74 ON THE ANATOMY OF THE FLY. 



on each side from the main trachea near the anterior thoracic spiracle, 

 to the same vessel near the posterior thoracic spiracle. These lie between 

 the lateral thoracic muscles which they partially supply they also send 

 branches to the legs. 



The cephalic branches of the lateral tracheae pass into the head and 

 expand into sacs which envelope the posterior and lateral portions of the 

 cephalic ganglion. Each of these sends a large branch forward to the 

 insertion of the antennas, which is continued along the side of the pharynx 

 to the proboscis ; and numerous offsets to the compound eyes, which they 

 almost surround. 



Another and smaller portion of the cephalic branch of the lateral 

 thoracic trunk passes over the cephalic ganglion and unites with its fellow 

 in the mesial line just behind the nerve to the ocelli. It sends off several 

 lateral offsets to the lateral portions of the cephalic ganglia, and supplies 

 the ocelli. 



The abdominal pulmonary sacs are connected by a narrow vessel with 

 the posterior extremity of the lateral thoracic trunk. They are very large 

 and spherical, occupying almost the whole base of the abdomen. 

 Each gives off several sets of vessels, kept open by a spiral fibre to the 

 abdominal viscera and dorsal vessel. With this exception, all the tracheae 

 already described are membranous in the adult insect. 



The abdominal spiracles are small round pores, about l-200th 

 of an inch in diameter although they vary considerably in 

 size in the underside of the dorsal arches. There are none 

 to the first segment, the dorsal arch being rudimentary. The 

 next four have each a pair ; but the sixth abdominal or first 

 segment of the generative organs has two pairs in the 

 female, one above the other, and only one in the male. The 

 remaining segments have none in the female, but the seventh 

 has two pairs, and the eighth a single pair, in the male. 



The peritreme of the abdominal spiracles is a mere thick- 

 ened rim, with a number of minute spiculas projecting from 

 its margin. The epitreme is situated about 1-1 5 Oth of an inch 

 within the spiracle, and is connected with the peritreme by a 

 short unbranched tracheal tube, with a few thread-like rings 



