Mystonectes Fery & Ribera, 2018

Alarie, Yves & Michat, Mariano C., 2025, Description of the mature larvae of the halophilic diving beetles Mystonectes coelamboides (Fall, 1923) and M. panaminti (Fall, 1923) and comparison with other known Deronectina (Coleoptera: Dytiscidae, Hydroporinae), Zootaxa 5666 (2), pp. 225-238 : 227-228

General morphological characteristics of the instar III larvae of Mystonectes Fery & Ribera, 2018

Diagnosis: Instar III larvae of Mystonectes can readily be distinguished from those of other genera of Deronectina described in detail (i.e., Deuteronectes Guignot, 1945 , Deronectes Sharp, 1882 , Hornectes Fery & Ribera, 2018 , Leconectes Fery & Ribera, 2018 , Nectoporus Guignot, 1950 , Neonectes J. Balfour-Browne, 1944 , Oreodytes Seidlitz, 1887 , and Scarodytes Gozis, 1914 .) by the following combination of characters: lateral margin of nasale convex along posterior half ( Figs 4 , 18 ); dorsal surface of prementum not fully sclerotized ( Fig. 12 ); lateral margin of prementum with elongate spinulae ( Figs 12–13 ); ratio L3/HW> 3.40 ( Table 1 ); natatory setae along dorsal margin of femora, tibiae, meso- and metatarsi present ( Figs 14–15 , 20–21 ), present (one only) or absent on protarsus; urogomphal annuli present ( Fig. 6 ); ratio U1/U2> 4.00 ( Table 1 ); ratio U1/HW> 2.50 ( Table 1 ).

Description, instar III ( Figs 3–15 , 17–21 )

Body ( Figs 3 , 17 ): Fusiformate, narrow. Measurements and ratios that characterize the body shape are shown in Table 1 .

Head ( Figs 4–13 , 18 ): Head capsule ( Figs 4 , 18 ) pear-shaped, longer than broad, maximum width posterior to stemmata at about level of occipital suture, with neck constriction distinctly delimited; ecdysial line well marked, coronal line short; occipital foramen broadly emarginate ventrally; frontoclypeus bluntly rounded, lateral margin of nasale convex along posterior half; ventroapical margin of frontoclypeus with 31–35 spatulate setae [= lamellae clypeales of Bertrand (1972)], not interrupted medially; gular suture not visible; ocularium present, stemmata not visible ventrally; tentorial pits visible medioventrally. Antenna ( Figs 7–8 ) elongate, four-segmented, about half as long as HW; A4 the shortest, A2 and A3 the longest, subequal in length, A3 with a ventroapical spinula, A3’ short. Mandible ( Fig. 9 ) falciform, curved inward and upward apically; mandibular channel present. Maxilla ( Figs 10–11 ) with cardo fused to stipes; stipes short, narrow; galea and lacinia absent; MP elongate, three segmented, slightly shorter than antenna, MP1 the longest, MP3 the shortest. Labium ( Figs 12–13 ) with prementum small, subquadrate to subtrapezoidal, slightly longer than broad, with elongate lateral spinulae, anterior margin slightly sinuated medially; LP slightly shorter than MP, two segmented, LP2 shorter than LP1.

Thorax ( Figs 3 , 14–15 , 17 , 20–21 ): Pronotum trapezoidal dorsally, ovate laterally, widest at posterior margin; subequal to meso- and metanotum combined; pronotum without transverse carina; both meso- and metathoracic terga with an anterotransverse carina; sagittal line visible on the three tergites; thoracic sterna membranous; mesothorax with a spiracular opening on each side. Legs ( Figs 14–15 , 20–21 ): long, composed of six segments (including pretarsus), L1 the shortest, L3 the longest; CO robust, elongate, TR divided into two parts, FE, TI and TA slender, subcylindrical; pretarsus with two long, slender, slightly curved claws, posterior claw shorter than anterior one on L1 and L2, claws subequal in length on L3; claws lacking basoventral spinulae; TI and TA with elongate spinulae along ventral margin, less developed on metatarsus.

Abdomen ( Figs 3 , 5–6 , 17 , 19 ): Eight-segmented, segments I–VI sclerotized dorsally, membranous ventrally; segments VII–VIII completely sclerotized, ring-like; tergites I–VII narrow, transverse, rounded laterally, without sagittal line, with anterotransverse carina, with long setae along lateral and posterior margins; segments I–VII with a spiracular opening on each side; segment VIII (= LAS) ( Figs 5 , 19 ) short, subtrapezoidal, lateral margin subparallel, slightly converging from base to point of insertion of urogomphi, abruptly converging thereafter, with anterotransverse carina dorsally and laterally; siphon short, bluntly rounded to acute apically. Urogomphus ( Fig. 6 ) elongate, two-segmented; U1 much longer than LAS, with several non-sclerotized annuli.

Chaetotaxy ( Figs 4–15 , 18–21 ): Head capsule with numerous secondary setae; lateroventral margin of PA with 6–8 spine-like secondary setae; primary seta PA3 elongate, much longer than setae PA1 and PA2 ( Figs 4 , 18 ); AN, MX, and LA lacking secondary setae; MN with one hair-like secondary seta on basoexternal margin; thoracic and abdominal sclerites with numerous secondary setae; natatory setae present on dorsal margin of femora, tibiae, and tarsi ( Figs 15 , 21 ); secondary leg setation detailed in Table 2 ; urogomphomere 1 with several secondary setae ( Fig. 6 ).

Figures

FIGURES 4–6. Mystonectes panaminti (Fall, 1923), instar III: 4. head capsule, dorsal aspect; 5. abdominal segment VIII, dorsal aspect; 6. urogomphus, dorsal aspect; PA, parietal. Numbers and lowercase letters refer to primary setae and pores, respectively. Scale bars = 0.2 mm.

FIGURE 18–19. Mystonectes coelamboides (Fall, 1923), instar III: 18. head capsule, dorsal aspect; 19. abdominal segment VIII, dorsal aspect; PA, parietal. Numbers and lowercase letters refer to primary setae and pores, respectively. Scale bars = 0.2 mm.

FIGURES 7–13. Mystonectes panaminti (Fall, 1923), instar III: 7. antenna, dorsal aspect; 8. antenna, ventral aspect; 9. mandible, dorsal aspect; 10. maxilla, dorsal aspect; 11. maxilla, ventral aspect; 12. labium, dorsal aspect; 13. labium, ventral aspect; A3’, lateral process of antennomere 3; AN, antenna; gAN, antennal group; LA, labium: gLA, labial group; MN, mandible: MX, maxilla; gMX, maxillary group; sp, spinula. Numbers and lowercase letters refer to primary setae and pores, respectively: *, secondary seta. Scale bars = 0.1 mm.

FIGURES 14–15. Mystonectes panaminti (Fall, 1923), instar III: 14. metathoracic leg, anterior surface; 15. metathoracic leg, posterior surface; CO, coxa; FE, femur; TA, tarsus; TI, tibia; TR, trochanter. Numbers and lowercase letters refer to primary setae and pores, respectively. Scale bar = 0.2 mm.

FIGURE 20–21. Mystonectes coelamboides (Fall, 1923), instar III: 20. metathoracic leg, anterior surface; 21. metathoracic leg, posterior surface. CO, coxa; FE, femur; TA, tarsus; TI, tibia; TR, trochanter. Numbers and lowercase letters refer to primary setae and pores, respectively. Scale bar = 0.2 mm.

FIGURES 2–3. Mystonectes panaminti (Fall, 1923): 2. adult habitus, dorsal aspect; 3. instar III habitus, dorsal aspect (specimen preserved in 75% alcohol for 36 years). Scale bars = 1 mm.

FIGURES 16–17. Mystonectes coelamboides (Fall, 1923): 16. adult habitus, dorsal aspect; 17. instar III habitus, dorsal aspect (specimen preserved in 75% alcohol for 36 years). Scale bars = 1 mm.