large rivers in the plains; these forms occur in large numbers in some 

 localities. Most of these potamobionts belong to the family Hydropsychidae; 

 there occurs also one species (several in the Amur) of Polycentropodidae 

 and one species of Brachycentridae. Species of Leptoceridae occur in some 

 localities, at the banks, on stones and on the bottom; these species are 

 lithophiles which inhabit also stony lacustrine littorals. The fauna of plant 

 thickets is present in some localities (rarely in large streams); this fauna 

 increases constantly when the stream becomes smaller and its current 

 slower. 



In the north of the Russian plains, the stream elements are represented 

 in the upper reaches of the Pechora River only by Lepidostoma hirtum. 

 Hydropsyche ornatula is distributed throughout the river; Brachy- 

 centrus subnubilus occurs in smaller numbers in the middle and 

 lower reaches of the river; Neureclipsis bimaculata is found in 

 some localities in the arms and channels of the river; Arctopsyche 

 ladogensis and Hydropsyche nevae are mainly found in the upper 

 and middle reaches of the river (Lepneva, 1953:187—192). In other northern 

 rivers (Neva, Svir') both the preceding species occur in large numbers; in 

 the lower reaches of the Svir' numerous adults of Arctopsyche lado- 

 gensis appear; numerous adults of Hydropsyche nevae appear every 

 year at the beginning of summer in the Petrodvorets area; they cause 

 inconvenience, and even harm, to the inhabitants (see p. 168). 



The characteristics of the typical fauna of Trichoptera of large rivers 

 were distinct along the Volga before its reconstruction; these characteristics 

 are still recognizable in parts of the Volga which were not reconstructed. 

 Stream elements were not found in the Volga; forms common to rapids and 

 northern rivers are also absent. The most common potamobiont of the Volga 

 is Hydropsyche ornatula, which has two generations as shown by 

 Zarechnaya (i960); in some localities H. guttata occurs in small numbers; 

 the larvae of Hydropsyche are distributed throughout the profile of the 

 river; they occur mainly in the littoral stripe, on stones, tree-trunks with 

 roots, branches, on clay bottom and sometimes in the burrows of the mayfly 

 Palingenia; 12,800 larvae per m of Hydropsyche ornatula were 

 found in the middle reaches of the Volga, near Sviyaga, with a biomass of 

 41.24 g/m (Zhadin, 1948:424); near the town Gorki (before building the dam) 

 the numbers of Hydropsyche ornatula reached 3,810 larvae per m 

 (Gun'ko, 1955:7); up to 5,000 larvae per m were found in the Volga near 

 Kuibyshev (Lyakhov, 1957:118). In summer, Hydropsyche ornatula 

 and the mayfly Polymitarcys virgo appear in masses; before the 

 reconstruction of the Volga, caddis flies and mayflies appeared in summer 

 along the whole river. Neureclipsis bimaculata occurred along the 

 whole Volga, from its upper reaches, on tree trunks with roots, on sunken 

 branches and among the vegetation; however, it occurred only sporadically, 

 and in smaller numbers than H. ornatula. It occurs in large numbers in 

 small tributaries of the Volga overgrown with vegetation (Figure 167, B); 

 146 larvae per m were found on the bottom in the Sviyagi area. Br achy - 

 centrus subnubilus occurs mainly in the middle and upper reaches 

 of the Volga. On the bottom, on stones or on tree trunks with roots at the 

 banks of the Volga Leptocerus annulicornis occurs as it does also 

 in the open littoral of lakes; sometimes, together with Neureclipsis 

 121 b i m a c u 1 at a, Oxy e t h i r a sp. and Orthotrichia tetensii occur in 

 plant thickets (Bening, 1924:254— 261; Lepneva, 1929b:155-161; Zhadin, 

 1948:423-424). 



112 



