46 THE INTERNAL STRUCTURE OF MUSCA DOMESTICA 



its length a trachea which supplies the palp of that side. On 

 reaching the haustellum it takes the form of a trachea proper, 

 having annular thickenings. Shortly after entering the haus- 

 tellum 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 tracheae running to the edges 

 of the oral lobes. Lowne, in his description of the tracheal 

 system of the blow-fly, 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, he claims, causes the inflation and tension of 

 the oral lobes. The mechanism of the proboscis will be discussed 

 later (p. 62), but it may be noticed here that in M. domestica 

 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 tracheae, and therefore incapable of 

 distension. 



The Posterior Thoracic Spiracle (figs. 6, 18, p.th.) is trian- 

 gular in shape and is guarded by dendritic processes. The 

 tracheal sacs of this system (fig. 18) 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 aerated to any great extent by those of 

 the other system. They supply chiefly the large muscles of 

 the thorax. Laterally a series of sacs (l.th.s.) extends antero- 

 dorsally in an oblique direction, external to the stemo-dorsales 

 muscles, to the humeral region. From the first of these sacs 

 a large number of tracheal twigs arise and supply the muscles 

 of the wing and the anterior sterno-dorsales muscles. Ventral 

 to this sac a large sac (m.v.s.) penetrates internally between the 

 anterior and median stemo-dorsales muscles and supplies the 

 lower dorsales muscles. 



From the dorsal side of the distributing sac a number of 

 sacs arise, some of which penetrate between the sterno-dorsales 

 muscles and supply the upper dorsales muscles. A more pos- 

 terior set supplies the posterior regions of the dorsales muscles, 



