18 
BULLETIN 329, U. S. DEPAETMEXT OF AGEICULTUEE. 
i t 9 
Fig. 15. — Sirnulium venustum. Tracheal 
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 Sirnulium 
larvse 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 110, 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: S. jenningsi, from TO to 
To ; S. venustum, from 55 to 65 ; S. 
bracteatum, from 60 to TO; S. vit- 
tatum, from 65 to 85; S. pictipes. 
from 130 to 110. 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 LARVAE. 
The tracheal system of the larva 
(fig. 15) consists of two main lon- 
gitudinal trunks, connected by 
commissures, which lead to the 
spiracular chamber. Extending 
from the spiracular chambers are 
thin chitinous rods (the initial 
threads), which lead to the oval 
