44 



THE INTERNAL STRUCTURE OF MUSCA DOMESTIC A 



fat-body is not greatly developed they occupy almost the whole 

 of the basal portion of the abdomen. They give off internally 

 a large number of tracheae which ramify among the viscera 

 and provide a large portion of the contents of the abdomen 

 with air. 



From the anterior side of the anterior thoracic spiracle a 



Fig. 18. The tracheal sacs supplied by the posterior thoracic spiracle. In this 

 figure the left side of the thorax has been removed, together with the wing 

 muscles and the posterior sterno-dorsales. It must be imagined that this 

 figure is superimposed on fig. 17. 



do. Dorsales. l.th.s. Lateral thoracic sac. m.v.s. Median ventral sac. 



p.th. Posterior thoracic spiracle. sc.s. Scutellar sac. st.do. Stenao- 



dorsales. 



flattened sac arises. On its ventral side this sfives off a branch 

 which supplies the muscles of the neck and the anterior leg. The 

 sac then narrows into a rather thick-walled cervical tracheal duct 

 {c.tr.), which passes through the neck alongside the cephalo- 

 thoracic nerve-cord and enters the head. 



Tracheal Sacs of the Head. The tracheal sacs of the head 

 occupy the greater portion of the head capsule. They entirely 



