thick, relatively long; intermediate seta situated near medioanal seta, thin, 

 black, moderately long; lateral seta short, thin; a group of setae between 

 the medioanal setae usually consisting of 2 black, moderately long, 2 short, 

 thin setae and a dark median seta, but number and form of these setae vary 

 considerably; surface with several secondary setae. 



Sclerite "b" of anal legs reddish brown, with large, dots laterally and 

 anteriorly, the median of the 3 distodorsal setae black, thick, long, 

 the 2nd the same, but shorter, the lateral thinner, black, moderately long; 

 2 black, moderately long setae at median margin; surface with 5—6 short, 

 brown, and several larger and smaller setae; ventral seta small, brown. 

 Sclerite "c" yellowish brown, with broad, black lateral margin; seta small, 

 brownish. 



Claw yellowish brown, apex reddish brown; dorsal denticle also dark; 

 seta 1 situated in basal part, small, thick; seta 2 brownish, large, situated at 

 end of suture; seta 3 the same, situated dorsally in distal part; seta 6 

 situated subdorsally and laterally; seta 4 straight, light, transparent, 

 relatively large, situated near suture, medial to seta 8; seta 5 small, situated 

 laterally near seta 8; ventral setae 7 and 8 transparent, pale yellowish, 

 seta 7 larger and curved distally (Figure 546). 



Case 12 — 15 mm long, tubular, slightly curved, slightly flattened ventrally, 

 made of smoothly arranged sand grains, those at anterior end sometimes 

 slightly larger; posterior end rounded, with a large, irregularly round, 

 central hole (Figure 54 7). 



Mode of life and habitats. Feeding on algae; pure, mountain 

 brooks and rivulets. 



Distribution. USSR: Caucasus, common. 



2. Drusus trifidus McLachlan 



Klapalek, 1888:22-25, Figure 8.- Ulmer, 1903:73-74, Figure 52; 

 1909:73-74.- Dohler, 1914:42-52, Figure 21.- Lestage, 1921:814-822, 

 Figure 295, b, e, f, i, j, k. - Fotius- Jaboulet, 1961:1-19, Figures I-XII. 

 315 Full-grown larva. Length 10— 1 1 mm, width 2.0— 2 .3 mm. Head sloping 

 sharply downward, convex dorsally in posterior part (Figure 548), slightly 

 flattened anteriorly; eyes situated in middle; antennae situated slightly 

 nearer to anterior margin than to eye. Dorsal side of head dark, especially 

 in transverse middle; anterior part lighter laterally and on ventral side; 

 posterior part of frontoclypeus with indistinct punctation. Gula dark. 

 Dorsal setae of head resembling those of Drusus caucasicus; setae 16 

 and 1 7 smaller than in D. caucasicus; seta 8 very small; seta 1 8 small. 



Labrum as in D. caucasicus. Mandibles short, massive, black; 

 distal margin without teeth; brush situated at dorsal margin; lateral setae 

 short, black, of equal length, situated basally. 



Sclerites of cardo dark in middle, with a black spot, a black stripe and 

 2 small setae laterally. Submentum with 2 transverse sclerites, divided by a 

 narrow, median stripe; primary setae short, one in middle of anterior 

 margin of each sclerite. Stipes wide, with projecting lateral margin; both 

 setae about the same length, situated anteriorly; apical seta at anterior 

 „^„ margin; basal seta at lateral margin. Apical and basal setae of palpifer 

 small, and of similar size. Maxillary palps with short segments, segment 2 



396 



