18 



BULLETIN 329, U. S. DEPARTMENT OF AGRICULTURE. 



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Fig. 15. — Simulium venustum. Tracheal sYjiT-qniilaT* chamber 

 system of larva : dc, dc, dc, Dorsal com- " 

 missures ; it, initial threads; sp(9), 

 spiracular chambers ; hg, hind gut ; rg, 

 rectal gills. Greatly enlarged. (Orig- 

 inal.) 



The thoracic proleg of Simulium 

 larvae consists of a single elongated, 

 truncate process, attached to the 

 second thoracic segment and bear- 

 ing concentric rows of minute 

 hooks at its apical end, which is 

 cup-shaped, and capable of muscu- 

 lar contraction, thus forming a 

 suckerlike organ, used by the larva 

 as means of attaching itself. A 

 somewhat similar but very much 

 larger suckerlike disk is situated at 

 the caudal extremity of the larva. 

 The rim is composed of rows of 

 strong chitinous hooks as in the 

 proleg. The number of rows 

 ranges in the different species from 

 50 to 140, and the number of hooks 

 in a row from 8 to 30. There is 

 so much individual variation in the 

 number of rows and the number of 

 hooks in a row that they are not 

 of much systematic value. The 

 average number of rows in the 

 caudal disks of five species is as 

 follows: 8. jenningsi, from 70 to 

 75 ; S. venustum, from 55 to 65 ; S. 

 bracteatum, from 60 to 70; S. vit- 

 tatum, from 65 to 85 ; S. pictipes. 

 from 130 to 140. S. pictipes has 

 by far the greatest number of rows, 

 very close together and from 25 to 

 30 hooks in each row. It is the 

 only one of these five species that 

 can be positively determined on 

 these characters alone. 



RESPIRATORY SYSTEM OF THE LARV^I. 



The tracheal system of the larva 

 (fig. 15) consists of two main lon- 

 gitudinal trunks, connected by' 

 commissures, which lead to the 



Extending 

 from the spiracular chambers are 

 thin chitinous rods (the initial 

 threads), which lead to the oval 



