Isoperla obscura

Isoperla obscura (Zetterstedt, 1840) Figs. 63–70 Material examined: Russia, Siberia, Republic of Buryatiya: 6 ♂ 3♀, Selenga River, 6 km from the Selenga Pulp Mill, 5.06.2007, coll. N. Bazova; 2♂ 1♀, Selenga River, Tologoy Rock, 09.07.2007, coll. N. Bazova; 7♂ 5♀, Chikoy River, settlement Povorot, Selenga R. basin, 8.06.2009, coll. N. Bazova; 15♂ 17♀, Selenga River, settlement Tataurovo, 1.07.2009, coll. N. Bazova; 22♂ 16♀, Selenga River, above confluence with Chikoy River, railway bridge, 3.07.2009, coll. A. Basov and N. Basova; Far East, Yakutiya: 4♂ 5♀ 4L, Aldan River, Lena R. basin, 10– 13.08.1987, coll. V. Bogatov; Amurskaya Oblast: 12♀, Zeya River, Amur R. basin, Ovsyanka settlement, 3.07.2013, coll. V. Teslenko; 2♀, Zeya River, upstream, near Mazanovo settlement, Amur R. basin, 5.08.2006, coll. T. Tiunov; 6♀, Zeya River, near Krasnoyarovo settlement, 24.06.2004, coll. V. Teslenko; 4♂ 2♀, Zeysky Nature Reserve, Gramatukha River, Zeya River basin, 9.09.2007, coll. E. Makarchenko; 6♂ 25♀, Zeya River, mouth, Blagoveshchensk, 22.06.1997, coll. T. Arefina; 5♂ 7♀, Zeya River, Alekseyevka-Alexandrovka settlement, Amur R. basin, 8.07.2013, coll. V. Teslenko; Khabarovsk Krai: 1♀, Bureya River, 10 km below the mouth of the Urgal River, Amur R. basin, 18.07. 2003, coll. T. Tiunova; 3♀, Tunguska River, near Danilovka settlement, Amur R. basin, 26.06.2007, coll. V. Teslenko; 3♂, 6L, Amur River, Susanino settlement, 23– 26.06.2000, coll. T. Tiunova; 2♀, Amur River, Zimmermanovka settlement, 18.06.2005, coll. T. Tiunova; 1♂ 1♀, stream without name in Amur R. basin, 2.07.2006, coll. N. Yavorskaya; Chukotka Autonomous Okrug: 1♀, Anadyr River basin, 31.07.1982, coll. I. Chereshnev. Egg. Shape oval, marginally asymmetrical with the posterior pole narrower than anterior one, which is widely rounded, cross-section concave (Figs. 63–64). Length 232‒246 µm, width at equator 165‒171 µm (n=3), chorion thickness 2.6 µm. Collar well developed, elevated with irregular and raised longitudinal ridges; base of collar embedded; ring around collar base short and slightly rough; rim flanged and wavy apically (Figs. 63, 65). Anchor mushroom-shaped with globular bodies concentrated into groups of 2‒5, mainly in a peripheral area along the anchor edge (Figs. 64, 66). Chorion structure with faintly visible hexagonal FCIs and ridges; chorionic surface smooth, evenly covered with numerous, small, and deep pits (Figs. 63–64, 67–69). Micropyles arranged singularly along one row near the anterior ⅓ of the egg and grouped in threes (Figs. 63, 67). Orifices present in oval, cup-like depressions; sperm guides are tunnel-shaped and visible (Fig. 69); in the Far Eastern population, the chorion is smooth without punctations around the orifices (Fig. 70). Eclosion line absent. Distribution. Transpalaearctic species was originally described from Lapponia (Umensis ad Umenaes, Sweden) and found in streams and large rivers in the northern part of the Palaearctic Region. Europe: Norway, Finland, Austria, Switzerland, France, Germany, Italy, and very rarely in the Balkan Region, including Croatia. Bulgaria. Russia: North-East, Komi Republic, and Nenets Autonomous Okrug; Asia, Siberia: Krasnojarski Krai, Altai, Sayan, and Transbaikalia; Far East: Southern Yakutia, Magadanskaya Oblast, Amurskaya Oblast, Jewish Autonomous Region, Khabarovsk Krai, Primorsky Krai, and Chukotka Autonomous Okrug. East Kazakhstan (Devyatkov 2022). Mongolia. Remarks. The eggs of the species vary slightly in size across its wide trans-Palaearctic range. The egg size ranges from 252–268 × 169–177 µm for the Mongolian specimen to 276–289 × 194–202 µm for the Norwegian specimen (Zwick & Surenkhorloo 2005) and 232–246 × 165–171 µm in specimens from the Russian Far East. The anchor is rarely observed in eggs of the Far Eastern population. The chorion thickness was 4 µm for the Mongolian (Zwick & Surenhkorloo 2005) and 2.6 µm in the Far Eastern specimens. Minor differences were also noted in the chorion punctuations surrounding the micropyle of the eggs from Siberia (the Selenga River basin) and Far Eastern specimens (Figs. 69–70).