Isoperla obscura ( Zetterstedt 1840 )

Teslenko, Valentina A. & Zhiltzova, Lidiya A., 2006, Nymphs of the genus Isoperla Banks (Plecoptera, Perlodidae) from the Eastern Palaearctic Region, Zootaxa 1130, pp. 1-33 : 21-23

Isoperla obscura ( Zetterstedt 1840)

( Figs. 54–60)

Description. The first time the nymph of I. obscura was described by Lillehammer (1978) from Fenoscandia and Denmark. Because the distribution of I. obscura occupies almost the whole of the Palaearctic, we expect some differences in nymphal pattern across its range. The association for Far Eastern nymphs was established by rearing nymphs from the Zeya River. Length of mature male nymph 7.8–8.4 mm, female 9.6–10.1 mm; cerci of male nymph 4.0– 5.2 mm, female 5.0–6.0 mm. General body color grayish­brown, distinct pattern consists of a row of light spots on head, thorax and abdomen; ventral side of body pale. Clothing hairs present.

Head with occiput spinule row almost complete medially; brown, M­shaped pigment mask found between antennal bases ( Fig. 54), reaching anterior margin of frontoclypeus and posterior margin of the head medially; pale U­shaped area anteromedially with two small darkened transverse spots anteriorly; interocellar area pale and enclosed; small diamond­shaped, pale spot medially on occiput; two pale patches from occiput to the antennae base between compound eyes and lateral ocelli; labrum with light areas medially and laterally; thin dark band from each eye to occiput. Lacinia bidentate ( Fig. 55 ), subtriangular; rounded basally; inner margin with five stout setae close to subapical tooth and several pale, short, thin setae below subapical tooth. Tip of galea reaches base of subapical tooth. Mandible similar to other species ( Fig. 56 ).

Thorax. Pronotum ( Fig. 54) transverse­oval, evenly fringed with pale, short setae; occasional longer setae may be present at anterior and posterior margins; pronotum with medial, pale, cross­shaped spot, lateral margins pale, anterior and posterior margins brown. Meso­ and metanotum ( Fig. 54) with near cross­shaped pattern of light and dark patches; wing pads with dark stripes and pale lateral margins; base of wing pads with a few stout, short, brown setae. Legs ( Fig. 57 ) pale, with long, dense fringe of silky swimming hairs along outer margins of femora, tibiae, and tarsi.

Abdominal terga ( Fig. 54) with medial pale stripe composed of a row of heart­shaped, spots, these divided partly by brown medial stripe from anterior edge; sublateral, longitudinal brown stripes with pigment on each segment being slightly widened anteriorly; posterior tergal margins with fringe of long pale setae; numerous intercalary bristles present along medial line. Cerci ( Fig. 58–60 ) light, in apical half with dorsal silky hair fringe arranged thickly, length of hairs exceeds length of the corresponding cercal segment; apical whorl in apical third of cerci with one ventral setae longer than others.

Material examined: Russia. Yakutiya, Aldan R., 10.VIII 1987, V. Bogatov, 3 nymphs; Amur Region, Zeya R., Set. Krasnoyarovo, 25.VI 2004, V. Teslenko, 26 nymphs (reared); Khabarovsky Region, Uda R., 13.VIII 2000, T. Tiunova, 3 nymphs; Maya R., Uda R. Basin, 9.VIII 2001, T. Tiunova, 1 nymph; Amur R., Set. Susanino, 26.VI 2000, T. Tiunova, 6 nymphs; Amgun R., 26.VI 1994, T. Tiunova, 1 nymph; Primorsky Region, Ussuri R., 30.VI 1960, I. Levanidova, 8 nymphs; Bolshaya Ussurka R., 8.VI 2004,V. Teslenko, 1 nymph.

Distribution. Europe; Russia: Altai Mnts., Sayan Mnts., Transbaikal Region, Amur River Region, Khabarovsky and Primorsky Regions, the Magadan Region; Mongolia.

Remarks. According to Lillehammer's Figure 93 (Lillehammer 1978), European populations of I. obscura have the same dark, M­shaped pigment mask between antennal bases, with anterior margin of frontoclypeus only slightly darkened; the posterior margin of the head also is darkened medially, missing the medial, diamond­shaped spot; interocellar area is pale and enclosed by pigment on all sides; the two pale patches from occiput to the antennal bases is present; the labrum has lateral margins slightly darker; the thin dark band from each eye to occiput is also present. The lacinia (Lillehammer 1978, Fig. 85) is similar the Far Eastern specimens. European specimens have the cerci (Lillehammer 1978, Fig. 99) as described in this work.

Figures

FIGURES 55 – 60. Isoperla obscura: 55. Right maxilla, ventral. 56. Right mandible, ventral. 57. Right leg, dorsal. 58 – 60. Basal, middle, and apical sections of right cercus, lateral. Scale (mm): 57 = 0.51; 55, 56, 58 – 60 = 0.4.