(90) 



530 o. GORDON HEWITT. 



the internal side of the portion of the lateral trachea! trunk 

 that is anterior to the commissure a branch arises, and running 

 ventral to the pharyngeal mass it supplies the anterior end of 

 the larva and the oral lobes. A branch that supplies the 

 muscles of this region is given off external to the origin of the 

 anterior commissure. Internal to the origin of the commissure 

 two tracheas arise ; the anterior branch enters and supplies the 

 pharyngeal mass, and the posterior branch (tr/) enters the 

 ganglion ventral to the cerebral lobes. In the fifth segment 

 another internal tracheal branch enters the ganglion (tr."). 

 These tracheas which supply the ganglion appear to run 

 chiefly in the peripheral regions, where they divide into a 

 number of branches, the fate of some of these being interesting. 

 These branches are extremely fine, and they arise, as I have 

 previously mentioned, in association with a number of the 

 segmental nerves with which they run to the body wall. 



6. THE YASCULAE SYSTEM AND BODY CAVITY. 



The relations and structure of the vascular system of the 

 larva are on the whole similar to those of the fly ; there are, 

 however, a number of modifications. 



The dorsal vessel, which includes the so-called " heart," is 

 a simple muscular tube lying on the dorsal side immediately 

 beneath the skin, and extending from the posterior tracheal 

 commissure to the level of the cerebral lobes of the compound 

 ganglion in the fifth segment. Its wall is composed of fine 

 striated muscle-fibres arranged transversly and longitudinally, 

 but chiefly in the latter direction. The swollen posterior 

 region (PI. 33, fig. 31), which is called the heart, lies iii the 

 last three or four segments, its anterior limit being hard to 

 define. It consists of three distinguishable chambers, which, 

 however, are not divided by septa. Three pairs of ostia (os.), 

 each provided with a pair of internal valves (v.), are situated 

 laterally, and place the cardiac cavity in communication with 

 the pericardium, in which this portion of the dorsal vessel 

 lies. There are three pairs of alar muscles controlling the 



