STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 529 
open to the exterior by a small pore which leads into a cavity 
having a clear lumen surrounded by branched cuticular pro- 
cesses, whose function is probably to prevent solid particles 
from penetrating the spiracular channel. The body of the 
fan-shaped spiracular process is filled with a fine reticulum of 
the chitinous intima, which Meijere (1902) calls the “ felted- 
chamber” (Filzkammer); through this meshwork the air can 
pass to the longitudinal tracheal trunk. 
The posterior spiracles (fig. 3, p. sp.) are D-shaped with the 
corners rounded off and their flat faces are opposed. Hach 
consists of a chitinous ring having internal to the flat side a 
small pierced knob. Hach chitinous ring encloses three 
sinuous slits, guarded by inwardly-directed fine dendritic 
processes; through these slits the air enters the small 
spiracular atrium, one of which is situated internal to each of 
the spiracles. ‘The spiracular atria communicate directly with 
the longitudinal tracheal trunks. 
The course and origin of the branches of each of the 
longitudinal tracheal trunks (fig. 26 J. tr.) is the same, so that 
of the left side will be described only. Immediately behind 
the spiracular atria the short posterior tracheal commissure 
(p. com.) connects the twotrunks. In the younger larve this 
commissure is situated more anteriorly, but in the adult it is 
situated so far back and so close to the spiracles that its 
presence might easily be overlooked. On the outer side of the 
tracheal trunk a large branch arises ; this, the visceral branch 
(v. tr.), bends ventrally to the lateral trunk, and thus becoming 
internal to it enters the convoluted visceral mass with its 
fellow of the other side. The visceral branches extend 
anteriorly as far as the seventh segment. In the twelfth and 
thirteenth segments the lateral tracheal trunk has a double 
appearance. A dorsal and a ventral branch arise in most of 
the segments, the dorsal branch chiefly supplies the fat body, 
and the ventral branch supplies the viscera; both give off 
branches to the muscular body wall. ‘The anterior commissure 
(a. com.) is situated in the fourth segment. It crosses the 
esophagus immediately behind the pharyngeal mass. On 
