The Casting of the Tracheal Gill 145 



The septa begin at the base of the stem, and die away 

 gradually upwards. Between the tortuous tracheae 

 within the thorax, and the strong chitinous septa, extend 

 a number of straight, parallel, and relatively weak 

 tubes (fig. III). 



It is here that the tracheae are ruptured wdien the 

 pupal cuticle is about to be cast. The vigorous contrac- 

 tions of the body exert a pull upon the tracheae, which 

 break across just where the abnijjt change to the chitinous 

 septa takes place, and the tracheal gill comes off with the 

 pupal skin. After the separation a number of short 

 threads (the broken ends of the tracheae) are seen to 

 project from the chitinous septa. This provision for 

 securing a clean fracture without undue violence reminds 

 us of the process by which an autumn leaf is detached 

 from the twig^. The sudden change in the strength of 

 the chitinous septa is in itself a cause of weakness ; the 

 base of the gill is weaker than if its tracheae were not 

 strengthened at all. 



Just behind the common stem of the gill the anterior 

 spiracle of the fly is formed, which opens to the air as 

 soon as the pupal cuticle is removed. 



The separation of the tracheal gill would seem likely 

 to leave openings by w^hich air could enter or leave the 

 tracheal system of the fly. Such openings, which would 

 be naturally incapable of regulation, would destroy the 

 efficiency of the tracheal apparatus, for air can only be 

 forced into the finer branches by closing all the outlets, 

 and then compressing the main trunks. The torn ends 

 of the tracheae are, how^ever, quickly sealed up. The 

 tubes themselves collapse, while their generating epithe- 

 lium, which, it will be observed, is an extension inwards 



* In a pupa which had just cast the larval skin no inequality in the 

 chitinous septa could be discovered. The generating cells, which secrete 

 the thickenings and plug up the outlets, were plainly seen in this st^iige. 



MIALL. L 



