Onychogomphus duaricus Fraser, 1924

Chainthong, Damrong & Boonsoong, Boonsatien, 2016, Description of two final stadium Onychogomphus larvae from Thailand (Odonata: Gomphidae), Zootaxa 4066 (5), pp. 561-570 : 566-569

2. Onychogomphus duaricus Fraser, 1924

( Figs. 2 , 13–22)

Material examined (4 F0, 2 ♀, 2 ♂): 1 F0 larva, 30.III.2013, Kang Som Meaw (13°24′28″ N 99°16′ 52″E, altitude 200 m), Suanpung district, Ratchaburi province, D. Chainthong leg.; 1 ♀ and its exuvia, emerged on 26.IV.2013. 2 F0 larvae, 4.II.2014, Huai Khayeng (14°36′20″ N 98°34′ 38″E, altitude 206 m), Thong Pha Phum district, Kanchanaburi province, D. Chainthong leg.; 1 ♀ and its exuvia, emerged on 18.V.2014; 1 ♂, emerged on 20.V.2014. 1 F0 larva, 14.XII.2014, Huai Pak Kok (14°39′34″ N 98°32′ 02″E, altitude 176 m), Thong Pha Phum district, Kanchanaburi province, D. Chainthong leg.; 1 ♂ and its exuvia, emerged on 23.III.2015.

Coloration: Larvae uniformly dark brown, body stout and bulky, integument finely granulose, body covered with short hair-like setae ( Fig. 2 ).

Measurements (n=4): Total body length 23.9 mm, length of abdomen 13.4 mm, abdominal maximum width 5.7 mm, head maximum width 4.6 mm, length of hind femur 3.3 mm, length of epiproct 1.5 mm, length of paraprocts 1.6 mm, length of cerci 1.4 mm.

Head: wider than long, wider than prothorax but narrower than synthorax, frontal part stouter, posterior lobe of head shorter than eye length, eyes large and broadest across, with three large ocelli. Antennae four-segmented, first two segments short and rather circular, third segment largest, twice as long as first two segments and weakly swollen externally (length 1.4 mm), fourth segment tiny (length 0.1 mm); all segments conspicuously spinulated, without setae ( Fig. 14 ). Mandibles as in Figs. 15–16 with formula: L 1234 0 a(m1–8 or m1–10)b / R 1234 y a(m1– 6 or m1–8)b with a>b in both mandibles. Maxilla: galeolacinia with seven moderately incurved teeth, three dorsal teeth nearly equal in length and robustness, three ventral teeth of same sizes, apical one largest; stipes and palp very strongly setose ( Fig. 17 ). Labium: flat and not protruding when at rest ( Fig. 18 ). Prementum-postmentum articulation reaching posterior margin of procoxae. Prementum subrectangular, longer than wide (width 2.3, length 3.4), sides convex, convergent basally, with small teeth and minute setae at lateral margins; apical margin convex, with ventral row of 29–31 short, subquadrate reddish brown teeth and dorsal rows of whitish piliform setae on apical border; labial palp with uniformly inflexed inner edge, yellowish brown, apical lobe reddish, rounded, its internal margin with row of 15–16 equidistant teeth. Movable hook reddish brown, sharp and moderately incurved (length 0.6 mm).

Thorax: Small, suboval, prothorax narrower than head, dorsal portion raised at sides forming two U-shaped ridges. Wing sheaths strongly divergent, reaching middle of S4 (inner wing pads length 5.3 mm, outer wing pads length 4.5 mm). Legs short and stout; fore and middle legs strongly curved; protibiae (length 1.9 mm) decidedly longer than profemora (length 1.6 mm). Mesotibiae (length 1.7 mm) slightly longer than mesofemora (length 1.6 mm); metafemur slightly longer than metatibia. Tarsal formula 2-2-3, tarsi yellowish. Minute short rows of setae scattered along femur, tibia, and tarsus of all six legs.

Abdomen: less convex dorsally, dark brown, without markings ( Fig. 19 ). Mid-dorsal spines absent on S1 and short, blunt spines present on S2–9 ( Fig. 20 ). Lacking lateral spines on S2–6. Lateral edges of abdomen serrated with projecting spines on S7–9 ( Figs. 20–21 ). Sternites yellow brown, with few scattered long setae, sternite 8 divided in 5 plates, remainder in 3 plates, median sutures of sternites slightly convergent apically on 2, parallel on 3–7 and gradually divergent on 8–9, posterior margin of sternites 1–9 straight, wavy on 10 ( Fig. 22 ). No lateral spine and no dorsal hook on S10. Anal appendages elongated. Epiproct same length as paraproct, cerci slightly shorter. Epiproct narrowly triangular with two black spots close to the tip, conical. Paraprocts and cerci with tips sharply pointed ( Fig. 19 ).

Biological notes. These two larvae of Onychogomphus occur in headwater streams, where they were found in the sandy gravel substrate. On each sampling date, physico-chemical parameters were measured. During the odonate sampling, physico-chemical parameters varied according to time. The streams ranged from 4.0– 9.6 m wide and 4.3–25.7 cm in depth. The water temperature ranged from 21.0–24.0 °C, pH from 7.1–7.5, dissolved oxygen from 6.5–7.5 mg /L, total dissolved solids from 6.3–20.0 mg/L, and conductivity from 90.3–297.0 µS/cm. These variations correlated of water chemistry generally followed the seasonal cycle.

Figures

FIGURES 1 – 2. Onychogomphus castor Lieftinck and O. duaricus Fraser, respectively: habitus of larvae, dorsal view.

FIGURES 14 – 19. Onychogomphus duaricus Fraser: (14) antenna, dorsal view; (15) right mandible, ventral and internal views; (16) left mandible, ventral and internal views; (17) right maxilla, ventral view; (18) ventral view of prementum; (19) dorsal view of anal appendages. Scale = 1 mm (14, 18, 19), 0.5 mm (15, 16, 17).

FIGURES 20 – 22. Onychogomphus duaricus Fraser: (20); right side of abdomen of exuvia, dorsal view; (21) lateral view of abdomen in exuvia; (22) ventral view of abdomen. Scale = 1 mm.