Acilius sinensis Peschet 1915

Acilius sinensis Peschet, 1915

(Figs 1–29)

Material examined. CHINA: YUNNAN: 2 Ô 2 ♀, 3 instar I, 4 instar II and 1 instar III larvae, Baoshan Prefecture, Tengchong County, Gaoligong Mts NNR, Dahaoping vill., 24°58.6’N 98°43.8’E, 2000 m, deep pool along road, 5.vii.2016, J. Hájek & J. Růžička leg. (NMPC, YALC, ZSMG); 1 Ô 1 ♀, Dehong Prefecture, Yingjiang County, ca. 25.09°N 98.23°E, 2000 m, 14.iv.2020, Zhao-Wei Guo leg. (SYSU); 1 ♀, Dehong Prefecture, Yingjiang [County], iv.2021, Mang Yun [leg.] (ZSMG). 1 ♀, Diqing Tibetan Autonomous Prefecture, Shangri-La County, Haba Snow Mountain (ȒUḦƜ), 20.v.2020, Wang Peng leg. (SYSU).

Larval morphology

General morphological characteristics of the larvae of Acilius (adapted from Alarie et al. 2011, 2023): Larvae of Acilius can readily be distinguished from those of other genera of Aciliini described in detail (i.e., Graphoderus, Sandracottus, Thermonectus) by the following combination of characters: primary pore MNa inserted at approximately the same level as pore MNb (Fig. 10), presence of less than 19 additional spiniform setae on the dorsal surface of stipes (instar I) (Fig. 8), all maxillary palpomeres with abundant spinulae (instar I) (Fig. 8), median process of prementum Y-shaped, bifid from the base (Figs 11–12), and presence of 13–23 anteroventral additional natatory setae on femur (instar I) (Fig. 13) (Table 2).

Description, instar I (Figs 1–2, 4–17)

Color: Body predominantly creamy white to pale yellow; head capsule yellow, creamy white around ocularium; head appendages creamy white except A4, apex of A3, MP3 and LP2 piceous; thoracic terga and legs pale yellow; abdominal terga pale yellow except segment VIII with black band mesally at about mid-length, gradually broadening towards apex; urogomphi black.

Body: Subcylindrical, bent at first abdominal segment, gibbous in lateral view (Figs 1–2). Measurements and ratios aimed to characterize body shape as in Table 1.

Head (Figs 4–12): Head capsule (Figs 4–5) flattened, subtriangular, longer than broad; maximum width at stemmata, constricted at level of occipital region; occipital suture absent, ecdysial line well marked; occipital foramen deeply emarginate both dorsally and ventrally; epicranial plates meeting ventrally, posterior tentorial pits visible ventrally on central region; surface covered with small scale-like microsculptures; frontoclypeus subtriangular, apical margin rounded medially, with one spine-like egg burster on each lateral side; anterolateral lobes (= adnasalia) rounded, not projecting beyond apical margin; six rounded dorsolateral stemmata at each side, stemmata protruding a short distance from head surface, two anterodorsal ones strongly developed. Antenna (Figs 6–7) short, robust, four-segmented, half as long as HW; A1 and A2 subequal in length; A3 longest, with a strongly developed ventroapical spinula; A4 shortest, with lateroventral process at mid-length, similar to that of A3; A3’ not protruding; A1–A3 covered with short spine-like spinulae. Mandible (Fig. 10) prominent, falciform, wide at base, sharp apically, with short spine-like spinulae along baso-external margin and short-hair-like spinulae distally along inner margin; mandibular channel present, inner margin slightly toothed ventrally. Maxilla (Figs 8–9) with cardo well developed; stipes strongly developed, subtrapezoidal, antero-internal angle projecting inwards, internal margin with short spinulae; palpifer very short, broad, incompletely sclerotized; palpus short, robust, three-segmented, MP1 shortest, MP3 longest, with lateroventral subapical process similar to those of antenna; palpomeres covered with scattered minute spinulae; galea well developed, spiniform, subconical, slightly curved inwards, surface covered with scattered minute spinulae. Labium (Figs 11–12) with prementum subtrapezoidal, somewhat pear-like, longer than broad, anterodorsal margin rounded, projecting forward into Y-shaped median process, deeply indented apically; dorsal surface of prementum densely covered with either minute (apical half) or elongate (basal half) spine-like spinulae; labial palpus short, robust, two-segmented, palpomeres subequal in length, with strong spine-like spinulae along external margin.

Thorax (Figs 13–14): Terga convex, pronotum about as long as meso- and metanotum combined, meso- and metanotum subequal; protergite subrectangular, margins truncated, more developed than meso- and metatergite; meso- and metatergite transverse, with anterotransverse carina; sagittal line well marked; venter membranous; spiracles absent. Legs (Figs 13–14) long, composed of six articles; L2 longest, slightly longer than subequal L1 and L3; CO robust, elongate, TR divided into two parts by an annulus, FE, TI and TA slender, subcylindrical, PT with two long, slender, almost straight claws, posterior claw shorter than anterior one; leg articles densely covered with minute spine-like spinulae (not represented); protarsus with a row of well-developed spinulae along ventral margin on distal half.

Abdomen (Figs 15–17): Eight-segmented, segments I–VI sclerotized dorsally, membranous ventrally; segments III–V widest, remaining segments progressively narrowing to apex; tergites I–VI similar to each other, narrow, transverse, laterally rounded, with anterotransverse carina, sagittal line present on anterior third; segment VII somewhat longer, completely sclerotized except of narrow sagittal line ventrally, without anterotransverse carina; spiracles absent on segments I–VII; LAS (Figs 15–16) longest, completely sclerotized, ring-like, without anterotransverse carina, covered with short spinulae (not represented); siphon reduced. Urogomphus (Fig. 17) short, one-segmented, covered with short spinulae (not represented).

Chaetotaxy (Figs 4–17): Similar to that of generalized Dytiscinae larvae (Alarie et al. 2011) except for following characteristics: frontoclypeus with large number of well-developed lamellae clypeales arranged in three/ four mixed rows; more dorsal setae often smaller, digitiform, more ventral setae with more or less well-impressed apical notches; parietal with pores PAl and PAm absent; maxilla with seta MX 6 absent; stipes with 14–19 elongate and spine-like additional setae on dorsal surface (Fig. 8); maxillary palpomere III with presence of two very small additional setae (Figs 8–9); prementum with two additional spiniform setae on each side of median process (Fig. 11); legs (Figs 13–14) with pores TAc, TAd, TAe, and TAf and setae TR2, FE4, FE5, PT1 and PT2 absent; seta TI4 elongate and hair-like; seta TI5 hair-like (difficult to see owing to presence of several additional setae); femora with additional hair-like natatory setae along anteroventral and posterodorsal margins, and 3–5 anterodistal spine-like setae (Table 2): tibiae with additional hair-like natatory setae along anteroventral and posterodorsal margins and 0–2 anterodistal spine-like setae (Table 2); LAS (Figs 15–16) with pore ABc absent; additional pore sporadically present dorsally; lateral margin with large number of additional natatory setae.

Description, instar II (Figs 3, 18–22)

As for instar I except as follows:

Color: Body predominantly yellow to pale brown (Fig. 3); head capsule yellow, brownish mesally; thoracic terga pale brown, darker mesally; femora broadly piceous proximally; abdominal terga I–VII pale brown, darker mesally; abdominal segment VIII creamy white anteriorly becoming dark brown to black posteriorly.

Body: Measurements and ratios aimed to characterize body shape as in Table 1.

Head (Figs 18–20): Head capsule: surface spinulae absent except mesally, over occipital region. Antenna: A4/ A3 = 0.21–0.22; Maxilla: PPF/MP1 = 0.19–0.29; GA/MP1 = 2.00–2.05; MP/LP = 0.51–0.53; LP2/LP1 = 0.69–0.84; antennomeres, maxillary palpomeres and mandible lacking spinulae.

Thorax (Figs 21–22): single ventral sclerite present on prothorax. Legs (Figs 21–22): L3/HW = 2.90–3.08; surface spinulae lacking; marginal ventral spine-like spinulae along distal half of all tarsi.

Abdomen: segment VII completely sclerotized, ring-like, with anterotransverse carina; LAS and U without spinulae.

Chaetotaxy (Figs 18–22): Dorsal surface of head capsule with numerous short secondary setae; ventral surface of parietale with 12–15 spiniform setae laterally and 4–7 mesally (Fig. 18); antennomere I with minute secondary setae dorsally (as in Fig. 24); mandible with a row of elongate secondary hair-like setae along basoexternal margin (Fig. 19); maxillary stipes with a row of 31–34 elongate spine-like setae dorsally (including both additional and secondary), and a row of secondary hair-like setae along dorsoexternal margin (Fig. 20); secondary leg setation detailed in Table 3;rows of secondary natatory setae present along posterodorsal margin and basal half of anteroventral margin of tarsi; posterior surface of femora and tibiae with linear row of minute secondary pores below rows of natatory setae (not represented); abdominal segment VII with row of natatory setae on lateral margin; LAS with secondary hair-like setae dorsally and numerous secondary spine-like setae ventrally.

Description, instar III (Figs 23–27)

As for instar II except as follows:

Color: Body predominantly dark yellow broadly brownish mesally; head capsule dark brown mesally.

Body: Measurements and ratios aimed to characterize body shape as in Table 1.

Head (Figs 23–25): Antenna (Fig. 23): A1/A3 = 2.00; A2/A3 = 1.09; A2 and A3 secondarily subdivided. Maxilla (Fig. 24): MP3 secondarily subdivided; MP3/MP2 = 1.18; GA/MP1 = 1.45.

Thorax (Figs 26–27): Spiracles present on mesothorax. Legs (Figs 26–27): L3/HW = 3.45.

Abdomen: Sagittal line and spiracles present on segments I–VII.

Chaetotaxy (Figs 23–27): Parietal with several secondary spine-like setae laterally and ventrally (Fig. 23); mandible with larger number of minute secondary setae distally; secondary leg setation detailed in Table 3 (Figs 26–27).

Habitat and collecting circumstances. Larvae and adults of Acilius sinensis from Dahaoping were collected together in a flooded ditch near the village (Fig. 30) The ditch was shady, rather deep (depth ca. 0.5 m), without any submerged vegetation, bottom was muddy with layer of decaying leaves; the ditch was frequently visited by domestic ducks from the nearby homestead; the only other dytiscid species occurring at the habitat was Laccophilus kempi holmeni Brancucci, 1983. A single female from Haba Xueshan (Figs 28–29) was collected in a mountain lake.

Distribution. Mountainous areas (ca. 1700–2000 m a.s.l.) of Sichuan and Yunnan provinces, China. The only record from Sichuan is imprecisely localized type specimens; historical records from Yunnan came from north-east part of the province, recent findings are all from western Yunnan (Fig. 31).