123 tenellus occurs on tree trunks with roots. The phytophilic forms in 

 plant thickets are: Orthotrichia tetensii, Cyrnus flavidus, 

 Triaenodes bicolor, Setodes t in e if or m i s, Mystacides 

 longicornis, Leptocerus aterrimus, Agrypnia pagetana 

 and Phryganea grandis. The potamobionts Hydropsyche orna- 

 tula and Neureclipsis bimaculata occur in the Voronezh River with 

 its more abundant plant thickets (Bukhalova, 1937:35; Saint Hilaire and 

 Bukhalova, 1937:75). 



Potamobionts are less common in the Northern Donets, some areas of 

 which form a transition between running and stagnant water with plant 

 thickets which fill almost the whole profile of the river and with a very 

 slow current. Hydropsyche ornatula was not found during a 

 systematic study of the river in 1924—192 6; single specimens of 

 H. guttata were found. Brachycentrus subnubilus and Neure- 

 clipsis bimaculata are also not common in this river, but N.bima- 

 culata occurs in large numbers in channels with a rapid current and 

 continuous plant thickets. The following phytophilic species predominated 

 in the Northern Donets: Hy droptila femoralis, Orthotrichia 

 tetensii, Oxyethira sp., Cyrnus flavidus, Leptocerus 

 aterrimus, Setodes tineiformis, Triaenodes bicolor, 

 Phryganea grandis, Agrypnia pagetana. Where the current 

 becomes slow, its caddis fly fauna is very similar to the lacustrine fauna 

 (Lepneva, 1929). 



Like other rheophilic Trichoptera, potamobionts are absent in marshy, 

 slowly flowing rivers and rivulets in the low Meshchera plain; the caddis 

 flies in plant thickets of these rivulets are the same as in lakes; Triaeno- 

 des bicolor and Oecetis lacustris are common, Ecnomus 

 tenellus also occurs, Molanna angustata and Phryganea 

 striata occur more rarely (Lepneva, 1929a:6— 9). 



Beyond the Urals, the species of potamobionts of the large Siberian rivers 

 change from west to east; Siberian and Japanese-Chinese elements become 

 more numerous, and the European forms become rarer and gradually 

 disappear or are represented by isolated forms. 



The faunistic border between the Biya with its clear water and the Ob 

 with its turbid water is rather sharp. Arctopsyche ladogensis and 

 Hydropsyche nevae, which are characteristic for the Biya, do not occur 

 in the upper reaches of the Ob; Hydropsyche ornatula occurs there, 

 but not in such large numbers as in European rivers, Neureclipsis 

 bimaculata is rare, Brachycentrus subnubilus is common in 

 some localities, and occurs in large numbers. The Siberian potamobiont 

 Cheumatopsyche czekanowskii appears; among the potamobionts 

 of the Ob, species of Macronematinae predominate, e.g., Macronema 

 radiatum and Aethaloptera rossica, which fly in large numbers 

 above the water in summer. Larvae of Mystacides sp. and Triaeno- 

 des interna are found in some localities on the riverbanks (Lepneva, 

 1930:156-163; 1948:96-98; 1960:175). 



Hydropsyche ornatula predominates in the lower reaches of the 

 Irtysh; large numbers of larvae are found on the bottom, especially on 

 clay bottoms. Brachycentrus subnubilus is found mainly on tree 

 trunks with roots and sunken branches, sometimes in large numbers. 



115 



