Stictochironomus bengalensis, Konar, 2021

Konar, Sanghamitra, 2021, A new member of the Stictochironomus caffrarius group from West Bengal, India with emendation of generic diagnosis (Diptera, Chironomidae), Zootaxa 5072 (2), pp. 173-181 : 174-176

Stictochironomus bengalensis sp. n.

( Figures 1–3 )

urn:lsid:zoobank.org:act:33BC206E-A3B8-43E2-88DC-298302EF91E0

Type material. Holotype, male with pupal and larval exuviae (reared), INDIA: West Bengal, Suri, Mayurakshi River (23° 92´N, 87°51´E), 25.x.2019, leg. S. Konar (emerged 29.xi.2019) . Paratypes, 3 males with pupal and larval exuviae, INDIA: West Bengal, Narajole, Kangsabati River (22°29’N 87°19’E), 11.xi.2019, leg. S. Konar , (emerged 18.xii.2019).

Etymology. From the type locality, Bengal.

Description. Male (n = 4). Total length 4.7–4.8, 4.8 mm. Wing length 1.9–2.0, 2.0 mm, width 0.44–0.51, 0.48 mm. Total length/wing length 2.40–2.47, 2.44, wing length/profemur length 2.30–2.36, 2.33.

Colouration. Head brown. Thorax, legs, and abdomen pale brown. Wing with dark spots on membrane; 3 spots on cell r 4+5, 1 on apex of cell m 1+2, 3 on cell m 3+4 and 1 on cell an.

Head. Eye with dorsomedial extension strong, parallel-sided, 158–163, 161 μm long; vertex with 11–17, 14 setae consisting of 5–7, 6 (3) IV; 4-6, 5 (3) OV and 2–4, 3 (3) PO. Clypeus with 20‒22, 21 setae. Corona with 3–5, 4 setae. Antenna 922–938, 930 μm long, with ultimate flagellomere 650–658, 654 μm long; AR 2.5–2.6, 2.6. Length (μm) of palpomeres 1–5: 46–52, 49; 62–66, 64; 94–98, 96; 112–116, 114; 208–212, 210. CA ratio 0.65–0.68, 0.67, CP ratio 1.44–1.62, 1.53. Frontal tubercle absent.

Thorax. Antepronotum with 1–3, 2 lateral setae. Scutum with conical tubercle; acrostichals 8–12, 10, uniserial; dorsocentrals 8–10, 9, uniserial; prealars 2–4, 3, uniserial. Scutellum with 3–5, 4 setae.

Wing ( Figure 1B ). FCu slightly distal to RM, and distance between RM and FCu 16 μm. R 2+3 well separated from R 1, and distance between apices of R 1 and R 2+3 160 μm. R with 10–12, 11 setae; R 1 with 7–9, 8 setae; R 4+5 with 5–7, 6 setae; R 2+3, M and Cu bare. Squama with 10–12, 11 setae. Brachiolum with 1 seta. VR 1.00–1.04, 1.02.

Legs. Apex of foretibia ( Figure 1D ) with broadly rounded scale bearing 3 strong setae. Mid- and hind tibiae each with fused combs bearing single pointed spur 22–27, 25 µm long. Foretarsus without beards. Pulvilli plumose on outer surface, reaching to middle of claw. Lengths and proportions of leg segments as in Table 1 .

Hypopygium ( Figure 1A ). Anal tergite with strong and separate bands; median tergite seta absent. Anal point 75–79, 77 μm long, nearly parallel-sided in dorsal view, bifid in apical 0.17–0.20, 0.18. Gonocoxite 186–190, 188 µm long. Superior volsella ( Figure 1C ) well developed, 76 μm long, gently curved, digitate, and apically rounded, with 2 long basal setae. Inferior volsella 119–122, 121 µm long, with 1 strong apical and 5 recurved subapical setae. Phallapodeme 78–82, 80 µm long; lateral sternapodeme 120–123, 122 µm long; transverse sternapodeme 38–42, 40 µm wide. Gonostylus 185–190, 188 µm long, distally flattened, with 1 apical and 4 subapical setae along inner margin. HR 1. 0, HV 2.5–2.6, 2.6.

Female. Unknown.

Pupa (n = 4). Total length 3.8–4.0, 3.9 mm.

Colouration. Exuviae mainly pale brown. Cephalic tubercle, margins of wing and antennal sheath, and hooklets on T II pale yellow. Anal comb on abdominal segment VIII dark brown.

Cephalothorax. Frontal apotome ( Figure 2A ) triangular; cephalic tubercle conical, 190–195, 193 µm high and 77–82, 80 µm wide at base, subapically bearing frontal seta 220–227, 224 µm long. Antennal sheath ( Figure 2B ) 1.1 mm long, narrowed at apex. Wing sheath 1.2–1.3, 1.3 mm long. Thoracic horn ( Figure 2C ) plumose; basal ring ( Figure 2D ) elliptical, 61–65, 63 µm long and 45–50, 48 µm wide.

Abdomen ( Figure 2E ). Pedes spurii B present on segment II. T I and IX bare; T II‒V with more or less extensive, rectangular shagreen; T VI with anterior and posterior shagreen bands and very weak median shagreen patch; TVII‒VIII with anterolateral shagreen patches. T II with posterior row of 41‒43, 42 hooklets, occupying 0.67‒0.71, 0.69 of segment width. Conjunctives III/IV and IV/V with continuous band of spinules. Anal comb on segment VIII ( Figures 2F‒G ) large, extending from apex of segment to base of L 4 -seta, composed of 2–4, (3) needle-like basal teeth and 3–5, (4) broad, short apical teeth. Segments II‒IV each with 3 L-setae. Segments V‒VI each with 3 LS-setae; T VII‒VIII each with 4 LS-setae. Relative position of LS-setae on abdominal segments V‒VIII as in Table 2 . Anal lobe ( Figure 2F ) 230‒265, 248 µm long, with 36‒52, 44 lateral taeniae; dorsal setae absent. Genital sac 160‒189, 175 µm long.

Larva (n = 4). Head capsule 250‒280, 270 µm long, cephalic index 0.82‒0.84, 0.83.

Colouration. All teeth of mentum, apex of mandible and postoccipital margin dark brown.

Antenna ( Figure 3A ). Total length 73‒77, 75 µm; AR 1.20‒1.29, 1.25. Basal segment 42‒45, 44 µm long and 9‒10, 10 µm wide, with ring organ situated 0.33‒0.36, 0.35 from base; blade 32‒36, 34 (3) µm long, accessory blade 3‒5, 4 µm long. Lengths (µm) of 2nd to 6th segments: 7‒9, 8; 10‒12, 11; 3‒5, 4; 8‒10, 9; 2‒4, 3, respectively. Lauterborn organ 11‒13, 12 µm long on 2nd segment, 3‒5, 4 µm long on 3rd segment.

Labro-epipharyngeal complex. SI ( Figure 3B ) and SII plumose on both sides. Seta premandibularis simple. Pecten epipharyngis ( Figure 3C ) tripartite; median platelet with 3 teeth, lateral platelet with 5 teeth. Premandible ( Figure 3D ) 60‒65, 63 µm long, with 2 apical teeth and 1 small inner tooth; brush dense.

Mandible ( Figure 3E ). Length 123‒126, 125 µm. 1 dorsal, 1 apical and 3 inner teeth present. Seta subdentalis slender and slightly curved apically.

Mentum ( Figure 3F ). Width 73‒77, 75 µm. Median teeth 3, large, arranged convexly; 1st to 6th lateral teeth gradually decreasing in size; median tooth much taller than others. Ventromental plate ( Figure 3G ) fan-shaped, 55‒58, 57 µm long, 131‒134, 133 µm wide, with fine and contiguous striae; distance between both plates 26‒30, 28 µm.

Distribution. Known only from West Bengal, India.

Remarks. The male resembles that of Stictochironomus obscurus in the largely pale-yellow wing with three dark markings on the cell r 4+5 and the slender, gently curved superior volsella in the hypopygium, but differs from it in the wing with markings on the apices of cells m 1+2 and m 3+4, the apically bifid anal point and the anal tergite without median setae. In S. obscurus , the wing cells m 1+2 and m 3+4 have no apical marking, the anal point is simple and the anal tergite has several median setae, according to Guha & Chaudhuri (1983: 637, figures 1, 3, as Polypedilum ) and Chaudhuri & Chattopadhyay (1990: 187, figs. 41, 43). The bifid anal point is unique in the genus Stictochironomus . Sometimes a bifid anal point also occurs in the male of Japanese Sympotthastia takatensis (Tokunaga) , belonging to the subfamily Diamesinae , and it is not rare (H. Niitsuma, pers. comm.). Although the two localities where the larvae of the present species were collected are far away from each other and the features are common to all the males emerged from the larvae, there is a possibility that the bifid anal point is an intraspecific variation. Further, the anal tergite without median setae is an atypical feature in the male of this genus. In the diagnosis for the male, Cranston et al. (1989: 416) wrote, “median anal tergite setae long, concentrated in field between ends of bands, separate from apical setae; exceptionally in S. crassiforceps without median anal tergite setae.” The present species differs from S. crassiforceps (Kieffer) in the slender superior volsella and the longer gonostylus in the male hypopygium. The male of the latter possesses stout superior volsellae and short gonostyli (see Cranston et al. 1989: 491, figure 10.65C). Therefore, the present species is distinct from the other members of this genus.

Within the S. caffrarius group, the pupa is similar to those of S. polystictus and S. caffrarius in the abdominal segments II‒III each with three pairs of L-setae, but separable from the pupa of S. polystictus by the slender cephalic tubercle, which is higher than the basal width, and from the pupa of S. caffrarius by the anal tergite without any shagreen. Stictochironomus polystictus has cephalic tubercles as high as the basal width (Blattacharyay et al. 1996: 323), and S. caffrarius possesses anterolateral shagreen on the anal tergite (Pinder & Cranston 1986: 456, fig. 10.91D, as Chironomini Genus F sp.).

The larva is most similar to that of S. polystictus described by Blattachayay et al. (1996) in the relatively high AR (1.20‒1.29), the mentum with three median teeth and the mandible with a dorsal tooth and three inner teeth, although Hazra et al. (2016) reported the larva of S. polystictus with lower AR (0.74‒0.92) and four inner teeth on the mandible under the name of Polypedilum clavipennae , which was synonymized with S. polystictus by Mukherjee et al. (2020). In any case, the larva of the present species is separated from both the larval types of S. polystictus by the smaller head capsule (length 250‒280 µm). The larval head capsule of S. polystictus is 285‒315 µm long in Blattachayay et al. (1996) and 380‒420 µm long in Hazra et al. (2016).

Figures

FIGURES 1A–D. Stictochironomus bengalensis sp. n., adult male. A, Hypopygium, dorsal view (left) and ventral view (right); B, wing; C, superior volsella; D, apex of foretibia.

FIGURES 2A–G. Stictochironomus bengalensis sp. n., pupa.A, Frontal apotome; B, antennal sheath; C, thoracic horn; D, basal ring; E, abdominal segments I–VII, dorsal view; F, abdominal segments VIII–IX, dorsal view; G, anal comb.

FIGURES 3A–G. Stictochironomus bengalensis sp. n., larva. A, Antenna; B, SI; C, pecten epipharyngis; D, premandible; E, mandible; F. mentum; G, ventromental plate.