Stenochironomus zonarius Borkent

Dantas, Galileu P. S., Hamada, Neusa & Mendes, Humberto F., 2016, Contribution to the knowledge of Stenochironomus Kieffer (Diptera, Chironomidae) from Brazil: seven new species and description of females and immatures of some previously known species, Zootaxa 4117 (1), pp. 1-47 : 41-44

Stenochironomus zonarius Borkent

( Figs. 31–32 )

Examined material. Brazil, Amazonas state: Itacoatiara, Madeireira Mil, 0 2 º 47’32,1’’ S, 58 º 27’42,4’’ W, in decayed wood, G.P.S. Dantas, S.C. Escarpinati ( INPA), 1 male with pupal exuviae, 08.iv.09; 1 male, 23.iv.09; 1 female with pupal and larval exuviae, 28.iv.2009.

Diagnostic characters. Male: lateral vitta with anterior mark extending medially, so that a transverse stripe is present across scutum; pigmentation on postnotum restricted to dorsum; pigmentation on T II extending from anterior to posterior margins ( Borkent, 1984). Female: thorax with a dark dorsolateral stripe, covering the posterior half of the medial vitta and anterior half of the lateral vitta; post-genital plate truncated at apex. Pupa: pedes spurii B present on T II; S II with 3 L setae and S III–IV with 4 L setae.

Female (n = 1).

Total length 5.01 mm. Wing length 3.34 mm. Total length/ wing length 1.50. Wing length/ length of profemur 1.42.

Coloration–Thorax: postnotum completely pigmented, except for a small median band; a dark stripe along the thorax, covering the posterior half of the medial vitta and anterior half of the lateral vitta. Legs: anterior leg with apical ⅓ of femur, tibia and tarsus light brown; median and hind legs yellowish. Abdomen: T I completely pigmented; T II with distal ½ pigmented. Eyes metallic green.

Head. Flagellomere lengths (in µm): 180; 178; 167; 150; 215. Longest seta 539 µm long. AR 0.30. Temporals 13. Clypeus with 58 setae. Tentorium 222 µm long; 26 µm wide at posterior tentorial pit. Stipes 274 µm long; 10 µm wide. Palpomere lengths (in µm): 69; 93; 391; 236; 413.

Thorax. Acrostichals 8 (extending until anterior margin of scutum); dorsocentrals 12 in single row; prealars 6 in single row. Scutellum with 13 setae in two rows.

Wing. VR 1.05. Brachiolum with 6 setae. R with 70; R1 with 95; R4+5 with 83; RM with 2; M with setae 8; remaining veins bare . Squama with 7 setae.

Legs. Scale of front tibia 69 µm long; spurs of middle tibia 48 µm and 51 µm long; spurs of hind tibia 48 µm and 52 µm long. Apex of fore tibia 87 µm wide, of mid tibia 95 µm wide, of hind tibia 107 µm wide. Lengths (µm) and proportions of legs as in Table 10 .

Genitalia ( Fig. 31 A, 31B): Tergite IX with 64 setae; gonocoxite IX with 8 setae. Cercus 197 µm long. Notum 265 µm long. Coxosternapodeme IX 242 µm long. Post-genital plate truncated at apex.

Pupa (n = 1). Total length 8.79 mm.

Coloration: cephalothorax and abdomen light-brownish; lateral margin from distal half of TIV to TVIII of abdomen brownish. Anal lobe with a large transverse band extending posteriorly on lateral margin until anal lobes. Cephalothorax ( Figs. 32 A, 32B) Frontal apotome elongated ( Fig. 32 A). Precorneal setae 68 µm long. Dorsocentrals (Dc) 101 µm long. Distance between Dc1 and Dc2 7 µm; between Dc2 and Dc3 325 (1) µm; between Dc3 and Dc4 6 µm. Median suture granulose.

Abdomen ( Figs. 32 C, 32D). T I bare ; T II–V with median, large field of shagreen; VI with a anterior field of fine shagreen T-shaped and a posterior field of thick shagreen; T VII–VIII with small anterolateral fields of scattered spinules (difficult to see); anal segment with an anterior pair of shagreen patches. T II–IV with fine shagreen on paratergites. T II with posterior row of hooklets extending to lateral margin of tergite, divided medially into two groups. Abdominal setation: S I without L setae; S II with 3 L setae; S III–IV with 4 L setae; S V–VII with 4 LS setae; S VIII with 5 LS setae. Pedes spurii B present on segment II. Spur on S VIII with 6 brownish teeth ( Fig. 32 E). Genital sac 596 µm long, extending posteriorly beyond anal lobe by 74 µm; anal lobe 535 µm long, rounded at apex, with fringe of about 140 filament

Systematic remarks. Borkent (1984) did not include the species in either of the two subgenera proposed since the immature and their substract were not known. The specimens analyzed here were obtained from submerged wood collected in black-water streams.

Notes on biology of species. The stream where S. zonarius was collected is similar to that described to S. bare sp. nov. A wood-mining larva was associated with a pupa and an adult, but it is very damaged and was not described here.

Remarks. In this study, 11 Stenochironomus species were analyzed. We describe the female, pupa and larva of S. impendens , female and pupa of S. zonarius , the pupa and larva of S. jubatus and female of S. palliaculeatus . In addition, seven new species were described; six of them had their pupae described and three had their larvae described. Thus, the number of Stenochironomus species from the Neotropical region increased from 29 to 36, for Brazil from 21 to 28 and from the Amazon from 18 to 25.

Taking into account the species of Stenochironomus described, it is noted that the pupal and adult stages have stable and marked characteristics to distinguish the species of this genus. At the larval stage, however, it is not easy to distinguish species, since they are morphologically very similar. The larvae of S. roquei and S. jubatus are very similar, with labrum and mentum almost identical. The larva of S. impendens resembles that of S. bacrionis and S. manauara sp. nov. An exception is the larva of S. amazonicus sp. nov. which is quite different from the larvae of all other described species of the genus, with the teeth of mentum and mandible fused.

Regarding the color pattern of the adults, was observed that in some species there is sexual dimorphism in this character, with females and males exhibiting a different pattern, a fact that makes difficult the association of males and females of the same species.

Borkent (1984) found a larger number of Stenochironomus species and specimens in wood that no longer had the bark. However, in the present study Stenochironomus larvae were not found in wood without the bark; they were mostly found mining just below the bark, but also in deep mines in the wood core.

In laboratory, it was observed that the life cycle of leaf-mining species is considerably shorter than that of the wood-mining species. Adults of the leaf miners emerged at most 20 days after collection in the field, while in adults of wood miners this time can exceed one year, as observed in S. amazonicus sp. nov. In general, the adults, pupae and larvae of leaf miners are considerably smaller than those of wood miners. This pattern is expected since the leaf is a substrate that is degraded more rapidly, and species that use this habitat therefore need to synchronize their life cycle to complete it before the leaves are completely degraded. Wood is a more stable substrate and remains available for a considerably longer time than leaves. Anderson and Cummins (1979) suggest that a long life cycle with low metabolic activity is one of the adaptations of xylophagous aquatic insects to the low nutritional value of wood.

The immature stages, especially the larvae, of Neotropical Stenochironomus species are still poorly known. Of the 36 described species for the Neotropical region (including those described here), less than 64% (n=23) are known as pupae and about 47% (n=17) are known as larvae. The description of these stages is essential to our understanding of the biology and ecology of the species in this genus. According to Cranston & Oliver (1988), the major impediment to understanding the role of the family Chironomidae in the decomposition of woody debris is lack of taxonomic information on immature stages.

Future studies on the genus Stenochironomus must focus effort on describing the immature stages of known species and on discovering or confirming the larval habitat of these species. As emphasized by Pinho et al. (2005), as long as most of the immature stages of Neotropical species remain unknown, caution is needed with respect to any discussion of the division of the genus into subgenera.

Figures

FIGURE 31. Stenochironomus zonarius Borkent, adult female. A, genitalia, dorsal view. B, genitalia, ventral view.

FIGURE 32. Stenochironomus zonarius Borkent, pupa. A, part of frontal apotome. B, thorax, lateral view. C, tergites I – V. D, tergites VI – IX. E, spur on S VIII.