2i8 TOPOGRAPHICAL ANATOMY OF THE 



pulmonary sacs ; these occupy the whole basal portion of the 

 cavity. The dorsal vessel lies immediately beneath the dorsal 

 integument, within the pericardial sinus ; and the posterior ex- 

 tremity of the chyle stomach and the proximal intestine lie in 

 the middle line immediately beneath the pericardium, as far 

 back as the posterior margin of the second abdominal segment ; 

 behind this the intestine is much convoluted. Below and 

 behind the pulmonary sacs is the great bilobed crop, which 

 corresponds with the honey-bag of the Lepidoptera ; when dis- 

 tended it occupies nearly one third of the ventral half of the 

 abdomen. 



Behind the honey-bag or crop the sexual glands are seen. 

 They are small in the immature female, but in the adult egg- 

 bearing insect the large ovaries occupy the greater part of the 

 abdominal cavity. The abdominal cavity of the male is much 

 smaller than that of the female, and it is encroached upon by 

 the great sac-like invagination or cloaca, which contains the 

 intromittent organ. 



The Muscular System. — It is not my intention to describe the 

 individual muscles of the imago in detail, as such a description 

 appears to me to possess little or no morphological interest, 

 and would occupy many pages. 



The general arrangement does not differ greatly from that 

 observed by Straus Durckheim [40J in the Cockchafer (Melo- 

 lontha), and Kiinckel d'Herculais [25] has described the muscles 

 of the genus Volucella, which scarcely differ from those of 

 Musca in number, origin, and insertion. 



It appears to me that the muscles may be conveniently 

 arranged in six groups, in the following manner : 



1. Muscles of the head and proboscis. 



2. Inter-segmental muscles. 



3. Muscles of the wings. 



4. Muscles of the legs. 



5. Respiratory muscles. 



6. Great thoracic muscles. 



The first five groups exhibit the same histological structure, 



