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isolated; seta 15 shorter and thinner than 14. Pronotum lighter than head, 

 long in the middle; large spinules at anterior margin light, not numerous. 

 Mesonotum with narrow, light, median stripe; spot in posterior angle small, 

 pinkish. Gills on abdominal segments 2—3. Case markedly narrower 

 posteriorly, flattened dorsoventrally, made of sand grains and a few small 

 stones. 



Mode of life and habitats. Open shore of Lake Baikal, mainly 

 on stones. 



Distribution. USSR: Lake Baikal. Absent outside USSR. 



1. Thamastes dipterus Hagen 



Bebutova, 1941:100-103, Figures 82-96.- Levanidova (Bebutova) 1945, 

 dissertation:41-49, Plate V, Figures 2-5.- Lepneva, 1964:669-676, 

 Figures 1, 5, 9, 15, 19. 



Full-grown larva. Length 10— 12 mm. Head (Figure 146, A) short, mainly 

 uniformly chestnut brown, dots light, indistinct. Frontoclypeus large, wide; 

 a small reddish brown speck instead of the tentorial pit; 3 pale, widely 

 separated large dots on each side of the median pit; several indistinct 

 dots in middle and posterior part of sclerite; wedge-shaped pattern absent. 

 Eyes on large light area; indistinct, light, large dots lateral to dorsal 

 stripes near branches of frontal sutures and coronal suture; similar dots 

 in longitudinal rows behind the eye. Ventral surface darker than dorsal 

 side, lighter near occipital foramen, with large, indistinct, pale dots. Gula 

 forming an almost equilateral triangle; posterior angle blunt, dark 

 (Figure 146, b). 



Median anterior -margin seta short, thick; intermediate seta brown, 

 nearly twice as long as median seta; lateral seta short, light. Anterior 

 frontal seta the same, mediofrontal seta small, thin, light, posterior frontal 

 seta thin, light brown; 2—3 small, light, secondary setae near the anterior - 

 margin setae; 2 similar setae inward to the posterior frontal seta. Short, 

 brown seta 7, situated before the antenna; seta 10 situated near eye, small, 

 light; setae 9 and 14 reddish brown, long, thicker and much longer than the 

 others; seta 14 slightly longer than 9; seta 11 thin, moderately long, light 

 brown, situated behind 9; seta 12 the same; seta 13 small, light; seta 15 

 thin, light brown, about Vs as long as 14; seta 16 small, procumbent, thin; 

 seta 17 near suture anterior to fork, light brown, moderately long, thick; 

 four small secondary setae near seta 9, one in the area of seta 14; ventral 

 seta 8 short, brown, almost as long as 7; seta 18 small, light, thick. 



Labrum, with broadly membranous anterior margin; membranous pads 

 on sides of anterior margin densely covered with thin light hairs; dorsal 

 sclerite reddish brown, anterior margin of sclerite with dark processes 

 between an oval median indentation and truncate lateral areas; posterior - 

 angle sclerites oblong, thin, reddish brown at the base, light at the end. 

 Seta 1 situated on the membrane, small, thin; seta 2 situated near process, 

 blunt, light; seta 4 similarly situated near process; seta 3 in anterior angle 

 at some distance from margin, yellowish; setae 5 and 6 large, brown, with 

 filiform end. 



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