Bunites distigma (Brullé, 1837)

Michat, Mariano C., Alarie, Yves & Balke, Michael, 2023, Bunites distigma (Brullé, 1837): discovery of the second- and third-instar larvae and phylogenetic relationships within the Colymbetinae (Coleoptera: Dytiscidae), Zootaxa 5353 (6), pp. 551-566 : 553-554

Bunites distigma (Brullé, 1837)

Diagnosis. The instar III of B. distigma can be distinguished from those of other Colymbetinae by the following combination of characters: large size (HL = 3.6–3.9 mm); mandible relatively short (ratio MNL/MNW <3.0) ( Figs 4 , 8 ); anterior margin of prementum straight or only slightly emarginate ( Figs 4 , 8 ); femur with ≤ 80 spine-like secondary setae (i.e., excluding natatory setae) ( Figs 10–11 ); protarsus with posteroventral secondary setae ( Fig. 11 ); urogomphi 3.8–3.9 mm in length (ratio U/HW <1.1), with a basal suture ( Figs 5 , 9 ). The instar II can also be distinguished by most of the characters mentioned above but see Tables 1 and 2 for measurements and ratios.

The instar I, on its part, was diagnosed by Michat (2005). We find worth, however, providing here an updated diagnostic combination for this instar: presence of additional setae on surface of frontoclypeus; presence of additional setae on ventral surface of parietal; presence of several additional setae on mandible; presence of two additional setae on maxillary stipes (one contiguous to seta MX 6 and one contiguous to setae MX 2 and MX 3); presence of additional setae on anterodorsal surface of tibia; presence of one additional spine-like seta on coxa (contiguous to seta CO10); presence of additional pores on dorsal surface of abdominal segment VIII.

Description, instar II ( Figs 6–7 )

Color ( Figs 6–7 ): Dorsal surface of cephalic capsule with color pattern as in Fig. 6 . Head appendages light brown except for distal third of mandible and distal portion of A4, MP3 and LP2 brown. Thoracic and abdominal tergites I–VI ( Fig. 7 ) light brown to brown with pattern of yellowish maculae, abdominal tergites VII–VIII brown ( Fig. 7 ). Legs and urogomphi ( Fig. 7 ) light brown.

Body: Measurements and ratios that characterize body shape are shown in Table 1 .

Head ( Fig. 6 ): Cephalic capsule. Flattened, subcircular, about as long as broad, maximum width posterior to stemmata, constricted at level of occipital region; covered with densely distributed microspinulae; occipital suture present, ecdysial line clearly visible. Frontoclypeus subtriangular, lateral margins curved, anterior margin rounded, lacking egg bursters; lateral lobes rounded, not projecting beyond anterior margin. Ocularium with four stemmata on upper side of head and two on underside, arranged in two convex vertical series. Gular suture not visible; posterior tentorial pits visible ventrally; occipital foramen deeply emarginate ventrally. Antenna. Slender, composed of four antennomeres, shorter than HW; A2 longest, A1 and A3 somewhat shorter than A2, subequal in length, A4 shortest; A3 with a ventroapical spinula; apical lateroventral process of A3 not protruding. Mandible. Prominent, falciform, wide at base, sharp apically; mandibular channel present. Maxilla. Cardo small, subrectangular; stipes well developed, subtrapezoidal; palpifer short, broad, palpomere-like, completely sclerotized; palpus composed of three palpomeres, MP1 longest, MP3 shortest; galea well developed, subconical, slightly curved inwards; lacinia absent. Labium. Prementum subrectangular, much broader than long, anterior margin shallowly emarginate, with microspinulae on dorsal surface; palpus long, slender, composed of two palpomeres, LP1 longer than LP2.

Thorax: Terga convex, lateral margins curved; pronotum about as long as subequal meso- and metanotum combined; tergites subrectangular, transverse, margins rounded, with sagittal line well marked; protergite more developed than meso- and metatergites, meso- and metatergites with anterior transverse carina; prothorax with two small, triangular sclerites ventrally on anterior half; mesothorax lacking spiracles. Legs. Long, composed of six articles, L3 longest, L1 shortest; coxa robust, elongate; trochanter divided into two parts by an annulus; femur, tibia and tarsus slender, subcylindrical; pretarsus with two long, slender, slightly curved claws, posterior claw shorter than anterior claw on L1 and L2, anterior claw shorter than posterior claw on L3; surface of legs covered with microspinulae except on trochanter and posterior surface of coxa; rows of well-developed spinulae present on ventral surface of protibia and protarsus, these absent on L2 and L3 except for few long spinulae on distal portion of tarsus; basoventral spinulae on claws present but reduced.

Abdomen ( Fig. 7 ): Eight-segmented; segments I–VII sclerotized dorsally, membranous ventrally, progressively narrowing to apex, lacking spiracles; tergites I–VI similar to each other, narrow, transverse, laterally rounded, with anterior transverse carina, sagittal line visible; segment VII completely sclerotized, ring-like, with anterior transverse carina; segment VIII longest, completely sclerotized, ring-like, with anterior transverse carina dorsally, covered with densely distributed microspinulae; siphon reduced. Urogomphus. Moderately long, composed of one urogomphomere, covered with microspinulae; basal suture present.

Chaetotaxy: Cephalic capsule with numerous hair-like secondary setae on dorsal and ventral surfaces, about 70–80 lamellae clypeales on anterior margin of frontoclypeus, 15–17 spine-like on the lateral margin of parietal, and 15–17 spine-like secondary setae on each side of ventral surface of parietal; mandible with one hair-like secondary seta on basoexternal margin, proximal to pore MNa, and several minute secondary setae; stipes with 4–6 minute secondary setae on ventral surface, near setae MX 2 and MX 3; prementum with one secondary pore on ventral surface, near seta LA1, and some spine-like secondary setae on anterodorsal margin; thoracic tergites with numerous secondary setae, mainly hair-like on protergite, mainly spine-like on meso- and metatergite; secondary leg setation detailed in Table 2 ; coxa with one secondary pore on posterior surface; trochanter with one secondary pore on ventroproximal margin; abdominal sclerites I–VIII with numerous, mainly spine-like secondary setae; urogomphus with several secondary setae.

Description, instar III ( Figs 4–5 , 8–11 )

As for instar II except as follows:

Color ( Figs 4–5 ): Darker in general. Dorsal surface of cephalic capsule with color pattern as in Fig. 4 . Distal portion of A4, MP3 and LP2 only slightly darkened.

Body: Measurements and ratios that characterize body shape are shown in Table 1 .

Head: Antenna. A3 somewhat shorter than A1. Maxilla. Stipes somewhat more elongate and subcylindrical.

Thorax: Spiracles present anteroventrally on mesothorax.

Abdomen ( Figs 5 , 9 ): Spiracles present laterally on segments I–VII.

Chaetotaxy: Cephalic capsule with hair-like secondary setae on dorsal and ventral surfaces more numerous, about 110–120 lamellae clypeales on anterior margin of frontoclypeus, 14–18 spine-like on lateral margin of parietal, and 31–36 spine-like secondary setae on each side of ventral surface of parietal ( Fig. 8 ); prementum with three secondary pores on ventral surface, near seta LA1; secondary leg setation detailed in Table 2 and Figs 10–11 ; coxa with 2–5 secondary pores on posterior surface ( Fig. 11 ); setation on abdominal segment VIII and urogomphi detailed in Fig. 9 .

Habitat. The habitat where the specimens used for this study were collected is located in the Yungas bioregion, a narrow band of forest along the eastern slope of the Andes Mountains from Peru and Bolivia to northwest Argentina. The climate in this area is rainy, humid, and warm ( Fig. 12 ). Larvae were collected from a semipermanent roadside pond in the montane forest area of the Yungas (which comprises altitudes from 1,500 to 3,000 m a.s.l.) ( Fig. 13 ). This pond has water for most of the year except during the winter when it is generally dry. It varies in size, reaching up to 30 m length, two meters in width, and 20 cm depth. Abundant terrestrial vegetation covers the water, leaving some open areas. It is situated near a small, shaded, intermittent creek, no more than at best half a meter wide that crosses the road, which seems to be the preferred habitat of adults as suggested by the large number of specimens collected there. The larvae, however, were strictly collected from the pond previously described (along with some adults). It is important to emphasize that the first stage larvae previously described ( Michat 2005) all originated from the same habitat. Adults and larvae of Rhantus calileguai , Platynectes curtulus (Régimbart, 1899) and Liodessus sp. were also collected, along with adults of Thermonectus alfredi Griffini, 1898 , Lancetes borellii Griffini, 1895 , Copelatus sp. , Desmopachria sp. , and Gyrinus argentinus Steinheil, 1869 .

Figures

FIGURES 1–7. Bunites distigma (Brullé, 1837). (1–3) Adults with different elytral color pattern, dorsal aspect: (1) elytra predominantly black with small ferrugineous macula on external margin very weakly marked; (2) elytra predominantly black with small ferrugineous macula on external margin well marked; (3) elytra black with large ferrugineous macula extending anteriorly and at base. (4–5) Instar III larva: (4) head, dorsal aspect; (5) abdominal segments VII–VIII and urogomphi, dorsal aspect. (6–7) Instar II larva: (6) head, dorsal aspect; (7) abdomen, dorsal aspect. Scale bars = 4 mm for Figs 1–3 and 1 mm for Figs 4–7. For a picture of instar I see Michat et al. (2017).

FIGURES 8–11. Bunites distigma (Brullé, 1837), instar III larva: (8) head, dorsal aspect (color pattern not represented); (9) abdominal segment VIII and urogomphi, dorsal aspect; (10) left prothoracic leg, anterior aspect; (11) right prothoracic leg, posterior aspect. CO: coxa; FE: femur; FR: frontoclypeus; LAS: last abdominal segment; PA: parietal; PT: pretarsus; TA: tarsus; TI: tibia; TR: trochanter; U: urogomphus. Scale bars = 1 mm.

FIGURES 12–13. Habitat of larvae of Bunites distigma (Brullé, 1837): (12) landscape in Abra de Salvear, Yala-Termas de Reyes road, Jujuy Province, Argentina; (13) portion of the roadside pond where the larvae were collected.