Bunites distigma (Brullé)

Michat, Mariano C., 2005, Larval Morphology and Phylogenetic Relationships of Bunites distigma (Brullé) (Coleoptera: Dytiscidae: Colymbetinae: Colymbetini), The Coleopterists Bulletin 59 (4), pp. 433-447 : 435-440

Bunites distigma (Brullé) , First-instar Larva

Diagnosis. First-instar larvae of B. distigma can be distinguished from those of other Colymbetini by the presence of additional setae and pores on some body regions, and by the geographic source of specimens. The presence of additional setae on the lateromedial portion of frontoclypeus, on the ventral surface of parietal, on the internal margin of maxillary stipes, and on the anterodorsal surface of tibia, and the presence of additional dorsal pores on urogomphus, separate B. distigma from every other known Colymbetini except Meladema . In addition, B. distigma and Meladema share the presence of five or more spine-like setae on each side of the anterodistal margin of prementum, while the other Colymbetini have three or less setae. On the other hand, B. distigma can be distinguished from every other Colymbetini species (including Meladema ) by the presence of one additional spine-like seta on the coxa (near seta CO10), and the presence of additional dorsal pores on the eighth abdominal segment. The presence of additional setae on mandible is also a distinctive feature of B. distigma , only shared with Neoscutopterus larvae.

Description. Body ( Fig. 1 ): subcylindrical, TL ¼ 5.50–8.20 mm; MW ¼ 1.50–1.70 mm. Color.

Dorsal surface of head with distinct color pattern, composed of a light brown background and several yellowish maculae of different shapes and sizes, located centrally on the frontoclypeus, behind each ocularium, anterior to the occipital suture, and medial to stemmata. Ventral surface light brown except for a few small, yellowish, posterolateral maculae on each parietal. Head appendages yellowish to light brown except for last maxillary and labial palpomeres and apices of mandible and fourth antennomere light brown. Thoracic tergites light brown with pattern of yellowish maculae. Abdominal tergites light brown, somewhat paler on the sides. Membranous parts and legs yellowish to light brown. Urogomphi light brown.

Head: HL ¼ 1.64–1.69 mm; HW ¼ 1.69–1.70 mm; FRL ¼ 0.76–0.78 mm; OCW ¼ 0.74–0.91 mm. Head capsule ( Figs. 2, 3 ). Flattened, subcircular, about as long as broad (HL/HW ¼ 0.97– 1.00); maximum width at stemmata, constricted at level of occipital region (HW/OCW ¼ 1.88– 2.27); occipital suture present, ecdysial line clearly visible (COL/HL ¼ 0.52–0.55); covered with densely distributed microspinulae. Frontoclypeus subtriangular, lateral margins curved, apical margin rounded, 0.45–0.48 times as long as HL, with 1 spine-like egg burster on each lateral, and 31–37 well developed spatulate setae on anterior margin, arranged in a single row; lateral lobes rounded, not projecting beyond nasale. Ocularium with four stemmata on upper side of head and two on underside, arranged in two convex vertical series. Parietal without temporal setae; gular suture not visible; posterior tentorial pits visible ventrally; occipital foramen deeply emarginate ventrally. Antenna ( Figs. 4, 5 ). Slender, 4-segmented, shorter than HW (A/HW ¼ 0.75–0.80); A1 the shortest (A1/A3 ¼ 0.76–0.80), remaining segments subequal in length (A2/A3 ¼ 1.00–1.06, A4/A3 ¼ 1.00–1.05); A3 with a ventroapical spinula; apical lateroventral process of A3 not protruding. Mandible ( Fig. 6 ). Prominent, falciform, wide at base, sharp apically, 1.88–2.24 times as long as broad, 0.36–0.41 times as long as HL; mandibular channel present. Maxilla ( Figs. 7, 8 ). Cardo small, subrectangular; stipes well developed, subtrapezoidal; palpifer short, broad, palpomere-like (PPF/MP1 ¼ 0.26–0.30), incompletely sclerotized; palpus 3-segmented (A/MP ¼ 1.37–1.39); MP1 slightly shorter; MP3 slightly longer, the narrowest (MP3/MP2 ¼ 1.06–1.12); galea well developed, subconical, slightly curved inwards (GA/MP1 ¼ 0.43–0.51); lacinia absent. Labium ( Figs. 9–12 ). Prementum subrectangular, much broader than long, anterior margin shallowly emarginate, with microspinulae on dorsal surface; palpus 2-segmented, long, slender (MP/LP ¼ 1.09–1.13); palpomeres subequal in length (LP2/LP1 ¼ 0.97–1.02).

Thorax: terga convex, lateral margins curved; pronotum about as long as meso- and metanotum combined, meso- and metanotum subequal. Tergites subrectangular, transversal, margins rounded; sagittal line well marked; protergite more developed than meso- and metatergites, meso- and metatergites with anterior, transverse carina. Thoracic venter membranous. Spiracular openings absent. Legs ( Figs. 13–15 ). Long, 6-segmented; L3 the longest (L3 ¼ 4.10–4.52 mm), L1 the shortest; L3/L1 ¼ 1.24–1.34, L3/L2 ¼ 1.11–1.14, L3/HW ¼ 2.41–2.68. Coxa robust, elongate; trochanter divided into 2 parts; femur, tibia and tarsus slender, subcylindrical; pretarsus with 2 long, slender, slightly curved claws, posterior claw shorter than anterior one on pro- and mesothoracic legs; on metathoracic leg, claws subequal in length or anterior claw slightly shorter than posterior one; each claw with basoventral spinulae. Legs covered with microspinulae except on posterior surface of coxa and most of trochanter; a row of well developed ventral spinulae present on protarsus and distal half of protibia; on meso- and metathoracic legs, only 1–2 long spinulae on ventrodistal portion of tarsus. L3: CO/FE ¼ 0.81–0.93, TI/FE ¼ 0.71–0.74, TA/FE ¼ 0.80–0.88, CL/TA ¼ 0.46–0.50.

Abdomen: 8-segmented; segments 1–7 sclerotized dorsally, membranous ventrally, progressively narrowing to apex, without spiracular openings. Tergites 1–6 similar to each other, narrow, transverse, laterally rounded, with anterior, transverse carina, sagittal line visible; segment 7 somewhat longer, with anterior, transverse carina. Segment 8 ( Figs. 16, 17 ) the longest, completely sclerotized, ring-like, with anterior, transverse carina dorsally (LAS ¼ 0.97–1.17 mm, LAS/HW ¼ 0.57–0.69), covered with densely distributed microspinulae; siphon reduced. Urogomphus ( Fig. 18 ). Moderately long (U ¼ 1.79–2.13 mm), 1-segmented (U/LAS ¼ 1.78– 1.91, U/HW ¼ 1.06–1.26), covered with densely distributed microspinulae.

Chaetotaxy: first-instar larva of B. distigma presents the following characteristics and differences with the generalized Colymbetinae larva ( Alarie 1995, 1998). Frontoclypeus with 6– 12 additional hair-like setae near each egg burster, and 0–1 medial to FR5; dorsal surface of parietal with 68–74 additional hair-like setae on each basal half and medial to each ocular area; ventral surface of parietal with 10–13 additional spine-like and hair-like setae on each basal half; 2 additional pores on ventral surface of A3; a very little structure (possibly a minute pore or spinula) on dorsal surface of fourth antennomere, distal to pore ANg; mandible with 11–12 and 9–10 additional minute setae on dorsal and ventral surface, respectively; pore MXa absent (in 5 of the 6 examined maxillae, 1–2 short spine-like setae present in the place where MXa is commonly located); 1 additional spine-like seta on maxillary stipes, near MX 5 and MX 6; 1 additional spinelike seta occasionally present contiguous to seta MX 4; 4 additional, minute, subapical setae on third maxillary palpomere; labium with 5–10 additional spine-like setae on each side of anterodorsal surface of prementum, and 2 additional, minute, subapical setae on second palpomere, near LA10 and LA12. Thoracic tergites with some spine-like and numerous hair-like setae; coxa with 1 additional spine-like seta near CO10; seta CO7 slightly more proximal on procoxa; trochanter without or with 1–2 additional spine-like setae on distal portion; femur with additional spine-like setae on anterodistal (0–1 on L1–2; 1 on L3), anteroventral (3–5 on L1; 4–6 on L2; 3–4 on L3) and posteroventral (2–4 on L1–2; 2–3 on L3) surfaces; seta FE5 short, spinelike; tibia with additional spine-like setae on anterodoral (1–2 on L1; 0–2 on L2; 2 on L3), anteroventral (2–4 on L1; 3–5 on L2; 4 on L3) and posteroventral (1–2 on L1–3) surfaces; setae FE6, TI6 and TI7 elongate, hair-like; pore TIa either absent, duplicated or replaced by a spine-like seta in one occasion of 18 examined tibiae; tarsus with additional spine-like setae on anteroventral

Taxa Source

AGABINAE Agabus anthracinus Mannerheim Alarie (1995, 1998 ) AGABINI

COLYMBETINAE Bunites distigma (Brulle´) This paper COLYMBETINI

Colymbetes dolobratus (Paykull) Nilsson (1988) ; Nilsson and Cuppen

(1988); Nilsson and Hilsenhoff

(1991); Alarie (1995, 1998)

Meladema lanio (Fabricius) Alarie and Hughes (in press)

Melanodytes pustulatus (Rossi) De Marzo (1974) ; Nilsson (1988);

Nilsson and Hilsenhoff (1991)

Neoscutopterus hornii (Crotch) Nilsson and Hilsenhoff (1991) ;

Alarie (1995, 1998)

Rhantus (Nartus) grapii (Gyllenhal) Galewski (1963) ; Nilsson (1988);

Nilsson and Hilsenhoff (1991);

Alarie (1995, 1998)

Rhantus calileguai Trémouilles M. C. Michat (unpubl.)

Rhantus orbignyi Balke M. C. Michat (unpubl.)

Rhantus signatus (Fabricius) M. C. Michat (unpubl.)

Rhantus suturellus (Harris) Galewski (1963) ; Nilsson (1988);

Nilsson and Hilsenhoff (1991);

Alarie (1995, 1998)

(2–3 on L1; 3–5 on L2; 4–5 on L3) and posteroventral (1–2 on L1; 2 on L2; 2–3 on L3) surfaces. Abdominal tergites 1–7 with several hair-like and numerous spine-like setae; eighth abdominal segment with 62–87 additional spine-like setae and 7–11 additional pores on dorsal surface, and 19–23 additional spine-like setae on ventral surface; urogomphus with 21–38 additional, more or less elongate, spine-like setae, 1 minute subapical seta, and 7–12 additional dorsal pores.

Character Analysis

Bunites distigma has 0–2 setae on the external margin of the stipes, in the place where ancestral pore MXa is usually located. Pore MXa is present in known larvae of the Agabinae but it is absent from every known Colymbetini larvae. There are some character states that suggest a derived condition of B. distigma within the Colymbetini (see below). Since MXa is absent in every other larva of Colymbetini , I consider it also absent in B. distigma , and non-homologous with the setae present in this species. I support this hypothesis in the fact that setae of the external margin of the stipes in B. distigma vary in number, which is uncommon among ancestral sensilla.

Thirty-five binary and six multistate characters (treated as unordered) were coded for first-instar larvae of 10 species belonging to seven genera and subgenera of Colymbetini ( Table 1). Many of these characters were previously used for the study of the phylogenetic relationships within this tribe and the subfamilies Colymbetinae and Agabinae (Nilsson and Hilsenhoff 1991; Alarie 1995, 1998; Alarie and Hughes in press). The characters used and their states are listed in Table 2. The analysis of the data matrix ( Table 3) using the program TNT resulted in four most parsimonious cladograms of length 63 (CI ¼ 0.75, RI ¼ 0.73). Five characters included in the matrix were uninformative (autapomorphies). Since their exclusion from the analysis produces no changes in topology, they were retained. The four cladograms differed in the relative positions of Melanodytes and the Rhantus species (except R. orbignyi ). For this reason, the strict consensus was calculated, in which some groups are collapsed in polytomies ( Fig. 19 ). Jackknife values indicate that most clades are well supported; other clades show lower support.

Figures

Figs. 1–3. Bunites distigma (first-instar larva). 1) Habitus, dorsal aspect; 2–3) Cephalic capsule, dorsal and ventral aspects, respectively. FR ¼ frontoclypeus, PA ¼ parietal. Numbers and lowercase letters refer to primary setae and pores, respectively. Additional setae not labeled. Scale bar ¼ 1.00 mm for Fig. 1 and 0.30 mm for Figs. 2–3.

Figs. 4–8. Bunites distigma (first-instar larva). 4–5) Antenna, dorsal and ventral aspects, respectively; 6) Mandible, dorsal aspect; 7–8) Maxilla, dorsal and ventral aspects, respectively. AN ¼ antenna, MN ¼ mandible, MX ¼ maxilla. Numbers and lowercase letters refer to primary setae and pores, respectively. Solid squares and solid triangles refer to additional setae and pores, respectively. Scale bars ¼ 0.20 mm.

Figs. 9–12. Bunites distigma (first-instar larva). 9–10) Labium, dorsal and ventral aspects, respectively; 11) Anterior margin of prementum, dorsal aspect; 12) Apex of second labial palpomere, ventral aspect. LA ¼ labium. Numbers and lowercase letters refer to primary setae and pores, respectively. Solid squares refer to additional setae. Scale bar ¼ 0.20 mm for Figs. 9–10.

Figs. 13–15. Bunites distigma (first-instar larva). 13–14) Metathoracic leg, anterior and posterior aspects, respectively; 15) Distal portion of metatarsus, anterior aspect. CO ¼ coxa, FE ¼ femur, PT ¼ pretarsus, TA ¼ tarsus, TI ¼ tibia, TR ¼ trochanter. Numbers and lowercase letters refer to primary setae and pores, respectively. Solid squares refer to additional setae. Scale bar ¼ 0.20 mm for Figs. 13–14.

Figs. 16–18. Bunites distigma (first-instar larva). 16–17) Eighth abdominal segment, dorsal and ventral aspects, respectively; 18) Urogomphus, dorsal aspect. AB ¼ eighth abdominal segment, UR ¼ urogomphus. Numbers and lowercase letters refer to primary setae and pores, respectively. Additional setae and pores not labeled. Scale bars ¼ 0.20 mm.

Fig. 19. Strict consensus cladogram obtained from four most parsimonious topologies of 10 terminal taxa of first-instar larvae of Colymbetini, with character changes mapped for each clade. Jackknife values indicated inside the boxes. * ¼ homoplastic character state transformation, ― ¼ reversal to the plesiomorphic condition.