STRUCTURE, DEVELOPMENT, AND TJTONOMICSOF HOUSE-FLY. 425 



vestibule wliicli communicateis with the rest of the spiracular 

 system through a valvular aperture. 



The anterior thoracic spiracles supply the whole of the 

 head, the anterior aud median regions of the thorax, the 

 three pairs of legs, and by means of the abdominal air-sacs 

 a large part of the viscera. 



Internal to the valve the tracheal system divides. The 

 tracheal sacs springing from the posterior side are as follows : 

 Ventrally a rather narrow tracheal duct leads into a sac — 

 the anterior ventral thoracic sac (fig. 13, n.v.s.) situated at 

 the side of the thoracic ganglion which it supplies. Above 

 the origin of this another tracheal duct leads to a vertical sac 

 supplying the anterior sterno-dorsales muscles. Dorsally 

 the ducts of two sacs take their origin ; the smaller and more 

 dorsal is a flat sac closely apposed to the anterior ends of the 

 dorsales muscles {do.) which it supplies; the more ventral of 

 the two is one of the two most important branches of the 

 anterior thoracic spiracle (the other being the branch 

 supplying the head). In the thorax it takes the form of an 

 elongated sac lying below the dorsales muscles, and by side 

 of the alimentary canal. From the dorsal side of this the 

 longitudinal thoracic sac {l.tr.s.) a number of branches arise 

 which supply the lower dorsales muscles. It is constricted 

 about the middle of its length and anterior to the constric- 

 tion ; a branch is given off which supplies the ventral portion 

 of the median sterno-dorsales muscles. In the posterior 

 region of the thorax another ventral branch is given off from 

 which branches arise, one supplying the ventral portions of 

 the posterior sterno-dorsales muscles, the other opening into 

 the posterior ventral thoracic sac {j^.v.s), which supplies the 

 intermediate and posterior legs. The longitudinal thoracic 

 sac then narrows, and passes through the thoraco-abdominal 

 opening into the abdomen. In the adomen it immediately 

 dilates to form one of the large abdominal air-sacs {a.b.s.). 

 The pair of abdominal air sacs in some cases occupy about 

 half the total space of the abdomen. When the fat-body is 

 not greatly developed they occupy almost the whole of the 



