Acilius sinensis Peschet, 1915

Alarie, Yves, Mai, Zuqi, Michat, Mariano C. & Hájek, Jiří, 2023, Larval morphology and new records of the iconic diving beetle Acilius sinensis Peschet, 1915 (Coleoptera: Dytiscidae: Dytiscinae) - a species well established in western Yunnan, China, Zootaxa 5301 (2), pp. 277-291 : 279-287

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 ).

Figures

FIGURE 31. Map of Yunnan province, China with marked records of Acilius sinensis Peschet, 1915. Yellow circles = historical records (<1925); red circles = recent records (2007–2021).

FIGURES 1–3. Habitus of Acilius sinensis Peschet, 1915 larva: 1. instar I, lateral view; 2. Instar I, dorsal view; 3. instar II, lateral view. Scale bar = 5 mm.

FIGURES 4–12. Acilius sinensis Peschet, 1915, head, instar I: 4. head capsule, dorsal aspect; 5. head capsule, ventral aspect; 6. antenna, dorsal aspect; 7. antenna, ventral aspect; 8. maxilla, dorsal aspect; 9. maxilla, ventral aspect; 10. mandible, dorsal aspect; 11. labium, dorsal aspect; 12. labium, ventral aspect. EB, egg bursters; FR, frontoclypeus; gAN, antennal group; gMX, maxillary group; gLA, labial group; LC, lamellae clypeales; PA, parietal; TP, tentorial pits; sp, spinula; * = additional sensilla. Numbers and lowercase letters refer to primary setae and pores, respectively. Scale bars = 0.5 mm (Figs 4–5); 0.1 mm (Figs 6–12).

FIGURES 13–17. Acilius sinensis Peschet, 1915, instar I: 13. mesothoracic leg, anterior surface; 14. mesothoracic leg, posterior surface. 15. abdominal segment VIII, dorsal aspect; 16. abdominal segment VIII, ventral aspect; 17. urogomphus, dorsal aspect. CO, coxa; FE, femur; TA, tarsus; TI, tibia; TR, trochanter. Numbers and lowercase letters refer to primary setae and pores, respectively; * = additional sensilla; spine-like spinulae not represented. Scale bars = 0.2 mm.

FIGURES 18–22. Acilius sinensis Peschet, 1915, instar II: 18. parietale, ventral aspect; 19. mandible, dorsal aspect; 20. maxilla, dorsal aspect; 21. metathoracic leg, anterior surface; 22. metathoracic leg, posterior surface. Scale bars = 0.5 mm (Fig. 18); 0.2 mm (Figs 21–22); 0.1 mm (Figs 19–20).

FIGURES 23–27. Acilius sinensis Peschet, 1915, instar III: 23. posterior half of parietal, ventral aspect; 24. antenna, dorsal aspect; 25. maxilla, dorsal aspect; 26. metacoxa and metatrochanter, anterior surface; 27. metacoxa and metatrochanter, posterior surface. Scale bars = 0.2 mm.

FIGURES 28–30. 28–29. Adult female of Acilius sinensis Peschet, 1915 from Haba Xueshan; 30. Habitat of A. sinensis larvae and adults in Dahaoping village.