Aethionectes oberthueri ( Régimbart, 1895 )

Alarie, Yves, Michat, Mariano C. & Dettner, Konrad, 2025, Testing the monophyly of the tribe Aciliini and the phylogenetic position of the Afrotropical genus Aethionectes Sharp, 1882 (Coleoptera: Dytiscidae, Dytiscinae) based on larval morphology, Zootaxa 5620 (2), pp. 335-352 : 337-343

Description of the instar III larva of Aethionectes oberthueri ( Régimbart, 1895)

( Figs 1–19 )

Color (alcohol preserved specimen): Body predominantly dark yellow to pale brown; head capsule dark brown, yellow around ocularium; head appendages yellow to pale brown; thoracic terga and legs yellow; abdominal terga yellow to pale brown, abdominal terga VII and VIII darker; urogomphi pale brown.

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

Head ( Figs 1–10 , 16–19 ): Head capsule ( Figs 1–2 , 16 ) flattened, subtriangular, longer than broad; maximum width at stemmata, slightly constricted at level of occipital region, HL/HW = 1.41–1.42; occipital suture present, 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 smooth except for crescentshaped microsculptures medially over occiput; frontoclypeus subtriangular, apical margin rounded medially; anterolateral lobes (= adnasalia) rounded, not projecting beyond nasale; six rounded dorsolateral stemmata at each side, protruding a short distance from head surface, two anterodorsal ones strongly developed. Antenna ( Figs 3–4 ) short, robust, four-segmented, about half as long as HW; A2 and A3 secondarily subdivided, A1 and A3 subequal in length, longest; A3 with a strongly developed ventroapical spinula; A4 shortest, with spinula at mid-length, similar to that of A3; apical lateroventral process of A3 (A3’) not protruding; antennomeres not covered with short spine-like spinulae. Mandible ( Figs 5–6 ) prominent, falciform, wide at base, sharp apically, with short-hair-like spinulae along inner margin; mandibular channel present, inner margin slightly toothed ventrally. Maxilla ( Figs 7–8 , 18 ) with cardo well developed; stipes strongly developed, subtrapezoidal, antero-internal angle projecting inwards, internal margin with short spinulae; palpifer very short, broad, completely sclerotized; palpus short, robust, three-segmented, MP1 shortest, MP3 longest, secondarily subdivided, MP1/MP2 = 0.77–0.92; palpomeres smooth, not covered with scattered minute spinulae; galea well developed, spiniform, slightly curved inwards, surface covered with scattered minute spinulae, GA/MP1 = 2.79–2.96. Labium ( Figs 9–10 , 18 ) with prementum subrectangular, longer than broad, anterodorsal margin lightly convex, projecting forward into a unifid median process, not indented apically, PMP/ LP1 = 0.32–0.37; lateral margin of prementum with elongate spinulae, dorsal surface densely covered with minute spine-like spinulae over apical and basal regions; labial palpus short, robust, two-segmented, MP/LP = 0.77–0.79; palpomere 1 longest; palpomeres 1 and 2 with strong spine-like spinulae along external margin.

Thorax ( Figs 11–12 ): 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 present on mesothorax. Legs ( Figs 11–12 ): 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 smooth, not covered with minute spine-like spinulae; ventral margin of tarsi with a row of well-developed spinulae along distal half; L3/HW = 2.74.

Abdomen ( Figs 13–15 ): Eight-segmented, segments I–VI sclerotized dorsally, membranous ventrally; segments III–V widest, remaining segments progressively narrowing to apex; tergites I–VII similar to each other, narrow, transverse, laterally rounded, with anterotransverse carina, sagittal line present on anterior third; segments VII–VIII completely sclerotized; spiracles present on segments I–VII; segment VIII (= LAS) ( Figs 13–14 ) longest, LAS/ HW = 1.25–1.28, subtriangular, without anterotransverse carina, not covered with short spinulae; siphon reduced. Urogomphus ( Fig. 15 ) short, one-segmented, not covered with short spinulae; U/HW = 0.42–0.45.

Chaetotaxy ( Figs 1–15 ): Dorsal surface of head capsule with tiny secondary setae ( Fig. 1 ); ventral surface of parietal with 7–10 spiniform setae laterally and 3–6 mesally ( Fig. 2 ); A1 with few minute secondary setae dorsally ( Fig. 3 ); mandible with a row of elongate hair-like secondary setae along basoexternal margin and a few tiny secondary setae distally ( Figs 5–6 ); dorsal surface of maxillary stipes with 17–21 elongate spine-like setae along inner margin ( Fig. 7 ); numerous secondary hair-like setae present along the dorsoexternal margin of stipes ( Fig. 7 ); secondary leg setation detailed in Table 2 and Figs 11–12 ; numerous pores on the coxae interpreted as setal sockets of either spiniform or hair-like setae dislodged through manipulation of preserved specimens; rows of natatory setae present along anteroventral and posterodorsal margin of femur, tibia and tarsi; posterior surface of tibiae with linear row of minute secondary pores below the rows of natatory setae (not represented); abdominal segments VII–VIII with row of elongate natatory setae on lateral margin ( Fig. 13 ); LAS with several secondary setae, short and minute dorsally ( Fig. 13 ), spine-like ventrally ( Fig. 14 ); urogomphi lacking secondary setae ( Fig. 15 ).

Remarks: Aethionectes oberthueri and Rhantaticus congestus are the only two Aciliini species recorded in Madagascar. In term of morphometry, instar III larva of A. oberthueri can easily be distinguished from that of R. congestus by larger size (HL> 2.70 mm vs <2.30 mm; L3> 5.30 mm vs <4.10 mm; LAS> 2.40 mm vs <2.10 mm), very large dorsal stemmata ( Fig. 1 ), shortly toothed inner ventral margin of mandible ( Fig. 6 ), and the following morphological ratios: HL/HW, MP3/MP2, GA/MP1, PMP/LP1, MP/LP, LAS/HW, and U/HW ( Table 1 ). In term of chaetotaxy, instar III of A. oberthueri differs from that of R. congestus by presence of two linear rows of secondary spines on the ventral surface of parietal ( Fig. 2 ) compared to non-linear and distributed over the entire surface ( Alarie et al. 2023b, Fig. 2 ); presence of single row of elongate spiniform setae on dorsal surface of stipes ( Fig. 7 ) compared to two ( Alarie et al. 2023b, Fig. 7 ); presence of four spiniform setae on prementum median process ( Fig. 9 ) compared to six ( Alarie et al. 2023b, Fig. 9 ), presence of hair-like and spiniform secondary setae on trochantera ( Figs 11–12 ) compared to spiniform only ( Alarie et al. 2023b, Fig. 11 ), and absence of secondary spiniform setae on dorsal surface of LAS ( Fig. 13 ) compared to present ( Alarie et al. 2023b, Fig. 13 ).

Figures

FIGURES 1–6. Aethionectes oberthueri (Régimbart, 1895), instar III: 1. head capsule, dorsal aspect; 2. head capsule, ventral aspect; 3. antenna, dorsal aspect; 4. antenna, ventral aspect; 5. mandible, dorsal aspect; 6. mandible, ventral aspect. A, antennomere; AN, antenna; MN, mandible; sp, spinula. Numbers and lowercase letters refer to primary setae and pores, respectively. Scale bars = 0.5 mm (Figs 1–2); 0.2 mm (Figs 3–6).

FIGURES 7–10. Aethionectes oberthueri (Régimbart, 1895), instar III: 7. maxilla, dorsal aspect; 8. maxilla, ventral aspect; 9. labium, dorsal aspect; 10. labium, ventral aspect. LA, labium; MX, maxilla; MP, maxillary palpomere. Numbers and lowercase letters refer to primary setae and pores, respectively. Scale bars = 0.2 mm.

FIGURES 11–15. Aethionectes oberthueri (Régimbart, 1895), instar III: 11. metathoracic leg, anterior surface; 12. metathoracic leg, posterior surface; 13. abdominal segment VIII, dorsal aspect; 14. abdominal segment VIII, ventral aspect; 15. urogomphus, dorsal aspect. AB, abdominal segment VIII; CO, coxa; FE, femur; TA, tarsus; TI, tibia; TR, trochanter; UR, urogomphus. Numbers and lowercase letters refer to primary setae and pores, respectively. Scale bars = 0.5 mm (Figs 11–12); 0.2 mm (Figs 13–15).

FIGURES 16–19. SEM of Aethionectes oberthueri (Régimbart, 1895), instar III: 16. head, dorsal view; 17. lamellae clypeales; 18. prementum and stipes, ventral view; 19. labial palpus, dorsal view.