STRUOTQEEj DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 427 



ganglia, communicates witli two tracheal sacs. An internal 

 duct leads into a large spherical sac, the anterior cephalic 

 sac {a.c.s.) situated in the anterior region of the head dorsal 

 to the fulcrum. From the dorsal side of this sac a branch 

 is given off which supplies the antenna of its side ; the ven- 

 tral side is continued down the fulcrum as a narrow tracheal 

 sac. The lateral portion of the tentorial tracheal duct opens 

 into the ventro-lateral cephalic sac {v.c.s.) situated posterior 

 to the optic ganglion. The lower end of this sac gradually 

 narrows as it enters the rostrum which it traverses, giving 

 off half-way along its length a trachea which supplies the 

 palp of that side. On reaching the haustellum it takes the 

 form oi: a trachea proper, having annular thickenings. 

 Shortly after entering the haustellum it gives off two branches 

 to the muscles of this region. The main trachea is continued 

 into the oral lobe of its side where it divides into anterior 

 and posterior branches, and these again divide into numerous 

 small trachccB running to the edges of the oral lobes. Lowne, 

 in his description of the tracheal system of the blowfly, 

 describes and figures the tracheal supply of the proboscis as 

 being of the nature of tracheal sacs and capable of distension ; 

 he also describes a trefoil-shaped tracheal sac at the base of 

 the oral lobes giving off very regular branches, the dilation 

 of which causes the inflation and tension of the oral lobes. 

 The mechanism of the proboscis will be discussed later 

 (p. 439), but it may be noticed here that in M. domes tic a 

 there is no trace of a trefoil-shaped sac at the base of the 

 oral lobes, and that all the tracheal structures of this the 

 haustellum region are definite annular trachea, and there- 

 fore incapable of distension. 



The posterior thoracic spiracle (figs. 6 and 15, p.th.) 

 is triangular in shape and guarded by dendritic processes. 

 It possesses a vestibule which leads into a distributing tracheal 

 sac. The tracheal sacs of this system (fig. 15) have not the 

 extended range of those supplied by the anterior thoracic 

 spiracle, but are confined to the thorax, chiefly in the median 

 and posterior regions which are not derated to any great 



