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. Each 

 consists of a chitinous ring having internal to the flat side a 

 small pierced knob. Bach 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 I. 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 two trunks. In the younger larvse 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 

 oesophagus immediately behind the pharyugeal mass. On 



