42 THE INTERNAL STRUCTURE OF MUS(JA DOMESTWA 



peritreme, and the opening is guarded by a number of dendritic 

 processes which prevent the entrance of dust and other foreign 

 bodies. It leads into a shallow chamber or vestibule which 

 communicates with the rest of the spiracular system through 

 a valvular aperture. 



The anterior thoracic spiracles supply the whole of the head, 

 the anterior and 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. 17, a.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 sup- 

 plying 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 supplymg the head). In 

 the thorax it takes the form of an elongated sac lying below the 

 dorsales muscles, and by the side of the alimentary canal. From 

 the dorsal side of this the longitudinal thoracic sac {l.tr.s.) a 

 number of branches arise Avhich supply the lower dorsales muscles. 

 It is constricted about the middle of its length and anterior to 

 the constriction 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 pos- 

 terior ventral thoracic sac (p.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 abdo- 

 men it immediately dilates to form one of the large abdominal 

 air-sacs (ab.s.). The pair of abdominal air-sacs in some cases 

 occupies about half the total space of the abdomen. When the 



