395 



imaginal liistoblasts, which now extend as a new coating right 

 along, and in close contact with, the larval tracheal intima 

 (fig. 85). 



But already before the epithelium has been completely 

 renovated, the liistoblasts at the anterior extremity of the 

 longitudinal tracheal trunks begin to grow forwards over the 

 numerous tracheoles which all open into the main trunks here; 

 as many a^ eight tracheoles may converge towards this region 

 and become enclosed together in the tracheal epithelium as it 

 extends forwards. This process, which begins in the larva 

 some eight hours after defaecation, advances greatly during 

 the next four hours, and, as a result, a distinct tube is formed, 

 which encloses the tracheoles, which now appear in a state 

 of degeneration. The appearance of the degenerating 

 tracheoles has already been described ; the products of degen- 

 eration evidently help to nourish the proliferating tracheal 

 histoblasts. 



The tracheal trunks, ceasing to extend straight forwards, 

 now begin to grow downwards, and in their further extension 

 travel quite independently of the tracheoles; they are seen four 

 hours later as two wide channels running vertically down the 

 head, parallel with and internal to the great head tracheoles, 

 and often separated from these by the great ascending column 

 of myoblasts — the developing musculature of the mouth 

 appendages (fig. 91). So rapid has been their development 

 that already at this period a "spiral" tracheal intima is partly 

 developed. 



Just before the tracheal trunks turn downwards two out- 

 growths are formed from them ; of these one grows forwards, 

 slightly outwards and upwa,rds, and supplies the anterior, 

 dorsal, and lateral regions of the head. The second branch 

 grows out from the descending trunk a short distance below 

 this dorso-lateral branch, and gives off a great branching 

 tracheole into the brain. In the fresh pupa other tracheoles 

 begin to grow out from this ''cerebral trachea" ; the structure 

 and development of the imaginal tracheoles will be described 

 later. 



Tracheoles also entered into the developing antennae, while 

 from the main tracheal trunks in the defaecating larva other 

 tracheoles extend outwards into the legs and wing^s. 



In the fresh pupa the .great dorso-lateral air sacs begin to 

 develop. The new tracheal epithelium just behind the first 

 stigmatic trunk on each side begins to grow upwards as a 

 slender column of cells. Cell division continues rapidly, and 

 the columns extend further upwards, then backwards and 

 slightly outwards, growing as a pair of narrow columns of 

 cells, already showing a very distinct lumen, along the 



