. Bulletin of the Department of Agriculture. Agriculture; Agriculture. 18 BULLETIN 329, U. S. DEPARTMENT OF Z t » 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-
. Bulletin of the Department of Agriculture. Agriculture; Agriculture. 18 BULLETIN 329, U. S. DEPARTMENT OF Z t » 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of the
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