galea well developed; lacinia subtriangular, fused to stipes, with inner margin bearing a group of stout setae; galea one-segmented, shorter than lacinia, elongate, with several apical setae. Palpus with four palpomeres, first palpomere the shortest, wider than long, second and third palpomeres slightly narrower, subequal in length, last palpomere slightly longer, narrower; first palpomere bearing one outer ramose seta, third palpomere with two setae, one on each anterolateral corner, last palpomere bearing several short apical setae and sensoria.

Labium ( Fig. 8 ) large, subdivided into a longer postmentum and a shorter prementum, forming together with maxillae the maxillo-labial complex; postmentum subrectangular, 1.76–1.79 times as long as wide, ventral surface with several short ramose setae at each side of midline, basal corners each with a large and stout ramose seta, distal corners each with a one long stout seta. Prementum short, poorly sclerotized, wider than long, distal margins densely setose; palps with two palpomeres, basal palpomere slightly shorter, distal palpomere with several distal setae and sensoria.

Thorax ( Fig. 9 ) strongly sclerotized; tergal plates with sagittal lines and with large setiferous tubercles arranged as follows: pronotum with an irregularly arranged row of tubercles on lateral margins; meso- and metanotum with one row of tubercles on each side of sagittal line, a group of tubercles admedially, and a row sublaterally. Lateral margins of thoracic segments sagittate, with large ramose setae. Prothorax the largest segment, 1.5–1.81 times as wide as long; pronotum subtrapezoidal, anterior and posterior corners rounded; ventral side with six sclerites: one large and irregularly shaped transverse anterolateral pair (? divided basisternum), one small, triangular lateral pair (? episterna), one large posterolateral pair (? epimera); coxal cavities open. Meso- and metathorax shorter than prothorax, 3.09 times as wide as long; each segment ventrally with five sclerites: one large anterior subpentagonal sclerite (? basisternum), and two smaller subrectangular sclerites on each side (? episterna and epimera); coxal cavities open. Legs: five-segmented; coxa the largest segment, subtriangular; trochanter smaller, subtriangular; femur and tibia elongate, femur slightly longer and wider than tibia; claw stout, slightly shorter than tibia; surface with several short spines.

Abdomen ( Figs 10–15 ) well sclerotized, composed of nine segments, tapering towards posterior end; segments I–VII with sagittal line poorly defined, subequal in length; segment IX the longest. Tergal plates with tubercles arranged in six longitudinal rows on segments I–IX, one central pair, one median pair and one lateral pair on each tergite; lateral tubercles less evident in the last segment. Pleural sclerites present on segments I–VII; sterna of segments I–VII subrectangular, wider than long. Segment VIII annular, ring-like. Segment IX elongate, 2.75–2.94 times as long as segment VIII, without dorsal keel, ventrally with several spines on distal half; sternal area with apical gill chamber, operculum subpentagonal, distal end smoothly pointed with several strong and ramose setae externally and marginal sharp setae internally, covering a pair of strong distal hooks with inner margin smooth and bearing several long setae on outer margin. Spiracles present on segments I–VIII.

Habitat. Nant y Fall stream is a 3rd order stream tributary of the Futaleufú river , and is located in a transitional mountain and piedmont area in the Northwest of Chubut province, Argentina. It belongs to the ecotone between the Subantarctic forest and the Patagonian steppe phytogeographical provinces. The substrate at the study site is composed of gravel, cobbles, and boulders, with sand and smaller fractions in pools and depositional areas. Aquatic vegetation is abundant and diverse, with Isoetes savatieri being the dominant species in riffles and Myriophyllum quitense and Callitriche lechleri in pools. Water flow ranged from 0.22 to 1.16 ms-1; water temperature ranged from 4 to 18 °C, with an annual average of 11.6 °C; and during the entire year the water remains well oxygenated (9.95–13.88 mgl-1 O2) and above saturation (114.1–121.4 %) (Brand & Miserendino 2012, Brand et al. 2012).

The Chubut river flows from west to east and drains to the Atlantic Ocean. Major part of the river is located in the Patagonian steppe, where the lack of precipitation on the Patagonian Plateau produces a xerophytic vegetation. Water temperature ranged from 5 °C to 22.2 ° C. Water flow ranged from 0.2 m s - 1 to 2.1 m s -1 ( Miserendino & Archangelsky, 2006). The specimens of A. nyctelioides were collected at Piedra Parada, which is near the middlebasin of the river. The substrate consisted mostly of boulders, cobbles, and pebbles ( Miserendino & Archangelsky, 2006).

Figures

FIGURES 1 – 4. Habitus of mature Austrolimnius larvae. 1) dorsal view of A. nyctelioides; 2) ventral view of A. nyctelioides; 3) dorsal view of A. elatus; 4) ventral view of A. elatus. Scale bars, Figs 1 – 4: 1 mm.

FIGURES 5 – 8. Mature larvae of Austrolimnius nyctelioides. 5) head capsule, dorsal view; 6) antenna, dorsal view; 7) right mandible, ventral view; 8) maxillo-labial complex, ventral view. Scale bars, Fig. 5: 0.1 mm; Figs 6, 8: 0.05 mm, Fig. 7: 0.025 mm.

FIGURES 9 – 15. Mature larva of Austrolimnius nyctelioides. 9) thorax, dorsal view; 10) tergum of fifth abdominal segment, showing central and median rows of tubercles and setiferous tubercles on posterior margin; 11) sternum of abdominal segment VII; 12) abdominal segment IX; 13) operculum, external view; 14) operculum internal view; 15) hook of gill chamber. Scale bars: Figs 9 – 11, 13 – 14: 0.01 mm; Fig 12: 0.25 mm; Fig 15: 0.05 mm.